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Pixie-500e User Manual

1. 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 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 group trigger mode may have an output count rate even though its input count rate is zero 3 Since LIVE TIME counters are paused when SDRAM or local 16 event buffers are full the input and output count rates should be considered as rates while active as opposed 44 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved to actual rates per elapsed lab time For input count rates this is the more intuitive case since the detector will not stop generating pulses when the channel becomes inactive due to a full SDRAM and the input count rate should closely correspond to the detector s rate For output count rates it is a matter of perspective should 1t mean the total number of counts per acquisition lab time or the number of counts processed while the module is active The former would produce unreasonably low count rates when e g the signal goes out of range periodically since it will not account
2. 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 41 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved 6 6 1 3 Dead time associated with host readout The final type of dead time comes from the readout of data from Pixie 500 Express 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 the FPGA While the host is reading the memory spectrum increments are queued in a buffer 2K long At maximum count rate 1t 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 data is buffered in a large SDRAM memory organized as a FIFO In a major improvement compared to the Pixie 4 or Pixie 500 the Pixie 500 Express therefore never stops the acquisition for data readout The host PC can read the memory from one end at the rate set by the PCIe interface max 800 MB s while new data is added on the other end Given the data bandwidth of the PXIe interface it is rather unlikely for the SDRAM to fill
3. 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 still above threshold An second 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 7G Knoll Radiation and Measurement J Wiley amp Sons Inc 2000 chapters 4 and 17 39 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved 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 3 Other In the Pixie 4 and Pixie 500 as well as in earlier versions of the Pixie 500 Express firmware there were additional dead times associated with reading out the data since only one event at a time was stored in the FPGA In the current firmware up to 16 events and or total 8K waveform samples are buffered in the FPGA Thus new events are accepted while captured ones are read out and processed further and these types of dead time are eliminated If the buffers fill up the channel pau
4. 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 500 Express User s Manual V3 20 XIA 2014 All rights reserved 3 2 Setup Group In the setup group there is a button to open the Start Up panel which is used to boot the modules The Open Panels popup menu leads to one of the following panels PARAMETER SETUP OsciLLoscopPE 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 button which will make all advanced panel controls visible as well The Pixie spectometer being a digital system all parameter settings a
5. 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 module 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 are separated by less than 1 0 us are thus seen to pileup in the present exa
6. OR to a bussed PXIe line The chassis has to be configured such that the leftmost segment drives all other segments to the right The Pixie 500 Express modules can be set up to operate in this mode using the chassis control panel of the Pixie Viewer The PXIe 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 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 500 Express modules with a suitable PXIe module to bring out signals from the backplane Please contact XIA for details 7 2 5 External Triggers Not yet implemented External triggers usually do not have the correct format and fast trigger vs event trigger timing required by the Pixie 500 Express trigger logic The Pixie 500 Express therefore includes specific logic to turn an external signal into distributed triggers External signals 3 3V TTL standard can be connected to a Pixie 500 Express front panel input 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
7. 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 and 32bit operating systems may impose a 4 GB limit 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 20 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 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 settings 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 SJS E orm 8 E E Wed Aug 19 2009 Wed Aug 19 2009 4 54 00 PM m m n 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
8. dead time simply given by the time to measure the pulse height is unavoidable unless pulse height measurements are allowed to overlap which would produce false results 38 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved 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 ICR max 1 Q 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 This filter dead time is recorded in the SFDT counter in each processing channel T TO T1 Pulse Filter sums gt SFDT a ae ae Piled Up ae Caen OO oo e nn oo Fig 6 7 Pulse dead time A pulse arriving at TO will incur slow filter dead time for energy measurement until T1 At T1 the pileup status is latched for a single pulse it is logic low and the event is accepted A second pulse arriving at TO0 will extend the dead time and cause the pileup status to be logic high Unless pileup rejection is disabled both events are rejected 2 Fast trigger dead time FTDT Input signal Sn
9. hit pattern set the threshold to zero disables triggers or uncheck the good channel check box The coincidence is based on triggers at the rising edge but a pulse can subsequently be rejected as piled up That may lead to coincidence records with missing channels For example if channels 0 2 were in coincidence and channel 0 saw a second pulse to be rejected as piled up then only channels 1 and 2 will be recorded but will show the actual 3 channel hit pattern bits 8 10 set for channels 0 2 To avoid such missing records disable pileup rejection 52 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved The List mone Trace display applies an independent coincidence window for viewing events Under the option show 4 pulses within events within that range are extracted from the specified file whereas the Coincidence Window described above limits the acquisition of events into the file To view coincidence events properly the range in the List mone Trace display should be set at least as large as the acquisition coincidence window Coincidence acquisitions can be conducted with independent triggers EACH channel is recorded when IT is hit AND all channels match the coincidence pattern or with distributed triggers ALL channels with the group trigger option set are recorded when ANY trigger enabled channel is hit AND all channels match the coincidence pattern In the former case waveforms
