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Pixie-4 User's Manual

1. In the current implementation of the clover mode the spectrum length is fixed to 16K The clover binning mode applies all runs but in list mode runs no sum energy is reported in the list mode data The clover mode is enabled by setting the corresponding checkbox in the Pixie Viewer s Module Control Register panel There is also the option of binning only those events in the individual channel spectra that do not have multiple hits Additional clover functions are under development 68 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 9 Troubleshooting 9 1 Startup Problems 1 Computer does not boot when Pixie module is installed in chassis This is usually caused by an incorrect clock setting on the Pixie module See section 7 1 for details The module needs to have a valid clock to respond to the computer s scanning of the PCI bus Computer reports new hardware found needs driver files Whenever a Pixie module is installed in a slot of the chassis for the first time it is detected as new hardware even if Pixie modules have been installed in other slots previously Point Windows to the driver files provides with the software distribution After driver installation the module should appear in Window s Device Manager as in the picture below 2101 File Action view Help cs m Hm t Batteries q amp Computer a Disk drives amp Display adapters gt DYD CD ROM drives U
2. Position Filter us Tau ps H H H H 38 39 40 41 42 43 44 Run number Figure 3 11 The Fite Series Resutts plot to analyze a series of files from a parameter scan 22 PIXIE 4 User s Manual V2 50 O XIA 2012 All rights reserved To analyze a series of mca files you can use the Fire Series Resutts panel Enter the base name and the start and end run numbers of the series then click Parse Files Start and end are inclusive i e for start 1 and end 13 the parsing covers files base0001 base0013 An ifm file is required to read the values for Tau and the filter settings The parsing routine reads the spectra and fits peaks with the options set in the MCA Spectrum Thus make sure the fit range is set appropriately In the plot the peak position and resolution is plotted as a function of run number together with the filter settings and tau for each channel selected 23 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 4 Data Runs and Data Structures 4 1 Run Types There are two major run types MCA runs and List mode runs MCA runs only collect spectra List mode runs acquire data on an event by event basis but also collect spectra List mode runs come in several variants see below The output data are available in three different memory blocks The multichannel analyzer MCA block resides in memory external to the DSP There is a local I O data buffer for list mode data located in the DSP c
3. An example is the on off cycle of a neutron generator and events may only be of interest if the generator is off This condition can also be accommodated by the GFLT function but is often described as vetoing the acquisition while the signal is on We thus use the names GFTL and VETO interchangeably In a third scenario it may be desirable to reject pulses that occur while a GATE signal is on or off Usually this is a dedicated signal for each channel for example derived from a BGO shield around the detector When the BGO shield sees a pulse not all of the energy was deposited in the detector and therefore this event should be rejected The GATE signal is thus coincident with the rising edge of the detector pulse give or take a cable delay in contrast to the GFLT function that contributes to the event validation a filter time after the rising edge GATE IN Gate Window GATE VETO GFLT Do Counter on ON NH Invert gt start finish hie required Valid L Veto i Gate Velay required Validation Counter or start finish Pileup on Filter Counter ADC data rhreshold H TRIGGER start finish PILEUP OK 61 PIXIE 4 User s Manual V2 50 O XIA 2012 All rights reserved TO T1 T2 T3 T4 TS T6 Input signal N N Gate input HN Gate Window ON a A Gate Delay ON Pp wn O e Veto input 7 Validation n ry Figure 7 3 Block and timing di
4. The concept behind cusp like filters is that since the points nearest the step carry the most information about its height they should be most strongly weighted in the averaging process How one chooses the filter lengths results in time variant the lengths vary from pulse to pulse or time invariant the lengths are the same for all pulses filters Traditional analog filters are time invariant The concept behind time variant filters is that since the y rays arrive randomly 36 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved and the lengths between them vary accordingly one can make maximum use of the available information by setting the length to the interpulse spacing In principal the very best filtering is accomplished by using cusp like weights and time variant filter length selection There are serious costs associated with this approach however both in terms of computational power required to evaluate the sums in real time and in the complexity of the electronics required to generate usually from stored coefficients normalized W sets on a pulse by pulse basis The Pixie 4 takes a different approach because it was optimized for high speed operation It implements a fixed length filter with all W values equal to unity and in fact computes this sum afresh for each new signal value k Thus the equation implemented is k L G k LV y V y V 6 2 i k 2L G 1 i k L 1 where the filter length is Land the
5. panel input labeled DSP OUT sic can be used to send the signal to the backplane The input signal must be LVTTL i e logic 0 OV logic 1 3 3V Only one module within a chassis may use this option to avoid conflicts in driving the backplane The option is enabled in software by setting the corresponding checkbox in the Pixie Viewer s Cuassis Setup panel Setting it for one module will automatically disable it for all other modules u n OOOO Pee KE The GATE signal is also distributed over the backplane using 4 PXI nearest neighbor lines Therefore a module to the left of a Pixie 4 can be used to input 4 GATE signals to the Pixie 4 XIA s PDM can provide this function inputs 8 5 for channel 0 3 The alternative is to use the VETO signal distributed to all modules and channels as the common GATE input for each channel 7 5 External Status Besides Veto a second function for the Pixie 4 s front panel input is to contribute to a wired OR backplane line called Status Several modules can be enabled to contribute to the Status line The backplane status line will be logic 1 whenever the DSP OUT input of any enabled module is high 3 3V The status of this line is read as part of the event acquisition and is stored in the list mode data It is also possible to send the hit status bit of channel 3 to the STATUS line so that all modules will include this channel s information in their event r
6. 24 to Td from table 6 2 C Events in buffer 1637 1637 682 50 5 8K Nwords D Buffer fill time 1 657 s 0 185 s 0 690 s 51 4 ms 6 5 ms C A E EM fill time 53 034 s 5 930 s 22 090 s 1 654 s 0 218 s 32 x D 0 30ms F Total readout time 39 6ms 39 6ms 39 6ms 39 6ms 39 6ms 32x 0 55 30 ms TT LT per spill Fraction time lost to lt 0 1 0 7 0 2 2 3 16 0 readout F E 30ms 6 6 2 Live and dead time counters The Pixie 4 firmware has been optimized to reduce the dead time as much as possible and a number of counters measure the remaining dead times as well as the number of counts to provide information for dead time correction The result of these counters is stored in the following DSP output variables TOTAL TIME The TOTAL TIME is an attempt to measure the real laboratory time during which the Pixie 4 module was requested to take data It essentially counts the time from the most recent command to start a new run i e in list mode runs the start of the first spill The TOTAL TIME includes the time spent for run start initialization and host readout Thus it can be used together with the LIVE TIME to determine the fraction of the real time the module was actively taking data However since it is based on the Pixie 4 s internal oscillator a part with 50 ppm accuracy from module to module it may not be as precise as a laboratory wall clock over long time spans e g the host PC s internal clock Also i
7. CSR Table 10 4 Control and Status Register of the Pixie 4 System FPGA 0x0001 0x0002 0x0004 0x0008 0x0010 0x0020 0x0040 0x0080 0x0100 0x0200 0x0400 0x0800 0x1000 0x2000 0x4000 0x8000 RunEna Unused PClactive Unused DSPReset SynchCtrl Unused Unused SynchFlag Live Unused Unused Unused Active LAMState Unused Set to 1 to start data acquisition or 0 to stop Automatically cleared when DSP de asserts Active to end run Reserved for future use Set to reserve external memory I O for host Reserved for future use Write only Set to reset DSP processor to initiate program download Read only If low module is busy with run initialization has filled its I O buffer with data or is finished with the run Reserved for future use Reserved for future use Read only Reserved for future use Read only If zero DSP is taking data Reserved for future use Reserved for future use Reserved for future use Read only If set there is a run in progress Read only If set LAM is set internally Reserved for future use
8. Gate Pileup Out of range Veto 28 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved Normally events that are piled up etc are rejected from the acquisition and do not appear in the list mode data However when the advanced options to allow pileup or out of range are enabled or when a VETO or GATE signal is present but rejection is not enabled such events are recorded The data in User PSA can be used to find such events and treat them in special ways 2 For compression 2 List Mode RUNTASK 258 CHANHEADLEN 4 and the four words are Table 4 6 Channel header for compression 2 format CHAN_TRIGTIME a 2 CHAN ENERGY EXA 1 2 3 3 For compression 3 List Mode RUNTASK 259 CHANHEADLEN 2 and the two words are Table 4 7 Channel header for compression 3 format CHAN TRIGTIME CHAN ENERGY 4 2 3 Reconstruction of list mode time stamps As discussed above in list mode the Pixie 4 records time information in three different locations The buffer header the event header and the channel header Below we describe how to combine these time stamps into full timing information In the Pixie 4 there is a 48 bit time counter that is reset to zero at boot time or at a run start with the synchronize clocks option selected It is incremented at the full processor clock rate of 75 MHz the unit of the LSB is one 13 33ns Hence the 48 bit word can span a time interval of 43 44 days before rolling
9. Pixie 4 Start Ur panel above and Main Panel right Update After the system is initialized successfully you will see the Mam control panel that serves as a shortcut to the most common actions and from which all other panels are called Its controls are organized in three groups Setup Run Control and Results In the Setup group the Start System button opens the Starr Ur panel in case you need to reboot the modules The Open Panels popup menu leads to four panels where parameters and acquisition options are entered They are described in more detail in section 3 and in the online help To get started select Parameter Setup which will open or bring to front the PARAMETER Setup panel shown in Figure 2 2 For most of the actions the Pixie Viewer interacts with one Pixie module at a time The number of that module is displayed at the top of the Mai panel and the top right of the Parameter Setup panel Proceed with the steps below to configure your system o Open Panels Note The More Less button next to the Help button on the bottom of the PARAMETER Setup panel can be used to hide some controls This may be helpful to first time Pixie users who only want to focus on the most essential settings For an initial setup go through the following steps 7 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved EM Parameter Setup Trigger Waveform Gate Coincidence Advanced Run Control Auto
10. Verify in Window s device manager that the modules are properly installed as PLX Custom OEM PCI 9054 Boards 32 or 64 and have no resource conflicts Currently the driver must be version 6 5 0 The previously used driver version 4 1 will identify the modules as Custom OEM PCI 9054 Boards without the PLX 2 1 3 Pixie User Interface The Pixie Viewer XIA s graphical user interface to set up and run the Pixie 4 modules is based on WaveMetrics IGOR Pro To run the Pixie Viewer you have to have IGOR Version 5 0 or higher installed on your computer By default IGOR Pro will be installed at C Program Files WaveMetrics IGOR Pro Folder The CD ROM with the Pixie 4 software distribution contains 1 an installation program Setup exe 2 the Pixie 4 software in the folder XIA Pixie4 and its subfolders The Pixie 4 software can be installed by running its installation program Follow the instructions shown on the screen to install the software to the default folder selected by the installation program or to a custom folder This folder will contain the IGOR control program Pixie4 pxp online help files and 8 subfolders Configuration Doc Drivers DSP Firmware MCA PixieClib and PulseShape Make sure you keep this folder organization intact as the IGOR program and future updates rely on this Feel free however to add folders and subfolders at your convenience For the latest version of the Pixie
11. Viewer software go to support xia com and search for Pixie release 2 2 Getting Started To start the Pixie Viewer double click on the file Pixie4 pxp in the installation folder After IGOR loaded the Pixie Viewer the Start Up panel should be prominently displayed in the middle of the desktop In the panel first select the chassis type and number of Pixie 4 modules in the system Then specify the slot number in which each module resides Click on the Start Up System button to initialize the modules This will download DSP code and FPGA configuration to the modules as well as the module parameters If you see messages gt For information on using the older PLX drivers version 6 3 1 with Windows 2000 see the readme file in the Drivers folder of the software distribution In the following SmaLL Caps are used for panel names italic font is used for buttons and controls 6 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved similar to Module 0 in slot 5 started up successfully in the IGOR history window the Pixie 4 modules have been initialized successfully Otherwise refer to the troubleshooting section for possible solutions If you want to try the software without a chassis or modules attached click on Offline Analysis gt gt P ON Pixie4 Start Up Panel E Pixie4 u E A NI 14 or 18 slot chassis 3U Open Panels Y Start System OI Stop Run Figure 2 1 The
12. button which will make all advanced panel controls visible as well The Pixie 4 being a digital system all parameter settings are stored in a settings file This file is separate from the Igor experiment file to allow saving and restoring different settings for different detectors and applications Parameter files are saved and loaded with the corresponding buttons at the bottom of the PARAMETER Setup panel After loading a settings file the settings are automatically downloaded to the module At module initialization the settings are automatically read and applied to the Pixie module from the last saved settings file In addition there are buttons to copy settings between channels and modules and to extract settings from a settings file Two large buttons at the lower left duplicate the buttons to call the START Up panel and the OsciLLoscorE 3 2 1 1 Trigger Tab The Trigger tab contains controls to set the trigger filter parameters and the trigger threshold together with checkboxes to enable or disable trigger to control trigger distribution see section 7 2 1 and to set time stamping options for each channel Except for the threshold the trigger settings have rarely to be changed from their default values The threshold value corresponds to 1 4 of the pulse height in ADC steps e g with a threshold of 20 triggers are issued for pulses above 80 ADC steps This relation is true if the trigger filter rise time is large compared to th
13. by the DSP is not completed before the FIFO is filled In the current firmware the impact of case b is practically eliminated by restarting the FIFO write process before the DSP readout is finished overwriting data already read with fresh pre trigger data 3 writes can be performed for one read and the writing is resumed after Y of the waveform is read In the current firmware implementation the FIFO logic does not allow the post trigger data of a second pulse to be stored while a first pulse is waiting for readout This means if waveforms are requested overlap of a second pulse with the FPGA readout is not possible because the waveform data of the second pulse can not be recorded Essentially the FIFO is dead for new events until the FPGA readout is complete This effect is usually more significant than the cases a and b above at least for systems like HPGe detectors and scintillators where the filter time typically 2 6 us is well below the FIFO limit of 13 6 us minus the pre trigger time In any case the FIFO dead time from a and b and the FIFO s effect to prevent overlap of a second pulse with the FPGA readout is only in effect when waveforms are required by the user i e only in list mode runs with non zero tracelengths It is therefore important to set the tracelength to zero in compressed list mode runs 0x101 103 if no pulse shape analysis is performed T TO T1 T2 T4 T3 Pulse Filter sums o Filter dead time o
14. each pulse The pulse decay time Tau is used to compensate for the decay of a previous pulse in the computation of the pulse height You can enter a known good value or click on 4uto Find Tau to let the Pixie 4 determine the best value The advanced controls in this tab contain functions to modify the energy computation and to acquire a series of measurements with varying filter settings and decay times to find the best settings For a detailed description of the filter operation see section 6 MM Parameter Setup Trigger Energy Waveform Gate Coincidence Advanced Run Control Module 1 Auto Find Tau Filter Computation Channel Rise Time us Flat Top us Tau us 0 5 973 SE 1 173 50 5 973 1173 50 50 50 Load C ave Extract Figure 3 3 The Energy tab of the PARAMETER SETUP Panel lt gt lt gt lt gt lt gt lt gt 4 gt 4 gt 4 gt 1 e 2 5 973 2 173 3 pe 5 973 1 173 12 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 3 2 1 3 Waveform Tab The Waveform tab contains the controls to set the length and pre trigger delay of the waveforms to be acquired Advanced options include parameters for online pulse shape analysis 3 2 1 4 Gate Tab The Gate tab contains the controls to set the window for gating acquisition with external signals We distinguish GATE a dedicated signal for each individual channel It is active for the rising edge of the pulse e g to suppress
15. experiment this added width can be increased up to a value of 870 microseconds 16 bit counter A difference in filter times between channels will cause even channels with simultaneous pulses to contribute to the hit pattern at different times The Pixie Viewer thus calculates the required minimum window width to compensate for any such difference and displays it The application 64 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved of this minimum value depends on the System global KEEP CW controlled by the popup menu below the Window Width The values of the popup menu have the following meaning Increase for E filter never decrease At each change of Window width or the energy filter times ensures that the Window width is at least the required minimum If a larger number is entered in Window width or if filter times are changed which would lower the required minimum keep the larger value When using this mode to modify the coincidence window as required by the experiment one should remember to verify its value after changes in the energy filter times In particular when energy filter times are increased in one channel after the other the increase in the first channel will increase the Window width but the increase in the last channel making filter times equal again will not decrease the Window width Keep at required E filter width At all times keep the Window width at the required minimum If a larger or smaller num