10. linked to the FPGA by a local bus At this time the interface does not issue interrupt requests to the host computer but 1t may do so in the future Instead for example to determine the run status the host has to poll a Control and Status Register CSR in the FPGA The FPGA links the PCIe IC 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 or list mode data while a run is in progress A dedicated I O FPGA distributes triggers and coincidence signals to other modules using the PXI backplane connections and the front panel connectors 30 PIXIE 500 Express User s Manual V3 20 XIA 2014 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 amplitude 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 e
11. 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 Key information bit of the hit pattern are decoded in checkboxes below the hexadecimal value 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 In the binary file events are stored as single channel records To display data from coincident pulses in multiple channels check the box Show 4 pulses and enter a coincidence window in clock ticks You will have to increment the Event Number up to 4 times to get new data The waveform from the current event is displayed in bold Arrow buttons allow changing of both module number and file name at the same time The Ref popup menu allows one of the current 4 channels to be saved for comparison check the corresponding box in the table to add its waveform to the plot For multi module runs select the appropriate file b for module 17 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 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 OsciLLoscorE since it uses the same full rate data same as the filters implemented in the module not the reduced rate sampled at the os
12. 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 47 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved 7 3 Run Synchronization It is possible to make all Pixie 500 Express modules in a system start and stop runs at the same time by using a wired OR SYNC line on the PXIe backplane The feature is enabled 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 500 Express 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
13. should only be read when the run has ended The data organization of one file is as follows The file always starts with a file header of length BUFHEADLEN Currently BUFHEADLEN is 32 and the 32 words 16 bit are Table 4 2 File header data format total 32 words 16bit 0 BikSize Block size 16 bitwords __ 1 Modum Modulemumber gt gt gt _____ 2 RunForma Format descriptor RunTask_ 3 ChanHeadlen ChannelHeaderLength____ 4 CoincPat Coincidence pattern CoincWin Coincidence window A MaxCombTraceLen Maximum length of traces from all 4 channels in blocks Following the file header the single channel event records are stored in sequential order Each event starts out with an channel header of length ChanHeadLen Currently ChanHeadLen 32 and the 32 words 16 bit are Table 4 3 Channel header data format 0 EviPattem Hitpatterm S O 1 Evtinfo Event status flags o O 2 NumTraceBlks_ Number of blocks of Trace data to follow the header 3 NumTraceBlksPrev____ Number of blocks of Trace data in previous record for parsing back _ 4 TrigTimeLO Trigger time low word S O JS TrigTimeMI Trigger time middleword o 6 TrigTimeHI Trigger time high word __ o O 7 TrigTimeX Trriggertime extra8bits S O 8 Energy Pulse Height o gt 9 Chano Channel number _ _ _ o S y O 10 User PSA Value Result of User specific pulse shape analysis 11 XIAPSAValue Re
14. stamp is recorded in every channel header Compressed data formats may store less bits in the future 27 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 5 Hardware Description The Pixie 500 Express 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 500 Express analog gain and offset analog gain and offset analog gain and offset Figure 5 1 Functional block diagram of the Pixie 500 Express 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 which are DC coupled to the input voltage range of the ADC which spans 2 V Input signals are adjusted for offsets and there is a computer controlled gain stage of switched relays A fine tuning of the gain is achieved by multiplying the calculated energy values with digital gain factors in the digital signal processor DSP Four options of termination and attenuation are selected by manual switches at the front end of the module The ADC is not a peak sensing ADC but acts as a waveform digitizer In order to avoid aliasing we remove the high f
15. the Open Panel popup menu in the Maw panel At the bottom of the Parameter Setup panel click on the Oscilloscope button This opens a graph that shows the untriggered signal input Fig 2 3 In the OscitLoscore 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 have falling leading edges toggle the Invert checkbox 0 067 10 3000 0369 0 067 10 3000 0369 0 067 10 3000 0363 0 067 10 3000 0389 oo m ee 12x10 AAA 14 bitADC Units Figure 2 3 OsciLLoscoPE panel 8 PIXIE 500 Express User s Manual V3 20 O XIA 2014 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 settings 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 overwrit
16. 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 21 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved 51x Filter limits Tau limits Filter Rane E JA Start Trise us Es JA Start Tau us End Trise us 26 453 8 End Tau us Step size us Step size us Start Tflat us 1 067 cra Tht IB Step size us 0 000 Set Scan Run Conditions Figure 3 10 The Fite Series Scan panel to acquire a series of files in which energy filter parameters and Tau are varied within user defined limits To set up such a parameter scan open the Fite Series Scan panel PARAMETER SETUP gt Energy tab gt Scan Settings 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 bu
17. up except for very high rates at very long waveforms If the SDRAM actually does fill up data acquisition is paused but as soon as the host frees up SDRAM memory by reading and storing data to disk the acquisition continues Any such pause is counted as dead time by turning off the livetime counter At the current firmware and SDRAM operation rate the SDRAM reads and writes data at about 0 9 GB s for a combined average throughput of 0 45 GB s and the PXI Express readout matches that rate transferring data from Pixie memory to host PC memory at up to 0 48 GB s With the buffering of up to 16 event in each channel s first processing stage and of up to 256 MB in the SDRAM temporary bursts of pulses creating higher data rates can still be captured without loss of data We note that streaming ADC data in real time amounting to 4 x 2 bytes x 500 MHz 4 GB s is still beyond the capacity of the PCIe x4 interface used by the Pixie 500 Express Few if any PXIe crates controllers and or hard drives can accommodate such rates Streaming ADC data is therefore only possible if some kind of data compression is applied please contact XIA for details 6 6 2 Live and dead time counters The Pixie 500 Express 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 i
18. virtually any radiation detector with exponentially decaying pulses Incoming signals are digitized by 14 bit 500 MSPS ADCs Waveforms of up to 8 0 us in length for each event can be captured in a first level FIFO and stored in 256 MB of on board SDRAM memory organized as a fast FIFO with DMA readout to the host PC The waveforms are available for onboard pulse shape analysis which can be customized by adding user functions to the core processing code 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 500 Express supports coincidence spectroscopy and can recognize complex hit patterns Data readout rates through the CompactPCI PXI Express backplane to the host computer can reach up to 800 MB s theoretical max for x4 connection Multiple modules can be read out in parallel with a suitable chassis and host PC The PXI backplane is also used to distribute clocks and trigger signals between several Pixie 500 Express modules for group operation With a large variety of CompactPCI PXI Express processor controller or I O modules being commercially available complete data acquisition and processing systems can be built in a small form factor Sooner or later there will even be hard drives with gt 10 GB s write capability to take full advantage of the Pixie 500
19. will not appear significantly shifted relative to each other even though they may be a few hundred ns delayed waveforms are shown vs time from first sample The time stamps carry the delay information The latter case will lead to multiple records per coincidence if the delays between channels are greater than a few dozen nanoseconds so triggers are recognized separately Event info bit 4 identifies such group trigger without local hit records also their energy is set to zero unless the estimate energy option is set If waveforms are of interest it is advised to make Trace Lengths long enough to cover at least half of the coincidence window That way waveforms in event N triggered by the first channel and recorded for the first channel and in event N 1 triggered by the first channel and recorded for the second channel will start at the same time and will contain the rising edge of trigger and delayed pulse respectively Otherwise matching events becomes somewhat more difficult 7 6 2 Coincidences Between Modules Not yet implemented 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 wi
20. 0 200 e e es Ce en P E EE E R cece E reverie NT AT A A EETA A Dr es PA H H H 3 NM eae aca A as ESEE pa e a eee asf 0 ro o a_a ee 0 0 5 10 15 20 25 30x10 Channel Figure 3 6 The MCA Spectrum display 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 500 Express User s Manual V3 20 XIA 2014 All rights reserved 3 4 2 List Mope Traces and List Moe SPECTRUM Bis List Mode Traces Sra Data File Hit Pattern Ox 00110F21 Channel Energy 16bit TimeStamp __XIA_F ll ern Ux Vl 12022 3719956000 0 a test0185 b02 Overall accept 99560 Local coincidence test ok palmer 5 E nes Local orale l Pulse piled up 3719955996 0 a E a Event Number 595 Pulse out of range Waveform FIFO full Veto logic high lt m V Show 4 pulses within 100 clock ticks Pulse Height ADC steps 0 100 200 300 400 500ns Time from first sample Figure 3 7 The List Mope Traces display The List Mope Traces display shows the data from the binary list mode files bin or b If waveforms were collected they are shown in the graph section of the
21. 2 or XJ4 backplane connector defined in the PXI e standard used by the Pixie 500 Express PXI J2 PXIe pin XJ4 pin C Connection Pixie pin function Type Left neighbor Right neighbor Bussed Bussed Bussed Left neighbor Right neighbor Right neighbor EAN Bussed Bussed Right neighbor ee ee Left neighbor Right neighbor Bussed Left neighbor Left neighbor Left neighbor GATE input channel 3 Right neighbor Event Trigger input chained OR Left neighbor GATE input channel 2 Star trigger to Hit pattern to slot 2 slot 2 Left neighbor GATE input channel 0 Right neighbor Sync input chained OR re Left neighbor Right neighbor Right neighbor Bussed Clock Left neighbor Right neighbor LBL9 LBR7 TRIGI TRIG2 TRIG3 LBL2 LBR4 a 3A I O 7A 6A SA 21A _ LBRO Oooo 7B SB Sai P al I6A 7A 17A _ 6A ISA SA 16B 16B 7 18B 5 TRIGO TRIG4 LBR5 B B LBL10 LBR8 18C TRIGS LBL3 LBLO 2 2 LBL7 LBR9 15D LBL6 f20c Ale 2D M 1 3D O 15D 8D 19D LBLa4 21D __ LBR2 o 8E 7E AAA SES gt 8E LBR6 TE TRIG7 CLK10 pI9E SCL BLS 218 f EBR not available in PXIe standard LBL8 3 3 3 3 LBR10 LA A iC C 0 D D E E
22. 3 External MA A A A A 50 TO Comcident Event ar da api 51 8 Using Pixie 500 Express Modules with Clover detectors oooonoconnococoocncoornconnncnnnnnnonocnonoronannnnnnconannnnos 53 OTTO ESOO IM ir AA AAA tees ATA BLA dd A EIA zi 55 OT Startup Problems it SAA AA E E dani 55 9 2 Acquisition Probe Msi A A AAA dia 55 LOA ppendixeA sex ese cae Gs seeds a svane ele eeacda saab e 8S dasaa eas ies Sa sede lana suleveascansvelicadadedeseane a a e i 57 10 1 Front end switches for termination and attenuatiOM ooooonnonccnonccionncnonnnooncnconcncono nono noncnnnnnnnnnncnanos 57 DOD LED Sissi OO ceils dntuate she TE 58 10 3 PXI backplane pin Pum Ct ons msi fasted Aes Covet tan died e iea as 59 2 PIXIE 500 Express User s Manual V3 20 XIA 2014 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 500 Express is a 4 channel all digital waveform acquisition and spectrometer card based on the CompactPCI PXI Express PXIe standard for fast data readout to the host It combines spectroscopy with waveform capture and on line pulse shape analysis The Pixie 500 Express accepts signals from
23. 5 5 Hardware Descriptor 28 5 1 Analog Signal Conditioning c cecccecssccssceessecseeeceeceeecenseceneeeseeceseeeeeeesaeeneeeessneeeeseneneeeeeenes 28 52 PULSE Processing t 29 5 3 Digital Signal Processor DSP and Event BuildiNg oooooooccnonocionocionccoonncnoncnnononcnnnncnnnoconnnnnnnnnnonos 30 4 POLEXpress Interface nisura a A AAA AA id 30 6 Theory of Operations is a A eh es eat 31 6 1 Digital Filters for y ray Detectors inc a a a conan conan on cannn A nn carr ronn nro R 31 6 2 Trapezoidal Filtering in a Pixie Module ooooononccnoncccnonaconanononoconnncooncnnononnononcononcnnn no nncnnnannnnnnnnnns 33 6 3 Baselines and Preamplifier Decay Times 0 cccccscesssseesneeeeneeceeeeeneecseecssaeensneeeeeeaeeeeessetteeeeeees 34 6 4 Thresholds and Pile up Inspection ccccecsccsssecssseesseeeeseecsseceeeecsseeesseecseeesseeesseeseseeseeeesseeeeeees 35 6 3 Filter ROCA A AAA A A aaa TCS ieta 38 6 6 Dead Time and Run StatistiCS o ooonccnncnnnnnnoconoccnnncnnncnoncnononnonnncnn conocio nono nn naar n cnn nc non c arc n naar nc nn cnn canas 38 7 Operating Multiple Pixie 500 Express Modules Synchronously oooconooccnonccconcccnonocnonocoononnoncnnoncnconnnnnns 45 ICO DISTIN lA iaa rita 45 12 Trigger Distr bution da A tai 45 LES R n Synchronizati n ori AA a A RA fi 47 7 4 External Gate and Veto GF D oooocccnocccoococoonncoonocoonncnoncnnononnon conan nnnon naco nn canon cnn n nc no nn ran nn nn nana nn nnnnnnnns 47 7
24. Driver Reference CD For simplicity it is recommended to install the software on these CDs in the default configuration 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 Fast data rates will avoid or reduce dead times associated with data readout from module to host PC but will only matter at high count rates Lower speed connections still work well for applications with lower data transfer requirements 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 gt At the time of writing these restrictions are Support for 64bit Windows is still under development 5 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 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 5 Install the Pixie 500 Express software provided by XIA see section 2 2 3 6 Unless already installed power down the host computer and install the Pixie 500 Express modules in the chassis Check the input switch settings for the appropriate signal termination 50 Q or 2 kQ see section 10 1 for details Then power up the system again chassis first 7