16. fill the raw data buffer faster than the DSP can process it a The maximum processing rate depends on the multiplicity of the event and whether pulse shape analysis is performed on the waveforms For standard processing as in MCA runs it is about 240 000 480 000 pulses s i e the DSP spends roughly 2 4 us per pulse to compute the energy Processing events containing pulses from several channels has less overhead and is thus faster per pulse This rate is much higher than the maximum throughput set by Poisson statistics for most typical filter times b The size of the raw data buffer is 8K words in mode 0x100 else about 4K words An event without waveforms consisting of 1 4 pulses from the 4 channels of a module contains 9 12 raw data words per pulse depending on the number of channels contributing This means up to 400 events can be stored in the raw buffer and it does not matter how closely they follow each other as long as on average the DSP can keep up with the processing For the standard processing in MCA runs as above the time to process a pulses is about 2 4 us Therefore no events are lost if for example the 400 pulses occur in 400 us followed by a pause of 400 1200 us i e 1M s bursts with a duty cycle of 50 25 or in 200 us followed by a pause of 600 1400 us 2M s bursts with a duty cycle of 25 12 5 and so on However if waveforms are read and pulses contain more raw data words the raw buffer can contain correspondingl
17. filter range 1 2 samples are averaged 2 samples in filter range 2 and so on Since the sum of rise time and flat top is limited to 127 decimated clock cycles filter time granularity and filter time are limited to the values listed in Table 6 1 Table 6 1 RTPU clock decimations and filter time granularity Filter range Filter granularity max Trise T na All filter ranges are implemented in the same FPGA configuration Only the FILTERRANGE parameter of the DSP has to be set to select a particular filter range 6 6 Dead Time and Run Statistics 6 6 1 Definition of dead times Dead time in the Pixie 4 data acquisition can occur at several processing stages For the purpose of this document we distinguish three types of dead time each with a number of contributions from different processes 6 6 1 1 Dead time associated with each pulse 1 Filter dead time At the most fundamental level the energy filter implemented in the FPGA requires a certain amount of pulse waveform the filter time to measure the energy Once a rising edge of a pulse is detected at time TO the FPGA computes three filter sums using the waveform data from T a energy filter rise time before TO to T1 a flat top time plus filter rise time after TO see section 6 4 and figure 6 7 If a second pulse occurs during this time the energy measurement will be incorrect Therefore processing in the FPGA includes pileup rejection which enforces a mini
18. gap is G The factor L multiplying Y arises because the sum of the weights here is not normalized Accommodating this factor is trivial While this relationship is very simple it is still very effective In the first place this is the digital equivalent of triangular or trapezoidal if G 0 filtering which is the analog industry s standard for high rate processing In the second place one can show theoretically that if the noise in the signal is white i e Gaussian distributed above and below the step which is typically the case for the short shaping times used for high signal rate processing then the average in Eqn 6 2 actually gives the best estimate of V in the least squares sense This of course is why triangular filtering has been preferred at high rates Triangular filtering with time variant filter lengths can in principle achieve both somewhat superior resolution and higher throughputs but comes at the cost of a significantly more complex circuit and a rate dependent resolution which is unacceptable for many types of precise analysis In practice XIA s design has been found to duplicate the energy resolution of the best analog shapers while approximately doubling their throughput providing experimental confirmation of the validity of the approach 6 2 Trapezoidal Filtering in the Pixie 4 From this point onward we will only consider trapezoidal filtering as it is implemented in the Pixie 4 according to Eqn 6 2 The resu
19. require a local coincidence of channels 0 1 2 3 or both as above set the coincidence pattern to 0x9008 in the ModuleRegisterPanel and check only the local test box in the Chassis RegisterPanel 2 To require coincidence of channels 0 and 1 in Module 0 and no other channel module matters in the ModuleRegisterPanel set the coincidence pattern in Module 0 to 0x8888 and in all other modules to OxFFFF In the Chassis Register Panel check the global test box for all modules and the local adds to global box for module 0 No PXI PDM is required 3 To require at least 3 channels to be active in all modules use a PDM module in slot 2 and set the PDM control pattern to 0x0013 for the module in slot 3 Make sure the Module writes control pattern box is checked for this module and the Send local hit pattern to PDM box is checked for all modules Then check the global test box for all modules 67 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 8 Using Pixie 4 Modules with Clover detectors When working with clover detectors the Pixie 4 can be operated in a specific clover mode In this mode the DSP will calculate the pulse height for each channel as in normal operation and in addition for events with hits in more than one channel calculate the sum of individual channel energies The result the full energy of gamma rays scattered within the clover detector is binned in an additional addback spectrum
20. set up to be driven from the leftmost segment 7 2 4 Trigger Distribution between PXI chassis In principle it is possible to distribute triggers between several chassis with Pixie 4 modules using XIA s PXI PDM module Please contact XIA for details 59 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 7 2 5 External Triggers External triggers usually do not have the correct format and fast trigger vs event trigger timing required by the Pixie 4 trigger logic The Pixie 4 therefore includes specific logic to turn an external signal into distributed triggers External signals 3 3V TTL standard can be connected to the Pixie 4 front panel input labeled DSP OUT The DSP variable XETDELAY Control field Validation delay in the Cuassis Setup panel controls generation of fast and event triggers If the value is zero no triggers are generated If the value is nonzero a fast trigger is issued to the backplane immediately after detection of a rising edge on the front panel and an event trigger is issued the specified delay thereafter As the triggers are sent to the backplane the external triggers appear as if an additional module with a pileup inspection time energy filter rise time plus flat top equal to XETDELAY had seen a pulse Sharing triggers over the backplane must be enabled even for the module connecting to the external signal 7 3 Run Synchronization It is possible to make all Pixie 4 modules in a system s
21. shown in Figure 3 10 A control field named Filter Range is repeated from the Energy tab In three groups of controls you can set the start end and step size for varying the energy filter rise time the energy filter flat top and Tau If the step size is zero that parameter will not be varied Two buttons assist in setting up the initial conditions Set Parameters to Start sets the current values of the energy filter and Tau to the start value defined in the File Series Scan panel If you omit to click this button the file series will begin with the current value this is useful to resume a file series Set Scan Run Conditions will set the checkboxes in the Data Record Options panel to the values required for the scan and set the run time to the total time required interval N in the Data Recorp Options panel times the number of settings At the bottom of the panel the button Start Scan starts the file series This is a different button from the standard Start Run button because it is starting a run which is modifying parameters All the updates during a run work the same as in a standard run though and the run can be stopped with the standard Stop Run button When the run is complete click on the File Series button to open the panel described in section 3 6 3 3 6 3 File Series Analysis IMFile Series Results E O x Basename Y Include channel 0 Parse Fil Hel cl O Include channel 2 LA Ered Kav O Include channel 3 FWHM 9
22. the read always option is set for this channel 58 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved Timestamp Normally in acquisitions with shared group triggers all channels record the identical timestamp of the last group trigger for this event Since waveforms are captured based on the group trigger this ensures that within a data record traces and timestamps are correlated However if no waveforms are recorded there is no time of arrival information for possible delays between channels from the timestamps alone In this case setting the local trigger only option preserves the time difference information by recording timestamps for each channel based on its local trigger only 7 2 2 Trigger Distribution Between Modules Both fast triggers and event triggers can also be distributed over the PXI backplane Each trigger uses a common backplane line for all modules which is set up to work as a wired OR Normally pulled high the signal is driven low by the module that issues a trigger All other modules detect the lines being low and send the triggers to all channels In other words the backplane line carries a system wide trigger that essentially acts as a 5 input to the trigger OR in the Communication FPGA of each module Each module can be enabled to share triggers over the backplane lines or not In this way a trigger group can be extended over several modules or each module can form its local sub gro
23. to slot 2 if a module is writing the global coincidence control word to a neighboring PDM and the control word to write There is also a checkbox for each module to send the hit bit of its channel 3 to the status line As the status line information is included in the event hit pattern in all modules this allows one specific channel to contribute information to the event records of all modules 66 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved E A Chassis Register Panel so e Jes Front Panel and Backplane Options Module 0 1 2 Trigger share mode 0 0 0 Al Front panel drives GFLT line one module only Oo Oo a Front panel contributes to STATUS line wire OR E O o Module writes control pattern to PDM to immediate left O O a Send local hit pattern to PDM in slot 2 O O a PDM control pattern 0x0000 Ox0000 0x0000 Validation delay for external fast trigger ns 0 0 0 y Module Coincidence Setup Module 0 3 2 Accept event if local hit pattern passes local test M M M Accept event if global hit pattern passes global test in PDM O a ul Module s local test adds to global test local fail causes global fail O a O Channel 3 Hit contributes to STATUS line wire OR O O a Coincidence Pattern for local test from Module Register OXFFFE OxFFFE OxFFFE Coincidence Window for both tests from Module Register 13 13 13 B Figure 7 6 Module coincidence setup in the Pixie Viewer Examples 1 To
24. trigger filter have only minor effect on the resolution However changing the trigger conditions might have some effect on certain undesirable peak shapes A longer trigger rise time allows the threshold to be lowered more since the noise is averaged over longer periods This can help to remove tails on the peaks A long trigger flat top will help to trigger better on slow rising pulses and thus result in a sharper cut off at the threshold in the spectrum 3 5 4 Decay Time The preamplifier decay time T is used to correct the energy of a pulse sitting on the falling slope of a previous pulse The calculations assume a simple exponential decay with one decay constant A precise value of Tt is especially important at high count rates where pulses overlap more frequently If tis off the optimum peaks in the spectrum will broaden and if Tis very wrong the spectrum will be significantly blurred The first and usually sufficiently precise estimate of t can be obtained from the Auto Find routine in the Energy tab of the Parameter Setup panel Measure the decay time several times and settle on the average value 19 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved Fine tuning of T can be achieved by exploring small variations around the fit value 2 3 This is best done at high count rates as the effect on the resolution is more pronounced The value of T found through this way is also valid for low count rates Manually enter Tt ta
25. two clock signals 7 2 Trigger Distribution 7 2 1 Trigger Distribution Within a Module Within a module each channel can be enabled to issue triggers Two kinds of triggers are distributed First a Fast Trigger indicating the trigger filter just crossed the threshold which is used to start pileup inspection and to stop the FIFOs for waveform acquisition among other things Second an Event Trigger indicating that pileup inspection was passed i e validating the event as acceptable The Event Trigger also tells the DSP that data is ready for readout in the Trigger Filter FPGA If channels are set to group trigger mode each trigger enabled channel issues both kinds of triggers to the central Communication FPGA which builds an OR of all triggers and sends it back to all channels The channels then use the distributed fast triggers and event triggers instead of their own local triggers to capture data In this way one channel can cause data to be acquired at the same time in all other channels of the trigger group The DSP then reads data from all participating channels and stores it as one event record Each channel trigger enabled or not always also generates a hit flag if pileup inspection was passed and DSP readout is conditional to this flag 57 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved Even in group trigger mode some data is captured based on the channel s local trigger For example the pulse he
26. would notify the DSP that there are raw data available now If a trigger was issued the data remain latched until the RTPU has been serviced by the DSP The third component of the RTPU is a FIFO memory which is controlled by the pile up inspector logic The FIFO memory is continuously being filled with waveform data from the ADC only stopped to avoid overwriting of data for valid events On a trigger the read pointer is positioned such that it points to the beginning of the pulse that caused the trigger When the DSP collects event data it can read any fraction of the stored waveform up to the full length of the FIFO 5 3 Digital Signal Processor DSP The DSP controls the operation of the Pixie 4 reads raw data from the RTPUs reconstructs true pulse heights applies time stamps prepares data for output to the host computer and increments spectra in the on board memory The host computer communicates with the DSP via the PCI interface using a direct memory access DMA channel Reading and writing data to DSP memory does not interrupt its operation and can occur even while a measurement is underway 33 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved The host sets variables in the DSP memory and if necessary calls DSP functions to apply them to the hardware Through this mechanism all gain and offset DACs are set and the filter settings are applied to the RTPUs The RTPUs process their data without support from the
27. 2 Unless already installed power down the host computer install the controller in both the host computer and chassis and power up the system again chassis first 3 Windows will detect new hardware the controller and should find the drivers automatically Verify in Window s device manager that the controller is properly installed and has no resource conflicts 4 Install Igor Pro In some systems scan for hardware changes in the Windows device manager may detect and install a remote chassis when the PC was booted first At the time of writing these restrictions are National Instrument s MXI 3 and MXI 4 controllers do not support Windows 7 64 bit on systems with more than 4 GB RAM Embedded PC controllers and the MXI Express series of controller bridges appear to be fully compatible with Windows 7 64 bit 5 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 5 Install the Pixie 4 software provided by XIA see section 2 2 3 6 Unless already installed power down the host computer and install the Pixie 4 modules in the chassis Check the input jumper settings for the appropriate signal termination 50 Q or 5 KQ see section 10 1 for details Then power up the system again chassis first 7 Windows will detect new hardware the Pixie 4 modules and should find the drivers automatically If not direct it to the drivers directory in the Pixie 4 software distribution installed in step 5
28. 7564 and the first event may occur at 8 56 456 With only the lower two words recorded in the event header the rule described in 2 above use BUF_TIMEHI 1 would lead to incorrectly using 4 as the highest 16 bit word in the computation of EventTime In this case CHAN REALTIMEHI should be used instead of BUF _TIMEHI It contains the highest 16 bit word at the time the event is processed by the DSP in this example the correct value 8 However this word is only available in run types 0x100 and 0x101 Since CHAN _REALTIMEHI is recorded several microseconds after the event time there is a in 1e6 chance of it being off by 1 31 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 5 Hardware Description The Pixie 4 is a 4 channel unit designed for gamma ray spectroscopy and waveform capturing It incorporates four functional building blocks which we describe below This section concentrates on the functionality aspect Technical specification can be found in section 1 2 Figure 5 1 shows the functional block diagram of the Pixie 4 FPGA MCA trigger List mode pileup memory FIFO Interface logic trigger pileup FIFO Figure 5 1 Functional block diagram of the Pixie 4 front end data acquisition and signal processing card 5 1 Analog Signal Conditioning Each analog input has its own signal conditioning unit The task of this circuitry is to adapt the incoming signals to the input vo
29. DSP once they have been set up When any one or more of them generate a trigger an interrupt request is sent to the DSP It responds with reading the required raw data from the RTPUs and storing those in an intermediate buffer It then returns from the interrupt routine without processing the data to minimize the DSP induced dead time The event processing routine works from the data in the buffer to generate the requested output data There are different implementations of the intermediate buffer for the different run types In standard list mode runs intermediate and I O buffer are the same to avoid moving long waveforms inside the DSP In other run types the intermediate buffer is circular i e old data is overwritten once it has been processed If the circular buffer fills up before the data can be processed no further raw data is read from the RTPUs In this scheme the greatest processing power is located in the RTPUs Implemented in FPGAs each of them processes the incoming waveforms from its associated ADC in real time and produces for each valid a event a small set of distilled data from which pulse heights and arrival times can be reconstructed The computational load for the DSP is much reduced as it has to react only on an event by event basis and has to work with only a small set of numbers for each event 5 4 PCI Interface The PCI interface through which the host communicates with the Pixie 4 is implemented in a PCI slave IC tog
30. DSP every 2 3ms if a run is in progress or not Their precision is in the order of 5 10 or 50 cps 3 2 3 Chassis SETUP The Cuassis Setup panel is used to set parameters that affect the system as a whole Examples are trigger distribution between modules coincidence settings between modules and the operation of the Pixie 4 s front panel input See sections 7 2 2 and 7 6 2 for details 3 2 4 Fices Patus The firmware files DSP files and settings files are defined in the Fites Patus panel Changes will take effect at the next reboot e g when clicking the Start Up System button in this panel or in the Start Up panel There is also a button to set the files and paths to the default relative to the home path of the file Pixie pxp ioj x File Names and Paths Firmware Firmware Directory C XIA Pixie 4 P9_v2 Firmuare Comm FPGA File Rev B C XIA Pixie 4 P4_w2 Firmware syspixie_RevB bin Find Find Find Comm FPGA File Rev C C XIA Pixie 4 P4_v2 Firmware syspixie_RevC bin Signal Processing FPGA File C XIA Pixie 4 P4_v2 Firmware pixie bin DSP DSP Code File C XIA Pixie 4 P9_v2 DSP PXIcode bin Find Find Find DSP Parameter Names File C XIA Pixie 4 P4_v2 DSP PXlcode war All Parameter Names File C XIA Pixie 4 P4_v2 DSP PXlcode Ist Settings File DSP Parameter Values File C XIA Pixie 4 P9_v2 Configuration default set Find Out
31. EN line and puts it in the event hit pattern The hit pattern also contains the status of the backplane STATUS line the result of the local coincidence test and the status of the front panel input at this moment Depending on user settings the event will be recorded or discarded if the TOKEN and or LOCAL bits are set in the hit pattern A full implementation of this feature thus requires an additional module in slot 2 of the chassis receiving hit patterns over the PXI STAR Trigger lines making a coincidence decision and signaling the result on the TOKEN line This can be XIA s PXI PDM module or any other compatible PXI module A limited coincidence decision can be made with Pixie 4 modules only e g one or more master modules can inhibit acquisition in all other modules based on their local hit pattern In the Pixie Viewer the module coincidence is configured in the Cuassis Setup panel Fig 7 4 With the checkboxes in the Module Coincidence Setup block each module can be set to accept events if a only the local coincidence test is passed check ocal b only the global coincidence test is passed check global c either the local OR the global coincidence test is passed check global and Jocal d the global test AND local tests from all master modules pass check global for all module and local adds to global for the master modules Other checkboxes and controls define if a module sends its hit pattern
32. Eas 71 IO Append Air ar dances A cok EA aa EE A aAA a agian E E e ES 73 10 1 Front end jumpers for termination and attenuatiON oooonnnoccnonncionnnnonecnonenconcnnononcnnnnonanncnnnnnnnnnnnnnos 73 10 2Clock Jumpers a Caden alba A tada e 74 10 3 PXI backplane pin fUNCtIONS oooooccconocoonccionecconencnnrnconnncnonocnon ono aa E aE cnn EA Ea 75 10 4 Control and Status Register BifS ooooooonocccnonccnonccconcncononcnonononnncnnnncnon conan nn con nn nan nn rro nn rn nn rn naar nnnnno 76 2 PIXIE 4 User s Manual V2 50 O XIA 2012 All rights reserved 1 Overview The Digital Gamma Finder DGF family of digital pulse processors features unique capabilities for measuring both the amplitude and shape of pulses in nuclear spectroscopy applications The DGF architecture was originally developed for use with arrays of multi segmented HPGe gamma ray detectors but has since been applied to an ever broadening range of applications The DGF Pixie 4 is a 4 channel all digital waveform acquisition and spectrometer card based on the CompactPCI PXI standard for fast data readout to the host It combines spectroscopy with waveform capture and on line pulse shape analysis The Pixie 4 accepts signals from virtually any radiation detector Incoming signals are digitized by 14 bit 75 MSPS ADCs Waveforms of up to 13 6 us in length for each event can be stored in a FIFO The waveforms are available for onboard pulse shape analysis which can be customize
33. Find Tau ii 49 149 49 lt gt Start System Oscilloscope Figure 2 2 The Parameter Setup Panel Energy tab shown 1 If not already visible open the Parameter Setup panel by selecting Parameter Setup from the Open Panel popup menu in the Maw panel 2 At the bottom of the Parameter Setup panel click on the Oscilloscope button This opens a graph that shows the untriggered signal input In the OsciLLoscorE panel click Refresh to update the display The pulses should fall in the display range 0 16K If no pulses are visible or if they are cut off at the upper or lower range of the display click Adjust Offsets to automatically set the DC offset If the pulse amplitude is too large to fall in the display range decrease the Gain If the pulses are negative toggle the Jnvert checkbox lt lt s lt lt lt s lt COC re J co wo E gt o a Ll 2 Figure 2 3 Osc LLoscore panel with typical pulses from a pulser 8 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 3 In the Energy tab of the Parameter Setup panel input an estimated preamplifier exponential RC decay time for Tau and then click on Auto Find Tau to determine the actual Tau value for all channels of the current module You can also enter a known good Tau value directly in the Tau control field or use the controls in the OsciLoscorE to manually fit Tau for a pulse 4 Save the modified parameter set
34. G This may be compared to analog filtered pulses whose tails may persist up to 40 of the rise time a phenomenon due to the finite bandwidth of the analog filter As we shall see below this sharp termination gives the digital filter a definite rate advantage in pileup free throughput ADC units 9 5 10 0 10 5 11 0 11 5 12 0 12 5us Time Figure 6 3 Trapezoidal filtering of a preamplifier step with L 1us and G 0 4ps 6 3 Baselines and Preamplifier Decay Times Figure 6 4 shows an event over a longer time interval and how the filter treats the preamplifier noise in regions when no y ray pulses are present As may be seen the effect of the filter is both to reduce the amplitude of the fluctuations and reduce their high frequency content This region is called the baseline because it establishes the reference level from which the y ray peak amplitude V is to be measured The fluctuations in the baseline have a standard deviation which is referred to as the electronic noise of the system a number which depends on the rise time of the filter used Riding on top of this noise the y ray peaks contribute an additional noise term the Fano noise which arises from statistical fluctuations in the amount of charge Qx produced when the y ray is absorbed in the detector This Fano noise Or adds in quadrature with the electronic noise so that the total noise in measuring V is found from sqrt of 0 3 38 PIXIE 4 User
35. Human Interface Devices Ca IDE ATAJATAPI controllers Keyboards A Mice and other pointing devices HE Modems E Monitors amp Network adapters Y NIMXI Devices lb Other devices lb PLX Custom OEM PCI 9054 Board 32 o Processors Sound video and game controllers lt Storage controllers q amp System devices F Universal Serial Bus controllers Tee mC mC mCmC mcs 3 Windows can not use drivers due to problem in digital signing This problem seems most common in Windows 7 The PLX drivers currently only inofficially support Windows 7 While this is being addressed a workaround solution is to install the complete PLX software development kit SDK which seems to install the PLX drivers in a way acceptable to Windows 7 The SDK is available for free from PLX http www plxtech com but registration is required 4 Drivers are installed but module does not boot The Pixie 4 modules require driver version 6 5 0 0 provided by XIA not the earlier versions 6 3 1 5 2 4 1 or 4 4 Modules should be listed in Window s device manager as PLX Custom OEM PCI 9054 Board 32 or 64 If the PLX is missing it indicates driver version 4 1 is used A picture of the driver information reported by Windows is shown below 2 x General Driver Details Resources M e PLX Custom OEM PCI 9054 Bo
36. T1 x will pass pileup inspection at time T4 x No dead time is incurred for the FPGA readout if it is completed at a time T3 before T4 T4 T1 T1 T0 the filter dead time The FIFO dead time is ignored As a consequence of the pileup inspection there is a delay of one filter time between the rising edge of the pulse and the decision to record the data i e the validation as not piled up This delay can be used to pipeline the subsequent processing steps the coincidence window and FPGA readout of a first pulse can overlap with the pileup inspection of a second pulse as described below 2 FIFO dead time The second unavoidable dead time comes from the waveform capture FIFO When the user requires waveform data from before time TO the FIFO must contain this pre trigger data at time TO else the event data is incomplete and the event is rejected This means that whenever the 7 G Knoll Radiation and Measurement J Wiley amp Sons Inc 2000 chapters 4 and 17 43 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved FIFO write process was stopped it takes a pre trigger time to refresh the pre trigger data and be ready for new events To minimize this effect the FIFO write process is stopped only in two cases when it is necessary to avoid overwriting potentially valid data a When the event validation takes longer than the time available in the FIFO 13 6 us minus the pre trigger time and b when the readout of a valid event
37. User s Manual Digital Gamma Finder DGF Pixie 4 Version 2 50 June 2012 XIA LLC 31057 Genstar Road Hayward CA 94544 USA Phone 510 401 5760 Fax 510 401 5761 http www xia com Disclaimer Information furnished by XIA is believed to be accurate and reliable However XIA assumes no responsibility for its use or for any infringement of patents or other rights of third parties which may result from its use No license is granted by implication or otherwise under the patent rights of XIA XIA reserves the right to change the DGF product its documentation and the supporting software without prior notice Table of Contents LOVE VIEW di e leo nes wel SOG Ia boon abl cana Palen Aoulgey a E A earn aa 3 TeV E A E eosin 3 AAA EAN 4 AS Opi EN 5 2T installations matinn i a Pani eto da dkPeae eh dae ca AE a E e e ee 5 2 2 Getting Madina a a a a a a T aaa A aa aa a e O 6 3 Navigating the Pixie Viewer onines ed an AE a s a do c 10 LION ad e a a a E EATA 10 3 2 SOUP GROUP ss a EEE taa 11 373 Run Control Groupen A A AA e E ET E a 16 314 Results OO o AE E A T iodo 16 3 5 Optimizing Parameters ui AAA a 18 IO FIE eres an A Rata 20 4 Data Runs and Data StrUC ULES ooooccnonocionncconecnonenonnnncononcnnn nono no ron nro n rn nan rr nan nr nan rn nan EE EEI ia 24 AUT RUN Types Taoroa A e A E A AA A Ad AA tin 24 4 2 Output Data Str ctureS it A A A ita 26 5 Hardware Description ii A A A A Dd Sale 32 5 T Analog Signal Condi
38. Valid event RR et dl sm eS FIFO write halted Coincidence Window FPGA readout incl trace EN FT SFDT ee OS SFDT neighboring ch A o Fig 6 8 Pulse dead time with trace capture The FIFO blocks new events as long there is a valid event in the FPGA Therefore the soonest arrival time for a second pulse is at T3 after the FPGA readout is complete now later than T4 The FIFO write resumes before T3 overwriting data already read out and thus contains pre trigger data for the second pulse at T3 44 PIXIE 4 User s Manual V2 50 O XIA 2012 All rights reserved 3 Coincidence window Each valid event is further subject to a coincidence test 1 e checking the hit pattern against the user defined pattern of acceptable events The test itself requires about 5 clock cycles overhead plus a minimum window of 1 clock cycle and thus adds about 80 ns of dead time Any time added by the user to the coincidence window increases the dead time accordingly However the full dead time is only incurred by the first channel that is validated since the coincidence window is counted from the first event validation For example if a second channel sees no pulse during the coincidence window it will be active during the full coincidence window If a third channel sees a pulse and is validated by passing its local pileup inspection 500ns after the first it was active for 500 ns longer than the first channel and incurred dead time of coin
39. a co a moment Sec i ul SAN W TOT MPU 56 58 60 62 64 66 68us Time Figure 6 6 A sequence of 3 y ray pulses separated by various intervals to show the origin of pileup and demonstrate how it is detected by the Pixie 4 The value V captured will only be a valid measure of the associated y ray s energy provided that the filtered pulse is sufficiently well separated in time from its preceding and succeeding neighbor pulses so that their peak amplitudes are not distorted by the action of the trapezoidal filter That is if the pulse is not piled up The relevant issues may be understood by reference to Figure 6 6 which shows 3 y rays arriving separated by various intervals The fast filter has a filter length Ly 0 1us and a gap G 0 1 us The slow filter has L 1 2us and G 0 35us Because the trapezoidal filter is a linear filter its output for a series of pulses is the linear sum of 1ts outputs for the individual members in the series Pileup occurs when the rising edge of one pulse lies under the peak specifically the sampling point of its neighbor Thus in Figure 6 6 peaks 1 and 2 are sufficiently well separated so that the leading edge of peak 2 falls after the peak of pulse 1 Because the trapezoidal filter function is symmetrical this also means that pulse 1 s trailing edge also does not fall under the peak of pulse 2 For this to be true the two pulses must be separated by at least an interval of L G Peaks 2 and 3 which ar
40. a detector pulse with a coincident pulse from a BGO shield VETO a signal distributed to all modules and channels but each channel is individually enabled to require or ignore this signal VETO is active during the validation of a pulse after pileup inspection an energy filter rise time plus flat top after the rising edge With suitable external logic the decision to veto a pulse can be made from information obtained at the rising edge of the pulse e g multiplicity from several channels and therefore this function is also called Global First Level Trigger GFLT For a detailed description of the GATE and VETO operation see section 7 4 3 2 1 5 Coincidence Tab The Coincidence tab contains the controls to set the acceptable hit pattern and the coincidence window after validation during which channels can contribute to the hit pattern There is a checkbox for each possible hit pattern For example if the checkbox with pattern 0100 is checked events with a hit in channel 2 and no others are accepted Selecting multiple checkboxes accepts combinations of hit patterns e g any event with exactly one channel hit For a detailed description of the coincidence operation see section 7 2 1 Controls for coincidences between modules are located in the Cuassis Setup Panel and described in section 7 2 2 3 2 1 6 Advanced Tab The Advanced tab contains the controls for modifying the pileup inspection histogram accumulation and baseline mea
41. agrams of the GATE and VETO circuitry in the trigger filter FPGA If required by the user pulses are validated only if VETO and or GATEBIT are present during validation GATEBIT reflects the status of the GATE input at the time of the trigger either directly or after a coincidence window is applied Names highlighted in green are controlled by the parameters in the Gate tab of the PARAMETER Setup Panel The Pixie 4 accommodates these scenarios in the following way figure 7 3 In each channel the VETO signal contributes to the validation of a pulse at time T3 if it is set by the user to be required to do so The polarity of the input can be optionally inverted The rising edge of the GATE signal optionally the VETO signal optionally edge inverted starts a counter of length Gate Window at time TO The time the Gate Window counter is ON is called GATE PULSE A trigger generated at the rising edge of a pulse from the detector starts a counter of length Gate Delay at time T1 When the Gate Delay counter is finished at T2 the status of the Gate Window counter is latched as GATEBIT Alternatively the pulsing logic can be bypassed and the status of the GATE input is latched directly as GATEBIT by the trigger If gating is required the GATEBIT optionally inverted also contributes to the pulse validation at T3 else it is only recorded in the output data stream in list mode data In all cases the trigger also starts the standard pileup counter that vali
42. ard 32 Driver Provider PLX Technology Inc Driver Date 6 1 2011 Driver Version 6 5 0 0 Digital Signer Not digitally signed Driver Details To view details about the driver files Update Driver To update the driver software for this device pa Mae deg e Disable Disables the selected device Uninstall To uninstall the driver Advanced OK Cancel 5 When starting the Pixie Viewer IGOR reports compile error or missing module For IGOR to start up properly a number of driver files have to be in the correct locations In particular the file pixie xop has to be located in the Igor Extensions folder usually C Program Files Wavemetrics Igor Pro Igor Extensions in a default installation and the file PIxApi650 dll has to be in C Windows System32 When starting up modules in the Pixie Viewer downloads are not successful This can have a number of reasons Verify that The files and paths point to valid locations run the UseHomePaths macro The slot numbers entered in the Startup panel match the location of the modules The correct drivers are used version 6 5 0 and modules are recognized in Window s Device Manager as shown above After starting up modules the ADC traces show only rectangular waves This problem can be caused by downloading the wrong Communication FPGA file e g a Rev B file to a Rev C module Verify the file names are correct 9 2 Acquisit
43. ay The MCA Spectrum display shows the spectra accumulated in on board memory or from a mca file saved at the end of a run Spectrum analysis is limited to fitting peaks with a Gaussian and computing the peak resolution There are several options to define the fit range as described in the online help Spectra can be saved as text files for import into other applications 16 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 3 4 2 List Mope Traces and List Mope SPECTRUM la File Pw2_Catraces0001 bin Find CO s0375 135534 omara ree J ose o a 2 wo m 2 3 o Figure 3 7 The List Mope Traces display The List Mope Traces display shows the data from the binary list mode files bin If waveforms were collected they are shown in the graph section of the panel Event and channel header information energy time stamps and hit patterns as described in section 4 1 2 are shown in the fields above the graph section After specifying a data file with the Find button you can select an event to view by entering its number in the Event Number field The button Digital Filters opens a new plot that shows the response of the trigger filter and energy filter computed from the list mode waveforms This plot is more precise than the related graph opened from the OsciLLoscoPE since it uses the same full rate 13 3ns data as the filters implemented in the module not the reduced rate samp
44. ber is entered it is ignored and changed back to the required minimum Use this mode if there are no coincidence requirements from the experiment and differences in the energy filter should be accommodated but unnecessary large values should not linger as in the first mode Disregard E filter requirement In this mode the energy filter requirement is ignored and any value can be entered for the Window width The entered value is not modified by the software Use this mode if channels are independent so there is no need to wait for delays between channels or if a specified Window width is deemed sufficient and should not be modified by the software Notes 1 Any added coincidence window width will increase the time required to process an event and thus reduce the maximum count rate 2 In run types 0x100 0x301 pulses contributing during the readout of data after the end of the coincidence window are lost The readout may take several microseconds longer if waveforms are to be recorded 7 6 2 Coincidences Between Modules If more than one module is operated in the same PXI chassis acceptance of events can also be subject to the results of a system wide global coincidence test The result of the global test is distributed over the TOKEN backplane line This module coincidence test takes place in the following steps After receiving a valid event trigger and waiting for the user defined coincidence window each module sends