25. Express readout bandwidth 1 1 Features e Designed for high precision y ray spectroscopy with fast radiation detectors e g scintillator PMT combinations Nal LaBr etc and many others e 14 bit 500 MHz ADC resulting in energy resolutions close to HPGe capabilities e Simultaneous amplitude measurement and pulse shape analysis for each channel e Programmable gain high low and input offset e Programmable pileup inspection criteria include trigger filter parameters threshold and rejection criteria e Triggered synchronous waveform acquisition across channels modules and crates e Supports x4 PCIe data transfers lt 800 Mbytes second 3 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved 1 2 Specifications Front Panel I O Signal Input 4x 4 analog inputs Selectable input impedance 500 and 2kQ 2V pulsed 2V DC Switch selectable input attenuation 1 8 and 1 1 for either impedance setting Logic Input Output General Purpose I O connected to programmable logic 1 MMCX coaxial connector and 1 high density 10 pin connector single ended or differential Backplane I O Clock Input Output Distributed 10 and 100 MHz clocks on PXIe backplane Triggers Wired or bussed trigger on PXIe backplane for synchronous event acquisition Synchronization Wired or SYNC signal distributed through PXIe backplane to synchronize timers and run start stop to 50ns Veto Global logic level to suppress event trig
26. Refresh results in no or bad data history window reports error of DMA timeout The SDRAM may have to be reinitialized Please reboot the module 9 2 Acquisition Problems l DUR 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 ci A 10 gt reduce gain and or adjust the offset If the detector output shows significant ringing or overshoots it can happen that the Pixie 500 Express 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 Data collection in list mode has low DAQ fraction SFD
27. T 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 time required to record an event and c the data transfer rate To reduce a shorten the tracelength as much as possible even in compressed list mode To reduce b in compressed list mode runs set the tracelength to zero if no pulse shape analysis is required To increase c avoid frequent updates of run statistics and spectra set the polling time to a small value 0 1 0 01 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 Signals are unusually noisy and show a consistent offset between even and odd samples Calibrate the ADC gain and offset matching of the cores Calibrations are reset at every power cycle or reboot of the module The process started with the button in the OsciLLoscoPE panel will measure th
28. User s Manual Digital Gamma Finder DGF Pixie 500 Express Version 3 20 July 2014 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 LIO e a AE ATEEN 3 A O ned sheave ateea es 3 L 2c SPECIES aid 4 DiSethin ge W pists sstesecctinzsocdcietvnwalecddh wate sSoatalca ast sidac shuts based dasa sensei oe oot cages E S Tadd sated otis oad che cee eeees 5 DM nstallati O RN 5 22 Getting Started aroan e a obhateetl loses A E E E A O ede oversea E eerie 6 3 Navigating the Pixie VIEW onice nno i nes T E E E E E aE a aa E ias 10 SV OVEEVICW ic A A E E 10 SUDAN A A A ARA a e ad 11 3 3 Run Control Group ccccecccccessccesseeeseeessceesseeceseecesnecssaecceaeessseeeeseeseseeeeseeseseesseecseaeensaeeseneeensenseeeeenes 16 3 4 Results Group is ii te 16 3 5 Optimizing Parameters acts a e Anidado ada 18 A RU ENE 20 4 Data R ns and DatarSITUCTTES etsene A A cutee AAA AA Ai 24 O O O ON 24 4 2 Output Data Structuressd ves poca A A A 2
29. Windows will detect new hardware the Pixie 500 Express modules and should find the drivers automatically If not direct it to the drivers directory in the Pixie 500 Express software distribution installed in step 5 Verify in Window s device manager that the modules are properly installed as Pixie500e under Jungo devices and have no resource conflicts 2 1 3 Pixie User Interface The Pixie Viewer XIA s graphical user interface to set up and run the Pixie 500 Express modules as well as other members of the Pixie family 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 500 Express software distribution contains the installation program for version XXX Pixie 500e XXX_ setup exe 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 Pixie 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 Viewer software go
30. al 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 a Pixie Module From this point onward we will only consider trapezoidal filtering as 1t is implemented in a Pixie module according to Eqn 6 2 The result 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 clear
31. 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 Start UP 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 Main panel and the top right of the Parameter Setup panel Proceed with the steps below to configure your system 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 7 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved EM Parameter Setup Trigger Waveform Gate Coincidence Advanced Run Control Auto Find Tau a v A A hd A yv Start System Oscilloscope Figure 2 2 The Parameter Setup Panel Energy tab shown For an initial setup go through the following steps l 2 If not already visible open the Parameter Setup panel by selecting Parameter Setup from
32. are to be synchronized at this point check 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 This is implemented as a pulse on an additional backplane line distributed to all modules From then on they will remain in synch until the next power cycle or reboot Whenever a module encounters an end of run condition and stops the run it will also drive the SYNC line low This will be detected in all other modules and in turn stop the data acquisition 7 4 External Gate and Veto GFLT Not yet implemented 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 500 Express also accepts a global first level trigger GFLT This signal acts as a validation for an event already recognized by the Pixie 500 Express 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 500 Express 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 An example is the on off cycle of a neutron generator and events may o
33. at temperature changes it may be required to recalibrate the cores An indication of mismatch are systematic offsets between odd and even samples These controls are repeated in the OsciLLoscorE panel 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 500 Express User s Manual V3 20 XIA 2014 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 Gat
34. ave 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 effect 1 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 incase pileup inspection is inverted or disabled it is not contributing to SFDT For the DAQ Fraction displayed in the Pixie Viewer the PC s time is used GDT GATE dead time 43 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved The dead time from 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 the optionally inverted signal on the VETO input is fed into VETO_ON While VETO_ON is high GDT is incremented every clock cycle and thus counts the time pulses can be rejected from acquisition The rejection has to be enabled independe
35. ay 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 7 6 Coincident Events 7 6 1 Coincidences Within a Module Y E E E El Sa Save Extract Figure 7 5 Coincidence Pattern and Coincidence Window Settings in the Pixie Viewer In any given event a single Pixie module will have up to four channels with a hit i e a rising edge of a pulse detected in the signal of a channel s ADC The four channels thus form one of 16 possible Hit Patterns described in a 4 bit word In this representation the Hit Pattern ranges from no channel hit 0000 over only channel 1 hit 0010 to all four channels hit 1111 The user can define a combination of allowed hit patterns the Coincidence Pattern to accept one or more hit patterns Usually this is done to reduce the recorded event rate omitting non 51 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved coincident events that are of no interest 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 For example allowing all except for H