45. ble 10 1 Analog conditioning selection jumpers on Pixie 4 modules x 1 4 for channel 0 3 Jumpers are marked with solid red 50Q and dashed blue attenuation arrows reference Remove only if you require attenuation Attenuation will be A 1 7 5 if JPx02 is set lt 4 _ 5K JPx05 Revision B only Set to VGA to enable contributions of the i variable gain amplifier to the overall system gain for fine tuning of the gain To ADC Figure 10 1 Simplified input stage of Pixie 4 showing jumpers input termination and attenuation and the overvoltage protection circuit 10 2 Clock Jumpers Table 10 2 On board jumper settings for the clock distribution on Pixie 4 modules Clock mode JP1 and JP2 JP3 PCB Label Daisy Chained Connect pins 2 and 3 of JP2 not set LOC to IN Clock Repeater Bussed Clock Connect pins 2 and 3 of JP2 Master Connect pinl JP1 to pin 1 JP2 OUT to BUS Slave PXI clock Clock Master for Connect pin2 JP1 to pin 2 JP2 not set PXI to IN PXI clock Connect pin3 JP1 to pin 3 JP2 LOC to BP Revision C only 10 3 PXI backplane pin functions Table 10 3 Pins of the J2 backplane connector defined in the PXI standard used by the Pixie 4 Pins not listed are not connected except for pull ups to 5V recommended by the PXI standard number name DI a NANA TRIGO TRIG4 LBR5 Right neighbor A AA AA AA A A PD AO uml A ee O ES 10 4 Control and Status Register Bits
46. c noise in the system you can view a Fourier transform of the incoming signal by selecting OsciLoscorE gt FFT The graph shows the FFT of the untriggered input sigal of the OsciLLoscorE By adjusting the dT control in the OscinLoscorE and clicking the Refresh button you can investigate different frequency ranges For best results remove any source from the detector and only regard traces without actual events If you find sharp lines in the 10 kHz to 1 MHz region you may need to find the cause for this and remove it If you click on the Apply Filter button you can see the effect of the energy filter simulated on the noise spectrum 18 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 3 5 2 Energy Filter Parameters The main parameter to optimize energy resolution is the energy filter rise time Generally longer rise times result in better resolution but reduce the throughput Optimization should begin with scanning the rise time through the available range Try 2us 4us 8us 11 2us take a run of 60s or so for each and note changes in energy resolution Then fine tune the rise time The flat top usually needs only small adjustments For a typical coaxial Ge detector we suggest to use a flat top of 1 2us For a small detector 20 efficiency a flat top of 0 8us is a good choice For larger detectors flat tops of 1 2us and 1 6us will be more appropriate In general the flat top needs to be wide enough to accommodate the longest t
47. cation of firmware settings and output files 3 2 4 Energy filter 3 2 1 2 Waveform capture 3 2 1 3 MCA Spectrum View energy histograms 3 4 1 List Mode Spectrum Recreate MCA from list mode data 3 4 2 Trigger distribution and Chassis Setup Oscilloscope Analog gain and coincidence between offset 3 2 2 modules 7 2 2 7 6 2 Parameter Setup 3 2 1 Trigger and threshold 3 2 1 1 Gate and Veto 7 4 Coincidence between channels 7 6 1 Run Control Options 3 2 1 7 Run Statistics Live times and count rates 3 4 3 6 6 File Series View results froma series of files 3 6 List Mode Traces View waveforms and event data 3 4 2 Figure 3 1 Block diagram of the major panels in the Pixie Viewer Numbers in brackets point to the corresponding section in the user manual All panels are described in detail in the online help 10 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 3 2 Setup Group In the setup group there is a button to open the Starr Up panel which is used to boot the modules The Open Panels popup menu leads to one of the following panels PARAMETER SETUP OscILLoscore CHassis SETUP FiLes PATHS 3 2 1 Parameter Setup Panel The Parameter Setup panel is divided into 7 tabs summarized below Settings for all four channels of a module are shown in the same tab At the upper right is a control to select the module to address At the bottom of the panel is a More
48. cidence window 500ns Similar to the FPGA readout described below the coincidence window may overlap with the pileup inspection of a subsequent pulse so it only adds to dead time if FPGA readout and coincidence window combined are longer that the filter time 4 FPGA readout dead time The current firmware only allows valid data of one event at a time in the FPGA The DSP reads out the data after receiving an event interrupt if the coincidence test was passed Thus once an event is validated the channel may incur dead time until the DSP finishes the data readout of hit pattern energy filter sums and possibly waveforms However the pileup inspection of a subsequent pulse can already begin while the first event is read out as long as it finishes after the DSP has completed the readout This means the DSP has a filter time minus coincidence window available for readout The readout creates additional dead time only if it exceeds this time For typical HPGe filter settings with no traces and minimum coincidence window the FPGA readout time usually fulfills this requirement and thus introduces no additional dead time However when traces are required the FIFO logic does not allow the post trigger data of a second pulse to be stored until the readout is completed so in this case the readout always creates dead time equal to the full readout time Details are as follows a Hit pattern readout At the beginning of the readout the DSP read
49. d by adding user functions to the core processing software Waveforms timestamps and the results of the pulse shape analysis can be read out by the host system for further off line processing Pulse heights are calculated to 16 bit precision and can be binned into spectra with up to 32K channels The Pixie 4 supports coincidence spectroscopy and can recognize complex hit patterns Data readout rates through the CompactPCI PXI backplane to the host computer can be over 100Mbytes s The PXI backplane is also used to distribute clocks and trigger signals between several Pixie 4 modules for group operation With a large variety of CompactPCI PXI processor controller or I O modules being commercially available complete data acquisition and processing systems can be built in a small form factor 1 1 Features e Designed for high precision y ray spectroscopy with HPGe detectors e Directly compatible with scintillator PMT combinations Nal CsI BGO and many others e Simultaneous amplitude measurement and pulse shape analysis for each channel e Input signal decay time as fast as 150ns and up to 10ms exponentially decaying e Wide range of filter rise times from 53ns to 109s equivalent to 27ns to 50us shaping times e Programmable gain and input offset e Excellent pileup inspection double pulse resolution of 50 ns Programmable pileup inspection criteria include trigger filter parameters threshold and rejection criteria e Digital oscil
50. dates a pulse a filter time after the rising edge of the pulse The validation thus always takes place a filter time after the rising edge of the pulse but is optionally subject to the current status of the VETO input and or the status of the GATE input stretched by Gate Window and latched a time Gate Delay after the rising edge of the pulse The action of Gate Window and Gate Delay is thus to set a coincidence window for the GATE signal and adjust for the delay between detector signal from the ADC and the GATE signal Mainly due to the pipelined processing inside the ADC it takes about 200 ns from a rising edge at the front panel analog input of the Pixie 4 until a trigger is issued by the Pixie 4 trigger circuit The GATE signal starting the Gate Window counter is therefore delayed by 200 ns inside the FPGA to compensate for this intrinsic delay Any delay due to cables or the physics of the experiment will be additional Both Gate Delay and Gate Window can range from 13 3ns to 3 4 us 62 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved f E E Parameter Setup lolol Run Control Module 0 9 uses no Delay us GFLTinput pulse 0 013 0 013 0 013 0 013 AAA A A _ Figure 7 4 Gate tab of the PARAMETER Setup Panel The VETO signal is distributed through the PXI backplane Using XIA s PDM module or a custom board external signals can be connected to the backplane In addition the Pixie 4 front
51. e V on a longer time scale the step will decay exponentially back to the baseline see section 6 3 When the y ray is absorbed in the detector material it releases an electric charge Q E e where e is a material constant Qx is integrated onto Cr to produce the voltage Vx Q C E C Measuring the energy E of the y ray therefore requires a measurement of the voltage step V in the presence of the amplifier noise 6 as indicated in Figure 6 1 b gt 3 5 a 5 O e Ss E A 0 00 0 02 0 04 0 06 a b Time ms Figure 6 1 a Charge sensitive preamplifier with RC feedback b Output on absorption of an y ray Reducing noise in an electrical measurement is accomplished by filtering Traditional analog filters use combinations of a differentiation stage and multiple integration stages to convert the preamp output steps such as shown in Figure 6 1 b into either triangular or semi Gaussian pulses whose amplitudes with respect to their baselines are then proportional to V and thus to the y ray s energy Digital filtering proceeds from a slightly different perspective Here the signal has been digitized and is no longer continuous Instead it is a string of discrete values as shown in Figure 6 2 Figure 6 2 1s actually just a subset of Figure 6 1 b in which the signal was digitized by a Tektronix 544 TDS digital oscilloscope at 10 MSPS mega samples per second Given this data set and some 35 PIXIE 4 User
52. e highest energy To scale the spectrum in keV enter the appropriate ratio in the field keV bin At this stage you may not be able to get a spectrum with good energy resolutions You may need to adjust some settings such as energy filter rise time and flat top as described in section 3 5 9 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 3 Navigating the Pixie Viewer 3 1 Overview The Pixie Viewer consists of a number of graphs and control panels linked together by the Main control panel The Viewer comes up in exactly the same state as it was when last saved to file using File gt Save Experiment This preserves settings such as the file paths and the slot numbers entered in the Start Up panel However the Pixie module itself loses all programming when it is switched off When the Pixie module is switched on again all programmable components need code and configuration files to be downloaded to the module Clicking on the Start Up System button in the Start Up panel performs this download Below we describe the concepts and principles of using the Pixie Viewer Detailed information on the individual controls can be found in the Online Help for each panel The operating concepts are described in sections 4 7 The controls in the Maw control panel are organized in three groups Setup Run Control and Results In the Setup and Results groups popup menus lead to the panels and graphs indicated in Figure 3 1 Files Path Lo
53. e pulse rise time and small compared to the pulse decay time A pulse shape not meeting these conditions has the effect of raising the effective threshold For a modeled behavior of the trigger you can open displays from the OsciLLoscore and the List Mone Traces panels that show trigger filter and threshold computed from acquired waveforms using the current settings The threshold value is scaled with the trigger filter rise time therefore it is not limited to integer numbers 11 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved MM Parameter Setup a Trigger Energy Waveform Gate Coincidence Advanced Run Control Module 1 Filter Options Enable Respond to Channel Rise Time us Flat Top us Threshold Trigger Group only 0 080 0 080 25 0 080 0 080 25 0 080 0 080 25 0 080 0 080 25 Start System Oscilloscope More About Help Figure 3 2 The Trigger tab of the PArameTER SETUP panel 4 gt 4 gt lt gt lt gt 4 gt 4 gt 4 gt lt gt 3 2 1 2 Energy Tab The Energy tab contains the settings for the energy filter and the subsequent computation These settings are most important for obtaining the best possible energy resolution with a Pixie 4 system The energy filter rise time or peaking time essentially sets the tradeoff between throughput and resolution longer filter rise times generally improve the resolution up to a certain optimum but reduce the throughput because more time is required to measure
54. e required to record an event and c the data transfer rate To reduce a run in compressed list mode run types 0x 101 103 shorten the tracelength as much as possible even in compressed list mode remove the read always and good channel option for unused channels To reduce b reduce the coincidence window to the minimum possible if no pulse shape analysis is required in compressed list mode runs set the tracelength to zero do not require pulse shape analysis To increase c run in 32x buffer or 16 16 double buffer mode avoid frequent updates of run statistics and spectra set the polling time to a small value 0 1 0 01 verify the number of events buffer is set to the maximum Bad energy resolution in MCA spectrum verify the decay time is set correctly increase energy filter rise time make the energy filter flat top approximately equal to the rise time of the pulse ensure the integrator is set to zero if integrator is set to 1 on purpose e g fast scintillator pulses make sure the energy filter flat top covers the entire pulse if integrator is set to 2 on purpose e g square pulses make sure the energy filter flat top covers the portion of the pulse that should be disregarded for the energy measurement e g the rising edge 10 Appendix A This section contains hardware related information 10 1 Front end jumpers for termination and attenuation Ta
55. e separated by less than 1 0 us are thus seen to pileup in the present example with a 1 2 us rise time This leads to an important point whether pulses suffer slow pileup depends critically on the rise time of the filter being used The amount of pileup which occurs at a given average signal rate will increase with longer rise times Because the fast filter rise time is only 0 1 us these y ray pulses do not pileup in the fast filter channel The Pixie 4 can therefore test for slow channel pileup by measuring the fast filter for the interval PEAKSEP after a pulse arrival time If no second pulse occurs in this interval then there is no trailing edge pileup and the pulse is validated for acquisition PEAKSEP is usually set to a value close to L G 1 Pulse 1 passes this test as shown in Figure 6 6 Pulse 2 however fails the PEAKSEP test because pulse 3 follows less than 1 0 us Notice by the symmetry of the 41 PIXIE 4 User s Manual V2 50 O XIA 2012 All rights reserved trapezoidal filter if pulse 2 is rejected because of pulse 3 then pulse 3 is similarly rejected because of pulse 2 6 5 Filter Range To accommodate the wide range of filter rise times from 0 053 us to 106 us the filters are implemented in the RTPUs using FPGA configurations with different clock decimations filter ranges The ADC sampling rate is always 13 3ns but in higher clock decimations several ADC samples are averaged before entering the filtering logic In
56. ecord see 7 6 2 The fourth function for the Pixie 4 s front panel input is to contribute one Front bit in the event hit pattern If the front panel is not used as Veto or Status input this allows recording of an externally created logic level in each module individually For example each module may be assigned to a detector or radiation source that is enabled disabled individually and so the status of that detector is recorded in the event data stream Notes The front panel input can be used for Veto Status and Front at the same time if necessary The wired OR backplane lines are of type active low i e logic 1 is OV 63 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 7 6 Coincident Events 7 6 1 Coincidences Within a Module EM Parameter Setup Trigger Energy Waveform Gate Advanced Run Control Disregard E filter requirement v lt lt lt 1 lt lt lt lt sett crane Start Systern Oscilloscope lt lt lt extract Figure 7 5 Coincidence Pattern and Coincidence Window Settings in the Pixie Viewer In any given event a single Pixie 4 module will have up to four channels with a hit 1 e a valid local pulse without pileup The four channels thus form one of 16 possible Hit Patterns stored in the lower 4 bits of a DSP parameter In this representation the Hit Pattern ranges from no channel hit 0000 over only channel 1 hit 0010
57. el time stamp and the recording of the event time stamps For example a channel may trigger at CHAN _TRIGTIME 65100 and the event may be recognized after that at EVT_TIMEHI EVT_TIMELO 5467 123 The correct absolute ChannelTime would be reconstructed from lt BUF_TIMEHI gt 5466 and 65100 i e using EVT_TIMEHI 1 instead of EVT_TIMEHI as the middle word So if ChannelTime comes out larger later than the EventTime it needs to be reduced by 65536 le 6 75s 2 The buffer start time is recorded before the event time Hence the time counter may have seen a 32 bit roll over change from high to low values of the middle word between the recording of the buffer time stamps and the recording of the event time stamps For example the run may have started at BUF_TIMEHI BUF_TIMEMI BUF_TIMELO 30 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 2 65510 34524 and the event may be recognized after that at EVT_TIMEHI EVT_TIMELO 7 7564 The correct EventTime would be reconstructed from 3 7 and 7564 i e using BUF _TIMEHI 1 instead of BUF_TIMEHI as the high word So if EventTime comes out smaller earlier than the BufferTime or the EventTime of a previous event in the same buffer it needs to be increased by 2 32 le 6 75s 3 If count rates are extremely low in the order of one count per minute or less the time counter may see multiple 32 bit rollovers events For example the run may have started at 3 153 1
58. ert gt start finish Use edge OR Pa GDT_ON count GFLT GDT Pas Su VETO ON allow Invert zz Veto mie live gt enable Fig 6 12 GDT counting logic shown for standard statistics mode In Gate statistics mode the channel LIVE signal is ignored for counting GDT and in turn LIVE is subject to GDT_ON To support the third case of gating mentioned above where the acquisition is only of interest when GATE or VETO are off there is an alternate GATE statistics mode to count GDT and livetimes In Gate statistics mode all time and rate counters except RUN TIME and TOTAL TIME are only active if GDT_ON is high This means GATE or VETO must be present for the channel to be live In turn GDT is counted independently from whether the module is live or not If the VETO input is unused but defaults to high so that GDT is equal to the live time invert the VETO polarity in the ChannNEL RecisTER Panel to only count the GATE PULSE time The VETO input can be used instead of the GATE input for the gating circuitry and 1f this option is used only the GATE PULSE derived from the VETO input is counted not the original VETO For the case that the Veto input is used for a GFLT type validation pulse it may be more useful to work with the number of pulses issued They can be counted by using the VETO input as the source for GATE PULSEs which are counted in the variable GCOUNT 6 6 3 Count rates Besides the live and dead times the P