36. cally cover the rise time of the incoming signal and makes the pulse height measurement less sensitive to variations of the signal shape The first two processing elements in the FPGA are thus a fast filter for triggering and a slow filter for pulse height energy measurements For a detailed description see section 6 These filters run continuously Triggers are issued at each detected rising edge latch time stamps and are used for the other processes The energy filter sums are latched the appropriate time after each trigger A third processing element is 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 flagged 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 The fourth processing component is the FIFO memory which is organized in two blocks A smaller delay FIFO 2K samples buffers ADC data to position captured waveforms appropriately for the user defined pre trigger delay A larger storage FIFO 8K samples captures waveforms of the user defined trace length Up
37. citoscore 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 Note that invalid events will have energy 0 which causes a large spike in the first bin of the spectrum Set Emin to a nonzero value to hide this feature 3 4 3 Run Statistics MM All Run Statistics SE 5 25 07 PM Fri Jun 26 2009 5 26 03 PM Fri Jun 26 2009 C X1A 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 500 Express 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 18 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 3 5 Optimizing Parameters Optimization of the Pixie 500 Express s run parameters for best resolution depends on the individual systems and usually requires some degree of experime
38. ction 7 4 the use of these signals may depend on the application a 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 b 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 they 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 c 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
39. d 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 500 Express User s Manual V3 20 O XIA 2014 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 Location 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
40. d 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 32 PIXIE 500 Express User s Manual V3 20 O XIA 2014 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 principle 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 500 Express 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 G L k LV y V y V 6 2 i i k 2L G 1 i k L 1 where the filter length is Land the gap is G The factor L multiplying Y arises because the sum of the weights here is not normalized Accommodating this factor is trivi
41. d 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 500 Express User s Manual V3 20 XIA 2014 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 and run statistics List mode runs acquire data on an event by event basis but also collect spectra and run statistics List mode runs come in several variants see below storing different amounts of data per event The output data are stored in three different memory blocks The MCA block resides in a dedicated spectrum memory List mode data is stored in 256 MB of SDRAM organized as a FIFO Run statistics are kept in local memory by the on board FPGA 4 1 1 MCA Runs If only energy spectra are of interest an MCA run should be used 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 its
42. e 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 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 500 Express 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 diagnose 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 modul
43. e 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 500 Express User s Manual V3 20 XIA 2014 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 0 080 0 080 25 1 0 080 0 080 25 2 0 080 0 080 25 3 0 080 0 080 25 Load d Copy ave rac Figure 3 2 The Trigger tab of the PArRAMETER SETUP panel lt gt lt 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 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 each pulse The pulse decay time Tau is used to compensate for the decay of a previous pulse in the computation of the pul
44. e mismatch then modify the gain and offset match in an iterative process 10 Appendix A This section contains hardware related information 10 1 Front end switches for termination and attenuation w XA S N 0110 Table 10 1 Analog conditioning selection switches on Pixie 500 Express modules Switches are marked with solid red 50Q and dashed blue attenuation arrows On the PCB inverse labels describe the switch positions Switch PCB Label Function reference S3 x a 1 1 8 Attenuation will be 1 1 or 1 8 A oo S3 x b 50 Input impedance will be 500 or 2KQ A To ADC Figure 10 1 Simplified input stage of Pixie 500 Express showing switches input termination and attenuation and the overvoltage protection circuit 10 2 LEDs The Pixie 500 Express has 3 LEDs on the front panel A green LED indicates the firmware has been booted correctly and clocks have been programmed A yellow LED indicates that a run is in progress This can be a very short flash e g for parameter I O or continuous for data acquisition runs A red LED indicates an error occurred Currently that will be one of two cases a the module has been powered up but not booted yet b in a list mode run the SDRAM has been filled with data so that the acquisition has been paused Acquisition will resume and the LED goes off if the host catches up with data readout 10 3 PXI backplane pin functions Table 10 3 Pins of the J
45. e 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 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 T22 3 2 1 6 Advanced Tab The Advanced tab contains the controls for modifying the pileup inspection histogram accumulation baseline measurements and ADC calibration The ADC used on the Pixie 500 Express actually consists of two ADC cores on a single IC which need to be calibrated for matched gain offset and phase Normally these calibration settings are read from the module s non volatile memory at boot time but sometimes for example
46. e settings 14 PIXIE 500 Express User s Manual V3 20 XIA 2014 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 DSP every 2 3ms independent of whether a run is in progress or not Their precision is in the order of 5 10 or 50 cps Calibrate the ADC gain and offset matching of its two cores Calibrations are reset at every power cycle or reboot of the module or by clicking the Reset button The process started with this button will measure the mismatch then modify the gain and offset match in an iterative process 3 2 3 Fites 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 3 All Files Panel C XIA Pixie500e Firmwasre syspixie_RevC bin C XIA Pixie500e Firmware syspixie_RevC bin C XIA Pixie500e Firmware pixie bin C XIA Pixie500e Firmware FippiP500 bin C XIA Redshift Redshift_B ZZ_redshift_psir bin C XIA Pixie500e DSP P500code bin C XIA Pixie500e DSP P500e ldr C XIA Pixie500e DSP P500e var C XIA Pixie500e DSP PXIcode Ist C XIA Pixie500e DSP P500e Ist C XIA Pixie500e Configuration debug set C XIA Pixie500e MCA C XIA Pixie500e P