59. ether with an FPGA The configuration of this PCI IC is stored in a PROM which is placed in the only DIP 8 IC socket on the Pixie 4 board The interface conforms to the commercial PCI standard It moves 32 bit data words at a time The interface does not issue interrupt requests to the host computer Instead for example to determine when data is ready for readout the host has to poll a Control and Status Register CSR in the interface logic also called communication FPGA The communication FPGA links the PCI slave with the DSP and the on board memory The host can read out the memory without interrupting the operation of the DSP This allows updates of the MCA spectrum while a run is in progress The communication FPGA also distributes triggers and coincidence signals to other modules using the PXI backplane connections 34 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 6 Theory of Operation 6 1 Digital Filters for y ray Detectors Energy dispersive detectors which include such solid state detectors as Si Li HPGe Hgl CdTe and CZT detectors are generally operated with charge sensitive preamplifiers as shown in Figure 6 1 a Here the detector D is biased by voltage source V and connected to the input of preamplifier A which has feedback capacitor Cr and feedback resistor Ry The output of the preamplifier following the absorption of an y ray of energy E in detector D is shown in Figure 6 1 b as a step of amplitud
60. event header of length EVENTHEADLEN Currently EVENTHEADLEN 3 and the three words are Table 4 3 Event header data format EVT PATTERN Hit pattern Bit 15 0 gate pattern hit pattern status read pattern EVT TIMEHI Event time high word EVT_TIMELO The hit pattern is a bit mask which tells which channels were read out plus some additional status information as listed in table 4 4 After the event header follows the channel information as indicated by the hit pattern in order of increasing channel numbers For example if bits 3 0 1001 the event header is followed by data from channel 0 then channel 3 27 PIXIE 4 User s Manual V2 50 O XIA 2012 All rights reserved Table 4 4 Hit pattern bit description Bit 0 3 12 15 4 Logic level of FRONT panel input 5 Result of LOCAL coincidence test 6 Logic level of backplane STATUS line lobal coincidence test see section 7 If set indicates that channel 0 3 has been hit in this event i e if zero energy reported is invalid or only an estimate Logic level of the GATE input of channel 0 3 for Rev D modules only The data for each channel are organized into a channel header of length CHANHEADLEN which may be followed by waveform data CHANHEADLEN depends on the run type and on the method of data buffering i e if raw data is directed to the intermediate Level 1 buffer or directly to the linear buffer Offline analysis programs should theref
61. fference e g ChannelTimeDiff CHAN_TRIGTIME o CHAN_TRIGTIME 1e 6 758 However it has to be kept in mind that the 16 bit word may overflow between being recorded in channel 0 and channel 1 For example channel 0 may trigger first at CHAN_TRIGTIME 65500 and channel 1 may trigger later at CHAN _TRIGTIME 105 and thus Channel TimeDiff would be 65395 le 6 75s even though the correct time difference would only be 141 le 6 75s If such large time differences can be known to be impossible from the physics of the experiment e g max time of flight delay or from the acquisition setup e g max coincidence window such overflows can be handled by simply adding 65536 to the smaller CHAN_TRIGTIME if the time difference is greater than the known maximum e g 63000 le 6 75s If an absolute ChannelTime is required it can be computed as follows ChannelTime CHAN_TRIGTIME ChannelTime EVT_TIMEHI pow 2 16 ChannelTime BUF_TIMEHI pow 2 32 ChannelTime le 6 75s For a small fraction of events the EventTime or the absolute ChannelTime will experience an overflow in a lower word in one header which is not reflected in the next higher word recorded in a different header These can typically be easily recognized and corrected for 1 The channel time is recorded before the event time Hence the time counter may have seen a 16 bit roll over change from high to low values of the low word between the recording of the chann
62. ffers of module 0 to N and so forth Table 4 1 Summary of run types and data formats List Mode Energies time stamps 6 PSA values and RUNTASK 256 standard wave forms in List mode block MAXEVENTS lt calculate gt Spectra in MCA block CHANHEADLEN 9 List Mode Energies time stamps and 6 PSA values in RUNTASK 257 Compression 1 List mode block MAXEVENTS lt calculate gt Spectra in MCA block CHANHEADLEN 9 List Mode Energies time stamps and 2 PSA values in RUNTASK 258 Compression 2 List mode block MAXEVENTS lt calculate gt Spectra in MCA block CHANHEADLEN 4 List Mode Energies and time stamps in List mode block RUNTASK 259 Compression 3 Spectra in MCA block MAXEVENTS lt calculate gt CHANHEADLEN 2 MCA Mode Spectra in MCA block RUNTASK 769 MAXEVENTS 0 4 1 3 Fast List Mode Runs The legacy fast list mode runs are no longer supported 4 2 Output Data Structures 4 2 1 MCA Histogram Data The MCA block is fixed to 32K words 32 bit deep per channel i e total 128K words The MCA block resides in the external memory which can be read out via the PCI data bus at rates 26 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved over 100Mbytes s If spectra of less than 32K length are requested only part of the 32K will be filled with data This data can be read even when a run is in progress to get a spectrum update In clover mode spectra for eac
63. g the incoming signal at twice the regular ADC clock frequency If both channels are fed the same input signal the RTPU can give the second ADC a clock with a 180 degree phase shift thus sampling the signal twice in one clock cycle Special firmware is required to combine the two input streams into one contact XIA for details The RTPUs apply digital filtering to perform essentially the same action as a shaping amplifier The important difference is in the type of filter used In a digital application it is easy to implement finite impulse response filters and we use a trapezoidal filter The flat top will typically cover the rise time of the incoming signal and makes the pulse height measurement less sensitive to variations of the signal shape Secondly the RTPUs contain a pileup inspector This logic ensures that if a second pulse is detected too soon after the first so that it would corrupt the first pulse height measurement both pulses are rejected as piled up The pileup inspector is however not very effective in detecting pulse pileup on the rising edge of the first pulse i e in general pulses must be separated by their rise time to be effectively recognized as different pulses Therefore for high count rate applications the pulse rise times should be as short as possible to minimize the occurrence of pileup peaks in the resulting spectra If a pulse was detected and passed the pileup inspector a trigger may be issued That trigger
64. gs and statistics are saved and then a new run is started This is equivalent to manually clicking first the Stop Run button and then the Start Run button It is recommended to enable the automatic increment and auto store options as shown in Figure 3 9 as well 20 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved a Data Record Options Y Y v m A Ly YA Ly E Mon Jun 22 2009 4 31 00 PM Mon Jun 22 2009 4 34 00 PM Figure 3 9 The Data REcorp Options panel with checkboxes set to acquire a series of files 3 6 2 File Series to scan filter parameters With some modifications the mechanism to create file series described in section 3 6 1 can also be used to scan through a range of energy filter or decay time settings This is equivalent to starting an MCA run with initial settings stopping the run incrementing the energy filter rise time restarting the run and so on The file series will thus contain spectra for a whole range of settings which can be analyzed manually or with the routine described in section 3 6 3 ON File Series Scan Set Scan Run Conditions Figure 3 10 The Fite Sertes Scan panel to acquire a series of files in which energy filter parameters and Tau are varied within user defined limits 21 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved To set up such a parameter scan open the Fite Series Scan panel PARAMETER SETUP gt Energy tab gt Scan Settings
65. h channel are 16K long and compressed into the first 64K of the external memory An additional 16K addback spectrum containing the sum of energies in events with multiple hits is accumulated in the second 64K of the external memory In 2D mode spectra for each channel are 16K long and compressed into the first 64K of the external memory The remaining 64K of the external memory contains a 256 x 256 bin two dimensional spectrum custom code required 4 2 2 List Mode Data The list mode data in external memory consists of 32 local I O data buffers The local I O data buffer can be written by the DSP in a number of formats User code should access the three variables BUFHEADLEN EVENTHEADLEN and CHANHEADLEN in the configuration file of a particular run to navigate through the data set It should only be read when the run has ended The 32 buffers in external memory follow immediately one after the other The data organization of one I O buffer is as follows The buffer content always starts with a buffer header of length BUFHEADLEN Currently BUFHEADLEN is six and the six words are Table 4 2 Buffer header data format Variable JF NDATA Number of words in this buffer JF MODNUM Module number JF FORMAT Format descriptor RunTask 0x20T0 T bits4 7 indicate channel 0 3 acquired waveforms in 4x trace mode JF_TIMEHI BUF_TIMELO Following the buffer header the events are stored in sequential order Each event starts out with an
66. he DSP can not keep up with processing section 6 6 1 2 1 and 2 and ends when the DSP encounters an end run condition e g memory full or host stop Subsequent spills add in the same way It is thus the time during which triggers are counted and can cause recording or pile up of data the best available measurement of the time the channel was active FTDT fast trigger dead time The fast trigger dead time counts the time the trigger filter is unable to issue triggers because the trigger filter output is already above threshold and can not recognize a second pulse It does not include the time triggers have been paused for a short time after a first trigger an advanced user option to suppress double triggering because the concept is that all triggers occurring during the pause are counted as only one trigger When computing the input count rate one should divide the number of triggers counted FASTPEAKS by the difference LIVE TIME FTDT since triggers are not counted during FTDT SFDT slow filter dead time The slow filter dead time counts the time new triggers will not lead to the recording of new data This includes effects 1 4 listed in section 6 6 1 1 as dead time associated with a pulse In detail e it includes the time the pileup inspection is taking place and the summation of energy filter sums is in progress e it includes the time the FIFO does not contain fresh pre trigger data or is unable to accept a new e
67. he bussed clock line does usually not connect over a PCI bridge in chassis with more than 8 slots whereas daisy chains usually do Always make sure that there is no shunt on JP3 in order to disconnect from the incoming daisy chained clock else there will be a conflict between the two clock signals 7 1 4 PXI Clock Mode A further option for clock distribution is to use the PXI clock distributed on the backplane The PXI clock is driven by default by the PXI backplane at a rate of 10 MHz too slow to run the Pixie 4 modules However clock signals are individually buffered for each slot and clock skew between slots is specified to be less than Ins making it the preferred distribution path for sensitive timing applications If a custom backplane provides 37 5 MHz or if a module in slot 2 overrides the default signal from the backplane this clock can be used as an alternative setting for Pixie 4 modules Such a module can be a Revision C Pixie module configured as PXI clock master shown in Figure 7 2 f a XIA PXI PDM power and logic module or any other suitable custom module The PXI clock master has to be configured as shown in Figure 7 2 f The PXI clock slaves are configured by connecting pin 2 on JP1 and JP2 together PXI to clock input as shown in Figures 7 1 e and 7 2 e Always make sure that there is no shunt on JP3 in order to disconnect from the incoming daisy chained clock else there will be a conflict between the
68. hen a preset number of events are reached The data has then to be read out by the host PC Runs can be resumed for longer data acquisitions as described in 4 1 2 3 There are three options for the data readout mode with different consequences for the readout dead time Modes 2 and 3 are only available for module revisions C and D a The default readout option available for all module revisions is to simply fill and read the local 1 O buffer However data readout from the local 1 O buffer is relatively slow and since acquisition is halted during the readout the readout dead time is relatively large 30ms per module and buffer b A more efficient readout mode is to transfer the data is to the external memory when the local buffer is full and resume the run right away This is repeated 32 times until the external memory is full and only then the run is halted and data is read out by the host PC in a fast block read 30ms per 32 buffers and module about 550us between buffers c A third readout mode is to transfer the data is to the external memory when the local buffer is full and resume the run as in mode 2 but after 16 times a flag is raised to the host PC to read out the external memory while new data is stored in the other half of the 25 PIXIE 4 User s Manual V2 50 O XIA 2012 All rights reserved external memory This allows almost uninterrupted data acquisition Readout dead time between buffers is about 550 us and readou
69. if read always Only read if read always always option selected option selected option selected Energy The pulse height of a pulse is best determined based on the trigger from the pulse itself not from a common group trigger that may be delayed Even if a channel is not trigger enabled energy filter values are latched some time after the local trigger filter crosses the user defined threshold The channel is then marked as hit for the DSP to read out Even channels without hit are read out if the read always option is set for this channel In this case the channel s energy is usually reported as zero since there was no valid local trigger to capture the value of the energy filter However since the energy filter is computed continuously setting the estimate energy option cause the energy filter value to be captured based on the last group trigger This might be useful for channels with occasional very small pulses below the threshold or possibly to obtain energy estimates on piled up pulses Note that since the timing of the group trigger is not precise with respect to the non triggering pulse the energy reported is only a rough estimate It might help to set the flat top time to a large value to make the capturing of the energy filter less time sensitive Waveforms The waveforms are always captured based on the last group trigger for this event Channels without hit are read out only if
70. ight of a pulse is best determined based on the trigger from the pulse itself not from a common group trigger that may be delayed if several channels in a group are trigger enabled always the last fast trigger before the first event trigger in this event is the one that counts The following table lists which quantities are based on local or group trigger in group trigger mode If not in group trigger mode all quantities are based on local triggers Note that for the trigger timing to be correct all channels in a trigger group should be set to the same energy filter rise time and flat top Furthermore to capture the energy of delayed channels ensure that the coincidence window see section 7 6 is long enough to include the maximum expected delay Channel Energy Waveform Timestamp Hit and Based on local trigger Based on last group trigger Based on last group trigger trigger unless local trigger only enabled option selected Always read out Always read out Always read out Hit but not Based on local trigger Based on last group trigger Based on last group trigger trigger unless local trigger only enabled option selected Always read out Always read out Always read out Not hit Based on group trigger if Based on last group trigger Based on last group trigger no pulse or estimate energy option unless local trigger only pileup selected else zero option selected Only read if read Only read
71. ill above threshold An additional type of dead time only affects the trigger filter Triggers are issued when the trigger filter output goes above the trigger threshold set by the user However depending on the length of the trigger filter and the rise time of the input signal the trigger filter output will remain above threshold for a finite amount of time During this time no second trigger can be issued Therefore triggers are not counted during this time and when computing the input count rate the time lost has to be taken into account FTDT is thus purely a correction for the computation of the input count rate 6 Summary As listed in Table 6 2 the dead time associated with each pulse is essentially in the order of the filter dead time unless waveforms are requested by the user Even with waveforms few counts are lost at low count rates for short waveforms Losses are significant with longer waveforms or at higher count rates To some extend this is due to the general purpose FIFO logic which allows waveforms up to the maximum available memory in the FPGA For application where only short traces are required lower dead times are possibly by reorganizing memory to buffer multiple events in the FPGA Please contact XIA for details The MAXWIDTH parameter can be used to define a maximum acceptable time over threshold and thus to reject events piled up on the rising edge 46 PIXIE 4 User s Manual V2 50 O XIA 2012 All
72. in section 2 3 the OsciLLoscorE Figure 2 3 is used to view untriggered traces as they appear at the ADC input and to set all parameters relating to the analog gain and offset There are controls titled dT us which sets the time between samples in the oscilloscope there are always 8192 samples in the oscilloscope window Offset which sets the target DC offset level for automatic adjustment Gain V V which sets the analog gain before digitization and Offset V which directly sets the offset voltage The traces from different channels are not acquired synchronously but one after the other Therefore even if coincident signals are connected to the Pixie 4 inputs the OsciLLoscorE will show unrelated pulses for each channel There are also buttons and controls to open a display of the FFT of the input signal which is useful to detect noise sources open a display of the waveforms of the trigger filter and energy filter computed from the traces in the oscilloscope repeat the action of the Refresh button until a pulse is captured This is useful for low count rates Fit the pulses in the OsciLLoscorE with an exponential decay function to determine the decay time Tau and to accept the fit value for the module settings 14 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved View the current input count rate and the current fraction of time the signal is out of range These values are updated in the
73. individual module see Figures 7 1 a and 7 2 a Configure the other modules in the chassis as clock repeaters by setting the jumpers as shown in Figures 7 1 b and 7 2 b i e remove all shunts from JP 1 and JP2 and set a shunt on JP3 located on top of the clock crystal U2 Note that the clock output is always enabled i e every board independent of its clock mode sends out a clock to its right neighbor as long as it has a clock itself Thus make sure that no other module sits to the right of a Pixie module that uses the PXI_LBRO line on the PXI backplane for other purposes 7 1 3 Bussed Clock Mode If there have to be gaps between a group of Pixie 4 modules the daisy chained clock distribution will not work since the chain is broken In this case the modules can be configured for bussed clock mode To do so configure one module in any slot as the bussed clock master as shown in Figures 7 1 c and 7 2 c i e set one shunt to connect pins 2 and 3 on JP2 and a second shunt to 56 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved connect Pinl of JP1 and JP2 together OUT to BUS The clock master will drive the clock signal to a line that is bussed to all others slots in the chassis The drive strength is limited to 3 4 modules All other modules should be configured as clock slaves i e set a shunt to connect pin 1 and 2 of JP2 BUS to clock input as shown in Figures 7 1 d and 7 2 d Note that t