47. e 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 6 in measuring Vx is found from sqrt of 0 3 34 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved The Fano noise is only a property of the detector materia
48. elf There is no data transferred between the Pixie module and the host PC except for the occasional manual update of MCA spectra and run statistics By design the MCA memory does not fill up each event simply increments a bin in the spectrum 4 1 2 List Mode Runs If on the other hand data should be collected on an event by event basis including energies time stamps pulse shape analysis values and wave forms a list mode run should be used In list mode pulse heights are still histogrammed into MCA spectra e g for monitoring purposes The list mode data is continuously transferred from the Pixie module to the host PC List mode runs halt data acquisition either when a preset time is reached or when a preset number of spills have been collected as determined by the Pixie Viewer A spill here means 2 MB of data read from the SDRAM FIFO Unlike the Pixie 4 or Pixie 500 the Pixie 500 Express never stops the acquisition for data readout List mode data is buffered in the SDRAM FIFO and read by the host PC on one end while being written by the firmware on the other end Given the data bandwidth of the PXIe interface it is rather unlikely for the SDRAM to fill up except for situations with very high rates and very long waveforms If the SDRAM actually does fill up data acquisition is paused but as soon as the host frees up SDRAM memory by reading and storing data to disk the acquisition continues The red light on the
49. en are controlled by the parameters in the Gate tab of the PARAMETER Setup Panel The Pixie 500 Express 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 1f 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 49 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved 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 validates 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 statu
50. ess modules into any peripheral PXIe or PXIe PXI hybrid slot with the chassis still powered down then power up the chassis Pixie 500 Express 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 supported on Windows 7 and still compatible to Windows XP and Vista Restrictions apply to the 64 bit version of Windows 7 Please contact XIA for details on operating Pixie 500 Express modules with Linux When the host computer is powered up the first time after installing the controller and Pixie 500 Express modules in the chassis it will detect new hardware and try to find drivers for it A Pixie 500 Express 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 500 Express 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
51. for the duty cycle of the signal source The latter would produce unreasonably high rates if the system is near its processing limit and be often paused for SDRAM readout though it may better reflect the pileup rejection statistics The choices made in the current firmware select the latter case but by qualifying the output count rate with the LIVE TIME TOTAL TIME the former can be recovered 45 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 7 Operating Multiple Pixie 500 Express Modules Synchronously When multiple Pixie modules are operating as a system it is usually 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 PXIe backplane 7 1 Clock Distribution Unlike the Pixie 4 or Pixie 500 the Pixie 500 Express uses the 10 MHz and 100 MHz clocks provided by the PXI Express chassis These clocks are routed on the backplane to tight tolerances and ensure that all modules receive the same clock with very little phase skew Every module in the chassis is therefore a clock slave to the backplane and no jumpers or switches are required to change the clock mode 7 2 Trigger Distribution 7 2 1 Trigger Distribution Within a Module Within a module each channel can be enabled to issue triggers Such a Fast Trigger indicates t
52. gering Data Interface PCI Express x4 connection to host PC Theoretical bandwidth 800 MB s per slot Actual bandwidth 450 MB s Digital Controls Gain Analog switched gain of 1 0 or 2 9 Digital gain adjustment of up to 10 in 15ppm steps Offset DC offset adjustment from 2 5V to 2 5V in 65535 steps Shaping Digital trapezoidal filter Rise time and flat top set independently in small steps Trigger Digital trapezoidal trigger filter with adjustable threshold Rise time and flat top set independentl Data Outputs Spectrum 1024 32768 bins per channel 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 Pulse height energy timestamps pulse shape analysis results waveform data and ancillary data like hit patterns 4 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 2 Setting Up 2 1 Installation 2 1 1 Hardware Setup The Pixie 500 Express modules can be operated in any standard 3U PXIe chassis The total system bandwidth will depend on the architecture of the chassis and controller for maximum bandwidth each slot should have an x4 connection 1 GB s and the controller should be capable of 1 GB s times the number of slots A PXIe controller embdded PC or bridge to desktop laptop must be placed in the system slot of your chassis Place the Pixie 500 Expr
53. ggered input signals Another tool is to create a file series where T is modified for each file in the series See section 3 6 for more details 3 5 5 Baselines and ADC calibration Between detector pulses the Pixie module continuously measures baselines which is ultimately used to correct for the DC offset Multiple baseline measurements can be averaged to reduce noise and a threshold can be set to exclude the occasional bad measurement from the average The controls to set these parameters are located in the Advanced tab of the PARAMETER SETUP panel The optimum values depend on the detector used but usually the defaults are good estimates and resolutions only improve slightly with manual fine tuning The 500 MHz ADC used on the Pixie 500 Express is actually a combination of two 250 MHz ADC cores on a single IC For best performance the two cores have to be calibrated to match in gain offset and phase Default ADC calibration values are stored on an on board EEPROM and are applied to the ADCs at boot time It may happen that the default values are not suitable e g due to significant temperature drifts This would manifest itself as a distinct offset between even and odd samples in the waveforms In such a case the ADCs can be recalibrated with a routine called from a button in the Advanced tab of the ParaMETER SETUP panel 3 6 File Series 3 6 1 File Series to break up long data acquisition runs When taking long data acquisitions it may
54. hat the trigger filter just crossed the threshold at the rising edge of a pulse and is used to start pileup inspection and to latch time stamps among other things Each trigger enabled channel issues triggers to the central system logic which builds an OR of all triggers and sends it back to all channels If a channels is set to group trigger mode it uses the distributed fast trigger instead of its own local triggers to capture data In this way one channel can cause data to be acquired at the same time in all channels of the trigger group However the DSP then reads data from all participating channels individually and stores it as one event record per channel Notes 1 Each channel trigger enabled or not always also generates a hit flag for a coincidence test when it detects a rising edge To disable this mark the channel as not good or set the trigger threshold to zero 2 In group trigger mode all data is captured based on the distributed trigger This may cause slight shifts in waveforms and timestamps due to the extra delay of routing signals through the central logic 7 2 2 Trigger Distribution Between Modules Fast triggers can also be distributed over the PXIe backplane The fast trigger signal 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 lo
55. he 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 Filter clock decimations and filter time granularity Filter range Filter granularity max Trise T nat 6 6 Dead Time and Run Statistics 6 6 1 Definition of dead times Dead time in the Pixie 500 Express 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 minimum 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
56. ie Viewer Examples 1 To 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 8 Using Pixie 500 Express Modules with Clover detectors Not yet implemented When working with clover detectors the Pixie 500 Express 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 55 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved individual channel energies The result the full energy of gamma rays scat
57. ing 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 the 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 goo
58. it Pattern 0000 makes the Coincidence Pattern OxFFFE Allowing only 0011 as in shown in Figure 7 5 makes the Coincidence Pattern 0x0008 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 OxXFFFF Each channel with a pulse above threshold whether trigger enabled of not contributes to the hit pattern A channel hit flag is set to logic high for a user specified time the Coincidence Window after the fast trigger The hit flags from all four channels of a module are continuously tested against the coincidence pattern local coincidence test Each channel latches the 4 Hit flags from all 4 channels and the result of the coincidence test in the middle of its coincidence window This data become part of the event status flags and the DSP can decide to accept or reject events based on this information The plot below shows the time of arrival difference histogram for an acquisition using one periodic and one quasi random pulser with a Coincidence Window of 800 ns The resulting distribution is basically flat with a sharp cutoff at 400 ns The recorded event rate dropped by a factor 60 compared to the acquisition without coincidence requirement 800 ns Coinc window 600 400 200 0 200 400 600 Time difference ns Notes To prevent a channel from contributing to the
59. l 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 2 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 downloaded to the module 6 4 Thresholds and Pile up Inspection As noted above we wish to capture a value
60. lectric 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 31 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved kind of arithmetic processor the obvious approach to determining V is to take some sort of average over the p
61. 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 19 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved 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 t 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 tcan 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 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 T take a short run and note the value of 7 that gives the best resolution Pixie users can also use the fit routines in the Oscitoscore to manually find the decay time through exponentially fitting the untri
62. ly 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 module 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 33 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved triangular and semi Gaussian filtering by stretching the triangular rise time a bit so that the true triangular rise 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 G This may be compared to analog filtered pulses whose tails may persist up to 40 of the rise time a phenomenon du
63. mes 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 The on board pulse processing by the DSP computes the pulse height energy from raw energy filter sums which is then stored in list mode memory and or binned into spectrum memory In the Pixie 500 Express the computation itself takes only a few DSP cycles but the readout and other overhead amounts to approximately 1 microsecond per pulse to read raw sums compute the energy and send it to the memory SDRAM Waveforms are transferred within the FPGA at 1 GB s Every microsecond of captured waveform thus takes one microsecond for SDRAM data transfer The measured processing limit is roughly 700 000 pulses s if a single channel is processed per event and about 1 700 000 pulses s for events with 4 active channels in MCA mode 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 In List mode with nonzero waveforms the limit is strongly dependent on the length of the captured waveform 3 Gating or Veto GFLT If an external signal prohibits acquisition using the GATE or VETO signals the channel is also dead disabled on purpose As further described in se
64. module s front panel indicates such a condition When the Pixie Viewer ends the data acquisition there may be data in the SDRAM that has not yet been stored to file The readout thus continues for a short while after the DSP stops collecting Rollovers from 2 to 0 may happen for extremely long runs and or extremely high count rates 24 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved new data adding one or more spills to the file The preset number of spills is therefore to be understood as a minimum request 4 1 2 1 Pulse Shape Analysis Pulse shape analysis comes in several varieties executing algorithms by XIA enabled by selecting options in the standard firmware software or algorithms programmed by users as plug in code for the DSP In the current firmware software the following algorithms are available from XIA Accumulation of 2 sums baseline subtracted near the rising edge of the pulse Capture of amplitude maximum minus baseline near the rising edge of the pulse Computation of the ratio of the 2 sums Please contact XIA for details 4 1 2 2 Compressed Data Formats The output data of list mode runs can be reduced by using one of the compressed formats described below The key differences are that as less data is recorded for each event there is room for more events in the SDRAM FIFO less time is spent per event to read out data to the host computer and data files are smalle
65. mple 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 module 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 37 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved symmetry of the 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 FPGA with different clock decimations filter ranges The ADC sampling rate is always 2ns but in higher clock decimations several ADC samples are averaged before entering the filtering logic In filter range 1 2 samples are averaged 2 samples in filter range 2 and so on Since t
66. n to be made captures the status of the TOKEN 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 cont
67. ndow each module sends 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 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 500 Express module The acceptance decisions implemented in the current PDM firmware are described in detail in 53 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 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 decisio