74. ion Problems 1 e Signal from PMT shows unusual pulse shape Verify the input jumpers are set to the correct termination When taking the signal directly from the PMT without a preamplifier the correct termination is usually 500 Missing peaks in spectra Unusually low count rate Unusually low Live Time Open the OsciLLoscorE and verify that the signal is in range i e that large pulses are not cut off at the upper end of the range 16K and that the baseline is above zero Low efficiency for high energy peaks in MCA spectrum At high rates pulses overlap with the decaying tail of a previous pulse When two or more pulses overlap in this way higher energy pulses are more likely to go out of range gt reduce gain and or adjust the offset If the detector output shows significant ringing or overshoots it can happen that the Pixie 4 triggers twice on the same pulse first on the rising edge then on the overshoot This would be more likely for higher energy pulses because the ringing or overshoot has a larger amplitude gt increase the trigger threshold and or the trigger filter rise time or use the advanced options to pause or for low count rates disable the pileup inspection EA Data collection in list mode has low DAQ fraction SFDT is a large fraction of the live time Rate at which list mode data is written to file is low The number of events collected in a given time depends on a the data per event b tim
75. ion resumes already at T5 while the memory is read out independently In a and b there is thus dead time in the Pixie 4 while it is waiting for the host to read out the data In b and c there is dead time while the data is transferred to external memory In all cases the run is considered stopped in the Pixie 4 as opposed to the dead times described in section 6 6 1 2 The readout dead times are omitted from the Pixie 4 live time and run time counters counters are stopped but included in the total time counter Examples of host readout dead times are shown in Table 6 3 Rows A and B are copied from Table 6 2 for comparison When no traces are recorded the fraction of time lost to readout is less than 1 even up to moderately high count rates and well below the fraction of events lost due to Td dominated by the filter time When traces are recorded the fraction of time lost increases but is still below the fraction of events lost due to Td 50 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved Table 6 3 Dead times from host readout in 32 buffer spill mode One channel active per event LM run 0x103 LM run 0x103 LM run 0x100 LM run 0x100 LM run 0x100 6us filter 6us filter 6us filter 6us filter 6us filter no trace no trace no trace 2ps trace 2s trace ICR 1k s ICR 10k s ICR 1k s ICR 1k s ICR 10k s A OCR 988 s 8869 s 988 s 973 s 7614 s B Fraction events lost 1 2 11 3 1 2 2 7
76. its channel hit pattern to slot 2 of the chassis using the PXI STAR trigger line Each module determines the result of the local coincidence test based on its own 4 channels If enabled to do so it signals the test result on the TOKEN line If the local test passed the TOKEN line is left pulled up 3 3V logic 1 else the TOKEN line is driven low logic 0 The module in slot 2 typically XIA s PXI PDM module uses the up to 48 bit hit pattern from up to 12 modules slots 3 14 to make an accept reject decision If the hit pattern is acceptable the 65 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved TOKEN line is left pulled up If not acceptable the TOKEN line is driven low logic 0 The decision criteria is based on a user defined control word downloaded to the PXI PDM by its neighboring Pixie 4 module The acceptance decisions implemented in the current PDM firmware are described in detail in the Pixie Viewer online help For example if the control word is 0x13 0x14 0x15 etc events are only acceptable if at least 3 4 5 etc channels are hit If the control word is 0x0200 channel 0 but not 1 2 and 3 must be hit in each module 0 and 1 The current firmware does not claim to cover all cases Please contact XIA to request additional cases or to obtain verilog source code to write custom PDM firmware Each module after waiting 100ns for the global accept reject decision to be made captures the status of the TOK
77. ixie 4 counts the numbers of triggers in each channel FASTPEAKS the number of valid events with one or more channels NUMEVENTS and the number of valid pulses stored for each channel NOUT In addition it counts the number of gate pulses for each channel GCOUNT FASTPEAKS and GCOUNT are inhibited when the live time is not counted NUMEVENTS and NOUT by nature only count events captured when the live time is counted Count rates are then computed in the C library as follows Input count rate ICR FASTPEAKS LIVE TIME FTDT Event rate ER NUMEVENTS RUN TIME Channel output count rate OCR NOUT LIVE TIME Gate count rate GCR GCOUNT LIVE TIME 53 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved The Pixie Viewer also computes a DAQ fraction in Igor defined as LIVETIME Igor run time which is an indication of the overall dead time lost to those processes not included in SFDT equivalent to the last row of Table 6 3 Users are free to use the reported values to compute rates and time better matching their preferred definitions e SFDT LT O FTDT LT m OCR fit OCR ICR exp 2Td ICR Td from fit 7 51us Pileup inspection time 7 57us FTDT LT SFDT LT A AE A A A e AAA g e 15 O 0H ee ee ee me ce ccna rre e Sassen ets A eee eee rr 2 TO 0 20 40 80 100 120 140x10 ICR cps Fig 6 13 OCR and Livetime fractions of FTDT and SFDT as a function of ICR in a Pixie 4 measurement with a ra
78. ke a short run and note the value of 7 that gives the best resolution Pixie users can also use the fit routines in the OscitLoscore to manually find the decay time through exponentially fitting the untriggered input signals Another tool is the Optimize routine in the Energy tab of the Parameter Setup panel Similar to the routine for finding the optimal energy filter times this routine can be used to automatically scan a range of decay times and find the optimal one Please refer to the Online Help documentation for more details A further option is to create a file series where Tt is modified for each file in the series See section 3 6 for more details 3 6 File Series 3 6 1 File Series to break up long data acquisition runs When taking long data acquisitions it may be beneficial to break up the run into smaller sub runs This helps to save data in case of power failure or system crashes since only the most recent sub run is lost Also list mode files tend to get large and unwieldy for analysis in longer runs The Pixie Viewer thus has a method to create a series of files at specified intervals In the Data Record Options panel opened with the Record button in the Run Control tab of the PARAMETER Setup panel there is a checkbox named New files every followed by a control field to enter a spill or time interval N If checked and a run is started every N spills or in MCA runs every N seconds the data file is closed spectra settin
79. l Gate Individual GATE to suppress event triggering for each channel with use of PXI PDM Rev D only Data Interface PCI 32 bit 33MHz Read Write memory readout rate to host over 100 Mbytes s Digital Controls Gain Analog switched gain from 0 97 to 11 25 in max 10 steps Digital gain adjustment of up to 10 in 15ppm steps Offset Shaping Digital trapezoidal filter Rise time and flat top set independently 53ns 109s in small steps Trigger Digital trapezoidal trigger filter with adjustable threshold Rise time and flat top set independently from 26ns to 853ns Data Outputs Spectrum 1024 32768 channels 32 bit deep 4 2 billion counts bin Additional memory for sum spectrum for clover detectors Real time live time filter and gate dead time input and throughput counts Event data Pulse height energy timestamps pulse shape analysis results peer waveform data and ancillary data like hit patterns 4 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 2 Setting Up 2 1 Installation 2 1 1 Hardware Setup The Pixie 4 modules can be operated in any standard 3U CompactPCI or PXI chassis Chassis following only the CompactPCI standard can be used to operate modules individually To operate several modules together a backplane following the PXI standard must be present Put the host computer or remote controller in the system slot of your chassis Place the Pixie 4 modules into any free slot
80. led at the osciLLoscope s dT However unless long list mode traces are acquired or energy filters are short there may not be sufficient data to compute the energy filter properly The List Mope Spectrum display is a plot similar to the MCA Spectrum but it is computed from the energies saved in the list mode data file Since energies are stored there in full 16 bit precision binning can be made finer than in the MCA Spectrum which is limited to 32K bins See the online help for a detailed description of the controls 17 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 3 4 3 Run Statistics E All Run Statistics SEE 5 25 07 PM Fri Jun 26 2009 5 26 03 PM Fri Jun 26 2009 C XIA Pixie 4 P4_v2 MCA test0001 ifm Update Figure 3 8 The Run Statistics panel The Run Statistics panel shows the live times and count rates measured by the Pixie 4 The numbers can be updated by clicking the Update button and read from or save to Files For a detailed description of the definition of these values see section 6 6 3 4 4 Fite Series See section 3 6 for a more detailed description 3 5 Optimizing Parameters Optimization of the Pixie 4 s run parameters for best resolution depends on the individual systems and usually requires some degree of experimentation The Pixie Viewer includes several diagnostic tools and settings options to assist the user as described below 3 5 1 Noise For a quick analysis of the electroni
81. loscope and FFT for health of system analysis e Triggered synchronous waveform acquisition across channels modules and crates e Dead times as low as 1 Us per event are achievable limited by DSP algorithm complexity Events at even shorter time intervals can be extracted via off line ADC waveform analysis e Digital constant fraction algorithm measures event arrival times down to a few ns accuracy e Supports 32 bit 33 MHz PCI data transfers gt 100 Mbytes second 3 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 1 2 Specifications 4 analog inputs Selectable input impedance 50Q and 5kQ 5V pulsed E2V DC Selectable input attenuation 1 7 5 and 1 1 for 500 setting Logic Input Output General Purpose I O connected to programmable logic Rev C D only Currently can be either used as input for global backplane signals Veto or Status as an input for module specific logic level reported in the data stream or as an external trigger Logic Output General Purpose output from Digital Signal Processor Function to be determined Backplane I O Clock Input Output Distributed 37 5 MHz clock on PXI backplane Triggers Two wired or trigger buses on PXI backplane One for synchronous waveform acquisition one for event triggers Status Token Synch Wired or SYNC signal distributed through PXI backplane to synchronize timers and run start stop to 50ns Veto Global logic level to suppress event triggering Channe
82. lt of applying such a filter with Length L 1us and Gap G 0 4uUs to a y ray event is shown in Figure 6 3 The filter output is clearly trapezoidal in shape and has a rise time equal to L a flattop equal to G and a symmetrical fall time equal to L The basewidth which is a first order measure of the filter s noise reduction properties is thus 2L G This raises several important points in comparing the noise performance of the Pixie 4 to analog filtering amplifiers First semi Gaussian filters are usually specified by a shaping time Their rise time is typically twice this and their pulses are not symmetric so that the basewidth is about 5 6 times the shaping time or 2 8 times their rise time Thus a semi Gaussian filter typically has a slightly better energy resolution than a triangular filter of the same rise time because it has a longer filtering time This is typically accommodated in amplifiers offering both triangular and semi Gaussian filtering by stretching the triangular rise time a bit so that the true triangular rise 37 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved time is typically 1 2 times the selected semi Gaussian rise time This also leads to an apparent advantage for the analog system when its energy resolution is compared to a digital system with the same nominal rise time One important characteristic of a digitally shaped trapezoidal pulse is its extremely sharp termination on completion of the basewidth 2L
83. ltage range of the ADC which spans 2V Input signals are adjusted for offsets and there is a computer controlled gain stage of switched relays This helps to bring the signals into the ADC s voltage range and set the dynamic range of the channel A fine tuning of the gain is achieved by multiplying the calculated energy values with digital gain factors in the digital signal processor DSP The ADC is not a peak sensing ADC but acts as a waveform digitizer In order to avoid aliasing we remove the high frequency components from the incoming signal prior to feeding it into the ADC The anti aliasing filter an active Sallen Key filter cuts off sharply at the Nyquist frequency namely half the ADC sampling frequency Though the Pixie 4 can work with many different signal forms best performance is to be expected when sending the output from a charge integrating preamplifier directly to the Pixie 4 without any further shaping 32 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 5 2 Real time Processing Units The real time processing units RTPUs one per two channels consist of a field programmable gate array FPGA which also incorporates a FIFO memory for each channel The data stream from the ADCs is sent to these units at the full ADC sampling rate Using a pipelined architecture the signals are processed at this high rate without the help of the on board DSP Note that the use of one RTPU for two channels allows samplin
84. lter rise time so that no system deadtime is produced by a capture and store operation This is a significant source of the enhanced throughput found in digital systems 3 32x10 31 30 29 ADC units 28 27 44 45 46 47 48us Time Figure 6 5 Peak detection and sampling in the Pixie 4 The peak detection and sampling in the Pixie 4 is handled as indicated in Figure 6 5 Two trapezoidal filters are implemented a fast filter and a slow filter The fast filter is used to detect the arrival of y rays the slow filter is used for the measurement of V with reduced noise at longer filter rise times The fast filter has a filter length L 0 1us and a gap G 0 1us The slow filter has L 1 2us and G 0 35us The arrival of the y ray step in the preamplifier output is detected by digitally comparing the fast filter output to THRESHOLD a digital constant set by the user Crossing the threshold starts a counter to count PEAKSAMP clock cycles to arrive at the appropriate time to sample the value of the slow filter Because the digital filtering processes are deterministic PEAKSAMP depends only on the values of the fast and slow filter constants and the rise time of the preamplifier pulses The slow filter value captured following PEAKSAMP is then the slow digital filter s estimate of Vx 40 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved ADC units ORO la A bu ina licenci
85. mum distance between pulses and validates a pulse for recording only if no more than one pulse occurred from TO to T1 in the previous firmware T to T1 Consequently each pulse creates a dead time Td T1 TO equal to the filter time This dead time simply given by the 42 PIXIE 4 User s Manual V2 50 O XIA 2012 All rights reserved time to measure the pulse height is unavoidable unless pulse height measurements are allowed to overlap which would produce false results Assuming randomly occurring pulses the effect of dead time on the output count rate is governed by Poisson statistics for paralyzable systems with pileup rejection This means the output count rate OCR valid pulses is a function of filter dead time Td and input count rate ICR given by OCR ICR exp ICR 2 Td 4 which reaches a maximum OCR max ICR max e at ICRmax 1 2 Td Simply speaking the factor 2 for Td comes from the fact that not only is an event E2 invalid when it falls into the dead time of a previous event El but El is rejected as piled up as well T TO T1 T2 T3T4 Pulse Filter sums et et gt Filter A Valid event 2 Rees FIFO dead time A tenpan A A A A ened eee e eee e eee errr tree tere C etree reer ere r ere rere rere eter errr etre cree rere rere Coincidence Window FA FPGA readout no trace Mn SFDT A SFDT neighboring ch Fig 6 7 Pulse dead time without trace capture A second pulse arriving at
86. n type 0x103 raw data from the energy filters and waveforms are temporarily stored in an intermediate buffer and only results are written to the output buffer In compressed list mode runs the following points are to consider When using a runtype that computes results of pulse shape analysis PSA computations make sure the total combined trace length from all four channels is less than 52us because the intermediate buffer used to temporarily store the trace data is limited to 4K samples If no PSA is required reduce the trace length to zero to avoid unnecessary data transfer time When the intermediate buffer is filled with events not yet processed for output data new events are rejected Whenever a new event occurs the DSP first checks if there is enough room left in the intermediate buffer then transfers the data from the FPGAs into its intermediate buffer or rejects it Consequently if the combined trace length is more than 26us only one event at a time can be stored This means that the effective dead time for an event is increased by the processing time If the combined trace length is such that N gt 2 events fit into the intermediate buffer the processing time does not add to the dead time as long as the average event rate is smaller than the processing rate and no bursts of more than N events occur 4 1 2 2 Data Readout Options List mode runs halt data acquisition either when the local 1 O data buffer is full or w
87. ndom pulse generator The measured OCR follows the expected behavior from Eq 4 with a fit value of Td very close to the pileup inspection time energy filter rise time plus energy filter flat top plus a few cycles Notes 1 Output pulse counters are updated whenever an event has been processed input gate and all time counters are updated every 7ms Therefore reading rates at random times e g clicking Update in the Pixie 4 Viewer might return slight inconsistencies between input rates and output rates At the end of the run all rates are updated and these effects should disappear 2 NOUT is counted for each event a channel is processed no matter if the channel had a valid hit or not Thus a channel that is processed in read always mode may have an output count rate even though its input count rate is zero 54 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 7 Operating Multiple Pixie 4 Modules Synchronously When many Pixie 4 modules are operating as a system it may be required to synchronize clocks and timers between them and to distribute triggers across modules It will also be necessary to ensure that runs are started and stopped synchronously in all modules All these signals are distributed through the PXI backplane 7 1 Clock Distribution In a multi module system there will be one clock master and a number of clock slaves or repeaters The clock function of a module can be selected by setting jum
88. nloaded to the module 6 4 Thresholds and Pile up Inspection As noted above we wish to capture a value of V for each y ray detected and use these values to construct a spectrum This process is also significantly different between digital and analog systems In the analog system the peak value must be captured into an analog storage device usually a capacitor and held until it is digitized Then the digital value is used to update a memory location to build the desired spectrum During this analog to digital conversion process the system is dead to other events which can severely reduce system throughput Even single channel analyzer systems introduce significant deadtime at this stage since they must wait some 39 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved period typically a few microseconds to determine whether or not the window condition is satisfied Digital systems are much more efficient in this regard since the values output by the filter are already digital values All that is required is to take the filter sums reconstruct the energy Vx and add it to the spectrum In the Pixie 4 the filter sums are continuously updated by the RTPU see section 5 2 and only have to be read out by the DSP when an event occurs Reconstructing the energy and incrementing the spectrum is done by the DSP so that the RTPU is ready to take new data immediately after the readout This usually takes much less than one fi
89. number of events MAXEVENTS is calculated by the Pixie Viewer when selecting a run type shown in the control Events Buffer and downloaded to the module as the preset number before starting a run This default value for MAXEVENTS is the maximum safe number of events That is given the maximum length of an event all good channels contributing in any case at least MAXEVENTS events will fit in the output buffer MAXEVENTS can be decreased by the user if desired MAXEVENTS can be set to zero to disable the halting at a preset number This makes the acquisition more efficient if MAXEVENTS takes into account 4 good channels per event but if in the acquisition only few multi channel events occur the buffer will only filled up to about 1 4 when MAXEVENTS is reached Setting MAXEVENTS to zero will always fill the buffer as much as possible 24 PIXIE 4 User s Manual V2 50 O XIA 2012 All rights reserved 4 1 2 1 Compressed Data Formats The output data of list mode runs can be reduced by using one of the compressed formats described below The key difference is that as less data is recorded for each event there is room for more events in the I O data buffer of the Pixie 4 module and less time is spent per event to read out data to the host computer For example if you need individual energies and time stamps but no waveforms select run type 0x103 energy and time only instead of 0x100 general purpose in the Pixie Viewer In ru
90. o Q Oo 00 ul o Q z o e z a z z 8 o E 9 D 3 a 2 a Individual b Clock c Bussed d Bussed e PXI clock f PXI clock or clock repeater clock master clock slave slave mode master mode master mode mode mode mode slot 2 only Figure 7 2 Jumper Settings for different clock distribution modes of Revision C modules In a group of modules there will be one daisy chained clock master a in the leftmost position and several repeaters b OR one bussed clock master c and several bussed clock slaves d OR one PXI clock master f in slot 2 and several PXI clock slaves e Modes a b c d or e f can not be mixed Mode e can also be used with a custom module in slot 2 or a backplane providing 37 5 MHz instead of the usual 10MHz 7 1 1 Individual Clock mode If only one Pixie 4 module is used in the system or if clocks between modules do not have to be synchronized the module should be set into individual clock mode as shown in Figures 7 1 a and 7 2 a Connect pin 2 of JP2 the clock input with a shunt to pin 3 of JP2 which is labeled LOC This will use the on board clock crystal as the clock source 7 1 2 Daisy chained Clock Mode The preferred way to distribute clocks between modules is to daisy chain the clocks from module to module where each module repeats and amplifies the signal This requires one master module located in the leftmost slot of the group of Pixie 4 modules with the same jumper settings as an
91. onsisting of 8192 16 bit words and an extended I O data buffer for list mode runs in the external memory holding up to 32 local buffers 4 1 1 MCA Runs If all you want to do is to collect spectra you should start an MCA run For each event this type of run collects the data necessary to calculate pulse heights energies only The energy values are used to increment the MCA spectrum The run continues until the host computer stops data acquisition either by reaching the run time set in the Pixie Viewer or by a manual stop from the user the module does not stop by itself Run statistics such as live time run time and count rates are kept in the Pixie module 4 1 2 List Mode Runs If on the other hand you want to operate the Pixie in multi parametric or list mode and collect data on an event by event basis including energies time stamps pulse shape analysis values and wave forms you should start a list mode run In list mode you can still request histogramming of energies e g for monitoring purposes In the current standard software one pulse shape analysis value is a constant fraction trigger time calculated by the DSP the other is reserved for user written event processing routines Other routines exist e g to calculate rise times and or to characterize pulses from phoswich detectors List mode runs halt data acquisition either when the local I O data buffer is full or when a preset number of events are reached The maximum
92. ore check the value of RUNTASK which is reported in the buffer header All currently supported data formats are defined below l For List Mode either standard or compression 1 RUNTASK 256 257 CHANHEADLEN and the nine words are Table 4 5 Channel header possibly followed by waveform data 0 CHAN NDATA____ Number of words for this channel 1 CHAN TRIGTIME Fast trigger time 2 CHAN ENERGY Energy 3 CHAN XIAPSA XIAPSA value 4 CHAN_USERPSA User PSAvalue S Unused NA 6 Unused NA 8 CHAN REALTIMEHI High word ofthe realtime Any waveform data for this channel would then follow this header An offline analysis program can recognize this by computing N_WAVE DATA CHAN _NDATA CHANHEADLEN If N WAVE _DATA is greater than zero it indicates the number of waveform data words to follow In the current software version the XIA PSA value contains the result of the constant fraction trigger time computation CFD The format is as follows the upper 8 bit of the word point to the ADC sample before the CFD counted from the beginning of the trace The lower 8 bits give the fraction of an ADC sample time between the sample and the CFD time For example if the value is 0x0509 the CFD time is 5 9 256 ADC sample steps away from the beginning of the recorded trace The User PSA value contains flags to indicate special events if it is not overwritten by User DSP code CHAN _USERPSA bits 3 2 1 0
93. over At the beginning of a data acquisition run the module looks at the counter and records the BufferTime as a 48 bit number using three 16 bit words These three words are the time stamps in the buffer header BUF_TIMEHI BUF_TIMEMI BUF_TIMELO Every time an event is recognized the DSP records the lower 32 bit of the time counter as two 16 bit words in the event header EVT_TIMEHI EVT_TIMELO Every time a channel triggers at the rising edge of a pulse the channel records the lowest 16 bit of the time counter which is reported in the channel header CHAN_TRIGTIME As the rising edge occurs a filter time before the event is recognized the channel time is typically a few microseconds ahead of the event time Using C parlance and enumerating beginning at 0 we would reconstruct the true time of arrival for any event as EventTime EVT_TIMELO For best precision use lus 75 in the conversion from clock ticks to seconds 13 33e 9s may lead to rounding errors 29 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved EventTime EVT_TIMEHI pow 2 16 EventTime BUF_TIMEHI pow 2 32 EventTime 1e 6 75s This EventTime can be used to match events between modules and to compute time differences between events ChannelTime serves as a refinement to compute the time of arrival differences between channels of the same event For most purposes it is sufficient and more convenient to simply compute a relative time di
94. pers JP1 JP2 and JP3 near the back of the board Pin 2 of JP2 is the input to the board s clock distribution circuitry It can be connected with a shunt to several other pins thus choosing a particular clock distribution mode The preferred clock distribution is the PXI clock mode is the chassis supports it else the daisy caned clock mode These jumpers differ slightly for modules of Revision B and Revision C D The clock functions themselves as described below are identical and compatible for both revisions Pixie 4 Revision B COCO ee 0e LOC LOC PXI ouT G59 BUS BUS JP1 JP2 JP1 JP2 JP1 JP2 JP1 JP2 JP1 JP2 a Individual or b Clock c Bussed clock d Bussed clock e PXI clock clock master repeater mode master mode slave mode slave mode mode Figure 7 1 Jumper Settings for different clock distribution modes of Revision B modules In a group of modules there will be one daisy chained clock master a in the leftmost position and several repeaters b OR one bussed clock master c and several bussed clock slaves d Modes a b and c d can not be mixed Mode e can only be used with a backplane providing 37 5 MHz instead of the usual 10MHz Jumper JP3 is located near the chip U2 it has no label 55 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved Pixie 4 Revision C D Left as 0 J e O 0 JP3 JP3 JP3 JP3 JP3 JP3 3 fl e e N TN AN AN NA AN ao aad aa aad aa aad o LOC
95. put File Directory MCA Spectrum Directory C X1A Pixie 4 P9_v2 MCA Pulse Shape Directory C XIA Pixie 4 P4_v2 PulseShape Figure 3 5 The Fites Patus Panel 15 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 3 3 Run Control Group The Run Control group in the Man control panel has the most essential controls to start and stop runs and to define or monitor the run time and the number of spills For more options use the Run Control tab of the PARAMETER SETUP panel 3 4 Results Group The Results group of the Main control panel displays the count rates of the current or most recent run Click Update to refresh these numbers The popup menu Open Panels leads to panels to view the output data from the data acquisition in detail These panels are the MCA Spectrum display the List Mone Traces display the List Mope SPECTRUM display the Run Statistics and a panel to display results from a series of files 3 4 1 MCA Spectrum loi x FWHM 96 FWHM ab Peak Area Fit peaks between cursors Y 33095 980 65 1 1914 11 684 640 04 A 0 4 3000 4000 968 64 1 153 11 168 639 55 1 1 0 4 o 0 1 973 2 1 1126 10 828 630 04 1 1 12 67 320 964 88 11993 14 572 639 41 2 11427 1656 1506 3 13 629 180 44 98193 y Update 250 200 e e ems jodo o Da aap eee Counts ee eee eee a a A Y aua ee be 0 5 10 15 20 25 30x1 o Channel Figure 3 6 The MCA Spectrum displ
96. rights reserved Table 6 2 Examples of dead times in typical conditions Assumes events with only one active channel MCA run MCA run LM run LM run LM run 2us filter 6us filter 6us filter 6us filter 6us filter any ICR any ICR no trace 2s trace 2s trace ICR 1k s ICR 1k s ICR 10k s C D Ous C D Ous mel C Coinc Window 0 133 us 0 133 us 0 13345 133 us 0 1335 133 us 0 133 us 133 us E eaeaee ON max A B C max A B C max A B C AFB B includes C D a ds C D 13 633 us 13 633 us SFDT per event Td 1 633 us 6 us 6 us Note 1 FIFO logic prevents overlap of second pulse with coincidence window and FPGA readout but only when waveforms are requested Additional dead time due to stopped FIFO write process occurs only when filter dead time gt 13 6 us pre trigger time 6 6 1 2 Dead time associated with external conditions There are three dead time effects that originate from outside the trigger filter FPGA The first two have the effect of stopping the Pixie 4 live time counter the last is counted separately Input signal N AU N Out of range n Processing overload JA Gate pp Gate dead time Live time AS AT Runtime Total time Fig 6 10 The live time counter is stopped when the signal is out of range and when events are rejected because of a processing backlog in the DSP or spectrum memory increment process in the FPGA SFDT and FTDT are only coun
97. s Manual V2 50 XIA 2012 All rights reserved The Fano noise is only a property of the detector material The electronic noise on the other hand may have contributions from both the preamplifier and the amplifier When the preamplifier and amplifier are both well designed and well matched however the amplifier s noise contribution should be essentially negligible Achieving this in the mixed analog digital environment of a digital pulse processor is a non trivial task however 3x10 2225 a a Se 7 pe ADC Output l gA l Filter Output 31 g E 5 O a lt 30 29 28 75 80 85 90 95ys Time Figure 6 4 A y ray event displayed over a longer time period to show baseline noise and the effect of preamplifier decay time With a RC type preamplifier the slope of the preamplifier is rarely zero Every step decays exponentially back to the DC level of the preamplifier During such a decay the baselines are obviously not zero This can be seen in Figure 6 4 where the filter output during the exponential decay after the pulse is below the initial level Note also that the flat top region is sloped downwards Using the decay constant tT the baselines can be mapped back to the DC level This allows precise determination of y ray energies even if the pulse sits on the falling slope of a previous pulse The value of t being a characteristic of the preamplifier has to be determined by the user and host software and dow
98. s Manual V2 50 O XIA 2012 All rights reserved kind of arithmetic processor the obvious approach to determining V is to take some sort of average over the points before the step and subtract it from the value of the average over the points after the step That is as shown in Figure 6 2 averages are computed over the two regions marked Length the Gap region is omitted because the signal is changing rapidly here and their difference taken as a measure of V Thus the value V may be found from the equation Vine L Wt L WN 6 1 i before i after where the values of the weighting constants W determine the type of average being computed The sums of the values of the two sets of weights must be individually normalized 4 i l a rita Sloot ae tan acl gt E Length 5 2 30 Gap Q 5 Len g th X 7 a A ll a 4 T l 20 22 24 26 28 30 Time us Figure 6 2 Digitized version of the data of Figure 6 1 b in the step region The primary differences between different digital signal processors lie in two areas what set of weights W y is used and how the regions are selected for the computation of Eqn 6 1 Thus for example when larger weighting values are used for the region close to the step while smaller values are used for the data away from the step Eqn 6 1 produces cusp like filters When the weighting values are constant one obtains triangular if the gap is zero or trapezoidal filters
99. s the hit pattern and determines which channel to read This together with some general overhead takes about 850 ns b filter sum readout Readout of filter sums takes about 600 ns per channel marked in the hit pattern c waveform readout _ Waveform readout occurs in the same processing step as the filter sum readout and increases the DSP readout time by 3x the recorded waveform length It can be avoided by setting the requested waveform length to zero Note that if the waveform length is nonzero waveforms are still read in the compressed run types 0x101 103 for possible pulse shape analysis only the storing of waveforms in the output data stream is disabled In MCA runs waveforms are never read so the waveform length is irrelevant In the current firmware the FPGAs are released to resume data acquisition only after all channels marked in the hit pattern have been read out At this time all channels are cleared after 45 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved readout which means that pulses validated after the end of the coincidence window and thus not contributing to the hit pattern are lost The readout therefore causes dead time in all other channels in the module as well 5 Fast trigger dead time FTDT Input signal Threshold Trigger filter output Trigger FTDT Fig 6 9 Fast Trigger Dead Time FTDT A second pulse is not detected if the trigger filter output is st
100. s with the chassis still powered down then power up the chassis Pixie 4 modules are not hot swappable If using a remote controller be sure to boot the host computer after powering up the chassis 2 1 2 Drivers and Software System Requirements The Pixie software is compatible with Windows XP Vista or Windows 7 Restrictions apply to the 64 bit version of Windows 7 Please contact XIA for details on operating Pixie 4 modules with Linux When the host computer is powered up the first time after installing the controller and Pixie 4 modules in the chassis it will detect new hardware and try to find drivers for it A Pixie 4 module will be detected as a new device every time it is installed in a new slot While there is no required order of installation of the driver software the following sequence is recommended users with embedded host computer skip to step 4 1 If you have a remote controller first install the driver software for the controller itself Otherwise skip to step 4 Unless directed otherwise by the manufacturer of the controller this can be done with or without the controller and Pixie 4 modules installed in the host computer and or chassis If the modules are installed ignore attempts by Windows to install drivers until step 7 NI controllers come with a multi CD package called Device Driver Reference CD For simplicity it is recommended to install the software on these CDs in the default configuration
101. surements 3 2 1 7 Run Control Tab The Run Control tab defines the settings for data acquisition The Run Type popup menu selects MCA or list mode runs see section 4 for a detailed description In addition there are controls to set the run time length of data acquisition as measured by Igor to set the polling time period for checking if list mode data is available for readout and or run time is reached to specify the data file name a base name plus 4 digit run number that can be made to increment automatically and 13 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved to specify the number of spills in list mode runs In list mode runs data is accumulated in on board memory until full at which time it is read out by the host PC We call each such readout a spill The number of spills thus sets the amount of data to collect The Start Run and Stop Run buttons from the Maw control panel are duplicated here as well Advanced options include settings for synchronizing acquisition between modules controls to set a timeout for each spill the number of events per spill and the spill readout mode and a button to open a panel with advance record options E Parameter Setup Trigger Energy Waveform Gate Coincidence Advanced 10000 S omo Stop Run et ome Start System Oscilloscope Figure 3 4 The Run Control tab of the PARAMETER SETUP Panel 3 2 2 OsciLLoscoPE As mentioned
102. t does not take into account the time required to send commands from the PC to the module For the DAQ Fraction displayed in the Pixie Viewer the PC s time is used RUN TIME The RUN TIME variable gives the time during which the DSP on the Pixie 4 module was switched on for data acquisition The usefulness of this variable is very limited It is less than the real time passed in the laboratory during acquisition e g the TOTAL TIME or the time measured by the host PC because it omits the time for a start run signals from the PC to reach 51 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved the DSP b communication between modules to synchronize the run start c clearing of memory and output variables before starting a run and d time spent waiting for readout It is larger than the time the FPGA is ready to take data because it does not account for the dead time from filter FIFO or DSP readout Thus for most purposes the LIVE TIME TOTAL TIME or the host PC s real time not the DSP variable named REAL TIME are more appropriate This variable is provided mainly to provide backwards compatibility to previous software revisions LIVE TIME The LIVE TIME is counted in the FPGA independently for each channel and measures the time the channel is ready for acquisition The LIVE TIME counter starts when the DSP finished all setup routines at the beginning of a run omits the times the ADC signal is out of range or t
103. t takes about 30 ms c In 16 16 buffer mode or double buffer mode the 8K buffer is also automatically transferred to external memory but only 16 times Then a flag is raised so that the host can read out the memory in a fast block transfer while acquisition continues and new data is stored in the second half of the memory However since again the memory has only a single port host readout can not happen at the same time as the DSP transfer Normally the readout will be finished before the DSP 8K buffer is filled but at high count rate or when recording waveforms the transfer may have to wait until the host finished reading the external memory which means the acquisition is stopped and there will be dead time in addition to the 300 us transfer time Case b is pictured in Fig 6 11 the difference for case c is shown with the dashed lines The Pixie 4 acquires several buffers of data T1 T2 T3 T4 until the external memory is full and read out by the host at T5 After readout a second spill resumes at T6 The live time counter is active while events can be acquired which excludes memory transfer and readout The run time starts a bit earlier and lasts a bit longer than the live time for each buffer and does not switch off for out of range or other conditions The total time counts all time from TO to the end of the last spill The Pixie 4 can not count the time between the user clicking the run start button and TO In case c the acquisit
104. t time for 16 buffers is again 30ms Note that at high cont rates and or for uncompressed data runs with long waveforms 16 buffers may fill up faster than 30ms Readout mode 3 is only efficient as long as the time to half fill the external memory is longer than the readout time 4 1 2 3 Multiple Spills Runs can be resumed by the host after the data is read out In a resumed run run statistics are not cleared at the beginning of the run 1 e it is possible to combine several buffer readouts spills into one extended run appended to the same file In the Pixie Viewer this is done automatically when requesting several spills Note that for runs with several modules the buffer ordering in the data depends on the readout option In mode 1 the data file begins with the first buffer readout of module 0 followed by first buffers of module 1 module 2 module N then the second buffers of modules 0 to N and so forth In readout mode 2 list mode runs are repeated 32 times before readout Therefore the data file will begin with the first 32 buffer readouts of module 0 followed by the first 32 buffers of module 1 module 2 module N then a second 32 buffers of module 0 to N and so forth In readout mode 3 above list mode runs are repeated 16 times before readout Therefore the data file will begin with the first 16 buffer readouts of module 0 followed by the first 16 buffers of module 1 module 2 module N then a second 16 bu
105. tart and stop runs at the same time by using a wired OR SYNC line on the PXI backplane In all modules the variable SYNCHWAIT has to be set to 1 If the run synchronization is not used SYNCHWAIT must be set to 0 The variable is set by checking the corresponding checkbox in the Run Control tab of the Pixie Viewer The run synchronization works as follows When the host computer requests a run start the Pixie 4 s DSP will first execute a run initialization sequence clearing memory etc At the beginning of the run initialization the DSP causes the SYNC line to be driven low At the end of the initialization the DSP enters a waiting loop and allows the SYNC line to be pulled high by pullup resistors As long as at least one of all modules is still in the initialization the SYNC line will be low When all modules are done with the initialization and waiting loop the SYNC line will go high The low gt high transition will signal the DSP to break out of the loop and begin taking data If the timers in all modules are to be synchronized at this point set the variable INSYNCH to 0 by checking the corresponding checkbox in the Run tab of the Pixie Viewer This instructs the DSP to reset all timers to zero when coming out of the waiting loop From then on they will remain in synch if the system is operated from one master clock Whenever a module encounters an end of run condition and stops the run it will also drive the SYNC line low This will be de
106. tected in all other modules and in turn stop the data acquisition Note that if the run synchronization is not operating properly and there was a run start request with SYNCHWAIT 1 the DSP will be caught in an infinite loop This can be recognized by reading the variables INSYNCH and SYNCHWAIT If after the run start request the DSP continues to show INSYNCH 0 and SYNCHWAIT 1 it is stuck in the loop waiting for an OK to begin the run Besides rebooting there is a software way to force the DSP to exit from that loop and to lead it back to regular operation See the Programmer s Manual for details 60 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 7 4 External Gate and Veto GFLT It is common in larger applications to have dedicated external electronics to create event triggers or vetoes Besides the external trigger described in section 7 2 5 the Pixie 4 also accepts a global first level trigger GFLT This signal acts as a validation for an event already recognized by the Pixie 4 Using multiplicities and or other information the external logic needs to make the decision whether to accept or reject a given event If that decision can be made within the filter time energy filter rise time plus flat top of all Pixie 4 channels involved then the GFLT function can be used By definition GLFT is a global signal that applies to all channels In a second scenario the acquisition may have to be inhibited for certain intervals
107. ted when the live time is on The gate dead time is counted in 47 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved a separate counter but also only when the live time is on Run time and total time are always on unless the run is stopped see below 1 Signal out of range When detector gains or offsets drift or an unusual large pulse is generated in the detector the analog input of the ADC may go out of range In this condition the FPGA can not accumulate meaningful filter sums and thus is considered dead This dead time is enforced during the actual out of range condition and several filter times afterwards until the bad ADC samples are purged from filter memory The live time counter is stopped during the out of range condition because no triggers can be issued and no pulses are counted 2 On board pulse processing limit Processing dead time includes all time lost in the acquisition due to the conversion of the raw energy filter sums into the pulse height which is stored in list mode memory and or binned into spectrum memory This conversion process begins after the raw filter sums have been read out from the FPGA The FPGA resumes its acquisition after the readout and raw filter sums for several hundred events can be buffered in DSP memory Therefore the computation of the pulse height in the DSP incurs no dead time as long as a average count rates are below the maximum DSP processing rate and b bursts of pulses do not
108. tings to file To do so click on the Save button at the bottom of the PARAMETER Setup panel to open a save file dialog Create a new file name to avoid overwriting the default settings file 5 Save the Igor experiment using File gt Save Experiment As from the top menu This saves the current state of the interface with all open panels and the settings for file paths and slot numbers the settings independent of module parameters 6 Click on the Run Control tab set Run Type to 0x301 MCA Mode Poll time to 1 second and Run time to 30 seconds or so then click on the Start Run button A spinning wheel will appear occasionally in the lower left corner of the screen as long as the system is waiting for the run to finish If you click the Update button in the Maw panel the count rates displayed in the Results group are updated 7 After the run is complete select MCA Spectrum from the Open Panels popup menu in the Results group of the Man panel The MCA spectrum graph shows the MCA histograms for all four channels You can deselect other channels while working on only one channel After defining a range in the spectrum with the cursors and setting the fit option to fit peaks between cursors you can apply a Gauss fit to the spectrum by selecting the channels to be fit in the Fit popup menu You can alternatively enter the fit limits using the Min and Max fields in the table or by specifying a Range around the tallest peak or the peak with th
109. tion via a iii eE 32 5 2 Real time Processing Units iii aa 33 5 3 Digital Signal Processor DSP oinnia nin tore a E E R E a io a i 33 APOLO tera tia T dc A E E EEE 34 6 Theory of Oper A A a id 35 6 1 Digital Filters for y ray Detecto TS ooooconionocionocionncnonnononcononocnon conan conan on ESE nan n non n OAS 35 6 2 Trapezoidal Filtering in the Pixle 4 0ooococnnococnonocionncoonncoonnnnonconononnon conan rn con nn cnn n ocn nr ran nn rro nan nnnncnnnnns 37 6 3 Baselines and Preamplifier Decay Times oooooocnonccnonccconcnconnnononocnnnncnonconononnonnncannncnnnnn rn rnnnnnnnnrnnnnnnns 38 6 4 Thresholds and Pile up Inspection ccccessccssscessseessneeseecesseceseeeseceeseecsseeessaeeseeeeeseeeesaeeseeeeeeees 39 NA 42 6 6 Dead Time and Run Statistisessa E ii e E ea 42 7 Operating Multiple Pixie 4 Modules Synchronously cccccesseeeteceeteeeeneeeeseeeeseeeseeeeesaeessaeeessnneeeeeees 55 Tel Clock DISTIN 55 1 2 Trigger A DutlOns 32 2 saveteeidd cai vases cuss a iaaa i e sa AE E A AEE E E 57 T R n Synchronizatiot senini E RN 60 T4 External Gate and Veto GELT is ccccsesacest conrad tad ee eo a n E idad 61 EMS A O RN 63 LO Coincident EVEN A A AS A A A a T 64 8 Using Pixie 4 Modules with Clover detecto S ooooocnoocononccnoncnconcncnonaconn conan conan nononcnnan nn con nn cnn n nora nrnnnnncinss 68 9 Troubleshooting ase NOR 69 Ol Startup Problems ci AEE T E E E R ea iE ee 69 9 2 Acquisition Problems seei A E E E a
110. to all four channels hit 1111 For each event the Hit Pattern is checked against the user defined Coincidence Pattern to determine if it is acceptable Local Test If acceptable the event is recorded and processed if not the event is rejected and data acquisition continues The user can define the Coincidence Pattern to accept one or more hit patterns For example in the Coincidence Tab of the Parameter Setup Panel there are 16 checkboxes for the 16 possible hit patterns and selecting one sets the corresponding bit in the coincidence pattern In the example shown in Figure 7 3 accepting only Hit Pattern 0001 makes the Coincidence Pattern 0x0002 Several of the check boxes can be set at the same time for instance to accept any pattern with two or more channels If all checkboxes are set any possible Hit Pattern is acceptable and the Coincidence Pattern is OXFFFF Each channel with a pulse above threshold whether trigger enabled of not contributes to the hit pattern the moment the pulse is validated as not piled up i e a filter time after the rising edge of the pulse The hit pattern is read for comparison with the coincidence pattern about 66 ns after the first pulse is validated If several channels contribute to an event the minimum coincidence window the time period in which delayed channels can contribute to the hit pattern is thus 66 13 ns If longer delays between channels are expected from the physics of the
111. uch longer 49 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved The final type of dead time comes from the readout of data from Pixie 4 memory to the host PC In MCA mode this is limited to the access arbitration for the spectrum memory The memory has only a single port for both the increments according to the pulse height computed by the DSP and for readout to the host PC arbitrated by an FPGA While the host is reading the memory spectrum increments are queued in a buffer 2K long At maximum count rate it will take the DSP at least 2K processing time to fill the buffer and correspondingly longer at lower count rates while the host readout typically takes 30 ms Thus host readout dead time is usually not an issue in MCA runs unless spectra are read very frequently In list mode runs the Pixie 4 memory fills up after a certain number of events are acquired Acquisition is stopped until the memory is read out Depending on the buffer per spill setting this is organized in one of three ways a In single buffer mode acquisition stops after the DSP s 8K buffer is filled and data is read out in a slow transfer mode b In 32 buffer mode the 8K buffer is automatically transferred to external memory The transfer incurs about 300us dead time this may be improved in the future After 32 transfers the external memory is full and acquisition is stopped The external memory is read out in a fast block transfer tha
112. up 7 2 3 Trigger Distribution across PXI segment boundaries In PXI chassis with more than 8 slots the PCI bus as well as the PXI bussed backplane lines are divided into segments with not more than 8 slots While the PCI bus is bridged between the segments the PXI bussed backplane lines are usually only buffered from one segment to the next i e the line in one segment drives the line in the neighboring segment Since this buffer is essentially a one way communication though the direction may be selectable no wire OR can be build across the segment boundary Note Sometimes there is no connection at all For applications with more than 7 modules the Pixie 4 have to be operated in a chained OR mode where trigger signals are passed from module to module using the PXI nearest neighbor lines which are not interrupted by the segment boundaries In this mode each module ORs the trigger signal from its right neighbor with its own contributions and passes it to the left The leftmost module issues the combined OR to a bussed PXI line The chassis has to be configured such that the leftmost segment drives all other segments to the right The Pixie 4 modules can be set up to operate in this mode using the chassis control panel of the Pixie Viewer The PXI backplane buffering has to be set up with the tools provided by the chassis manufacturer the lines named PXI TRIGO fast trigger PXI TRIG1 event trigger and PXI TRIG2 synchronization have to be
113. vent e it includes the time from event validation until one filter time before the end of the DSP readout This includes DSP readout of an event in a different channel after which data in all channels are discarded the end of dead time counting before the end of the DSP readout is implemented as a delayed start of the counting GDT GATE dead time The dead time from GATE or VETO GFLT is counted separately from SFDT for each channel As mentioned above and further described in section 7 4 the use of these signals may depend on the application In the current firmware a rising or optionally falling edge at the Gate input creates a GATE PULSE with length GATE WINDOW see section 7 4 Both this GATE PULSE and the optionally inverted signal on the VETO input are combined into GDT_ON While GDT_ON is high GDT is incremented every clock cycle and thus counts the time pulses can be rejected from 52 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved acquisition The rejection has to be enabled independently It is possible to instead count the time during which pulses are allowed by inverting the combined signal In the standard statistics mode the GDT counter is additionally subject to the channel being live i e GDT is only counted if a run is in progress signal in range etc GATE IN Gate Window VETO GFLT GATE counter onH GATE PULSE GDT Counter EZ Inv
114. y fewer events For example when collecting 2 us waveforms a pulse consists of 160 words and thus there is only room for 25 pulses Assuming no pulse shape analysis the time to process a pulse is again roughly 2 4 us Thus the buffering limit changes from less than 400 pulses in 800 1600 Ls to less than 25 pulses in 50 200 us Obviously the average rate stays the same but the length of bursts is reduced In any case burst rates are still limited by Poisson statistics filter dead time and the FPGA readout time if it exceeds the filter time or if waveforms are read 48 PIXIE 4 User s Manual V2 50 XIA 2012 All rights reserved 3 Gating or Veto GFLT If an external signal prohibits acquisition using the GATE or VETO signals the channel is also dead though on purpose As further described in section 7 4 the use of these signals may depend on the application a b c On one hand they may be used to reject an individual pulse e g externally summing multiplicities from several channels and issuing a short validation pulse at the right time in the validation process In this case the actual length of the pulse does not correspond to a dead time The VETO input is set up for this purpose and we call this mode of operation GFLT global first level trigger for validation On the other hand GATE or VETO may block validation of events for certain amounts of time e g changing sources or activating beams In this case the
115. y should be counted clock cycle by clock cycle as dead time Both the VETO and the GATE inputs are available for this purpose VETO as a global signal for the whole system GATE as a dedicated signal for each channel VETO acts at the time of pulse validation GATE acts at the time of the rising edge of the pulse However the VETO input can be routed to replace the GATE input with a user option In a third class of application the acquisition may only be of interest when GATE or VETO are off The pulse rejection logic would be similar to b but livetimes and count rates should only be counted when GATE Veto are off as count rates would be different in on and off times In b one would be more interested in an overall livetime and average count rate and additionally the time inhibited by GATE or VETO to make corrections The appropriate way to count GATE or VETO dead time may thus depend on the experiment See below GDT for the current methods implemented in the firmware 6 6 1 3 Dead time associated with host readout TO T1 T2 T3 T4 T5 T6 Input signal TransfertoEM AC EM readout OOO rn TN Run tme i ii ee Total Time Fig 6 11 After a run start command arrives from the host at T0 the Pixie 4 module acquires several buffers of data T1 T2 T3 T4 until the external memory EM is full and is read out by the host at T5 For single buffer mode not using external memory the transfer is replaced by a host readout which however takes m
116. ypical signal rise time from the detector It then needs to be wider by one filter clock cycle than that minimum but at least 3 filter clock cycles Note that a filter clock cycle ranges from 0 026 to 0 853us depending on the filter range so that it is not possible to have a very short flat top together with a very long filter rise time The Pixie Viewer provides a tool which automatically scans all possible combinations of energy filter rise time and flat top and finds the combination that gives the best energy resolution This tool can be accessed by clicking the Optimize button on the Settings tab Please refer to the Online Help documentation for more details A second option is to create a file series where the energy filter parameters are modified for each file in the series See section 3 6 for more details 3 5 3 Threshold and Trigger Filter Parameters In general the trigger threshold should be set as low as possible for best resolution If too low the input count rate will go up dramatically and noise peaks will appear at the low energy end of the spectrum If the threshold is too high especially at high count rates low energy events below the threshold can pass the pile up inspector and pile up with larger events This increases the measured energy and thus leads to exponential tails on the ideally Gaussian peaks in the spectrum Ideally the threshold should be set such that the noise peaks just disappear The settings of the

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