68. nly 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 48 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved Validation Pileup Filter Counter y ADC data rhresholg TRIGGER start nish ILEUP O TO T1 T2 T3 T4 TS T6 Input signal Gate input Gate Window ON Gate Delay ON GATEBIT Veto input Validation Figure 7 3 Block and timing diagrams 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 gre
69. ntation 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 electronic noise in the system you can view a Fourier transform of the incoming signal by selecting Oscittoscope gt FFT The graph shows the FFT of the untriggered input sigal of the OsciLoscorE By adjusting the dT control in the OsciLLoscoPE 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 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 6u
70. ntly 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 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 Pixie 500 Express counts the numbers of triggers in each channel FASTPEAKS the number of valid single channel events 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 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 as well as to run start stop overhead Users are free to use the reported values to compute rates and time better matching their preferred definitions
71. 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 35 PIXIE 500 Express User s Manual V3 20 XIA 2014 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 500 Express the filter sums are continuously updated in the FPGA 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 FPGA is ready to take new data immediately after the readout This usually takes much less than one filter rise time so that no system deadtime is produced by a capture and store opera
72. oints 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 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 weighte
73. ot account for the dead time from the pileup inspection out of range condition and energy filter in each individual channel Thus its main uses are to compute an average event rate total output counts of all channels RUN TIME and to compute the fraction of time the SDRAM was not full and acquisition not paused which is the DAQ Fraction displayed in the Pixie Viewer RUN TIME TOTAL TIME 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 each channels local 16 event buffer is full or the SDRAM memory is full and ends when the DSP encounters an end run condition e g host stop 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 The difference between LiveTime and RunTime can be used to determine how long the local 16 event buffers were full and waiting for readout or other events prevented the channel from data taking e g out of range 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 h
74. r These compressed data formats are currently under development Table 4 1 Summary of run types and data formats Output data DSP Variables MCA Mode Spectra in MCA memo RUNTASK 0x301 List Mode Energies time stamps PSA values and wave RUNTASK 0x400 standard forms in List mode memory Spectra in MCA memory CHANHEADLEN 32 4 2 Output Data Structures 4 2 1 MCA Histogram Data The MCA memory currently uses 32K words 32 bit deep per channel i e total 128K words The memory can be read out via the PCIe data bus at any time though not at the full burst rate If spectra of less than 32K length are requested only part of the 32K will be filled with data The total MCA memory size on the Pixie 500 Express is 512K words It can be reorganized for special applications e g 2D spectra or channel sum spectra 25 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved If enabled the histogram data is automatically read and saved to file at the end of the run The file has the extension mca 4 2 2 List Mode Data The list mode data in the SDRAM FIFO consists of a series of single channel data records For each module the host readout process creates an individual file for these records The extension of these files is b 2 digit module number The records can be written by the DSP in a number of formats User code should access the data in the file header to navigate through the data The file
75. re 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 OsciLLoscope 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 the pulse rise time and small compared to the pulse decay time A pulse shape not meeting these conditions has the effect of raising th
76. requency 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 28 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved Though the Pixie 500 Express 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 500 Express without any further shaping 5 2 Pulse Processing Real time pulse processing is implemented in a field programmable gate array FPGA which also incorporates FIFO memory for each channel The data stream from the ADCs is sent to these units at the full ADC sampling rate While modern FPGAs can capture high speed data streams internal processing is limited by the complexity of the logic Therefore the FPGA on the Pixie 500 Express internally de serializes each channel s 14 bit 500 MHz data stream into a 56 bit 125 MHz data stream for processing Using a pipelined architecture the signals are processed at this high rate without the help of the on board DSP The processing applies 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 typi
77. rols define if a module sends its hit pattern 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 54 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved H E Chassis Register Panel so e Jes Front Panel and Backplane Options 0 1 2 Trigger share mode 0 0 0 Aaj Front panel drives GFLT line one module only a E a Front panel contributes to STATUS line wire OR a oO a Module writes control pattern to PDM to immediate left a O a Send local hit pattern to PDM in slot 2 a O a PDM control pattern 0x0000 0x0000 0x0000 Validation delay for external fast trigger ns 0 0 0 y Module Coincidence Setup Accept event if local hit pattern passes local test Al Accept event if global hit pattern passes global test in PDM ul a Module s local test adds to global test local fail causes global fail a a a Channel 3 Hit contributes to STATUS line wire OR a a O Coincidence Pattern for local test from Module Register OxFFFE OxFFFE OxFFFE Coincidence Window for both tests from Module Register 13 13 7 gt Figure 7 6 Module coincidence setup in the Pix
78. s 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 500 Express until a trigger is issued by the Pixie 500 Express 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 De ay and Gate Window can range from 13 3ns to 3 4 us Save Etec F igure 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 500 Express front 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 automa
79. s 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 module was requested to take data It essentially counts the time from the command to start a data acquisition to the command to end it The TOTAL TIME includes the time spent for run start initialization and host readout However since it is based on the PXI chassis internal clock a part with 50 ppm accuracy from module to module and only updated periodically 1 ms it may not be as precise as a laboratory wall clock over long time spans e g the host PC s However writing data to hard disk is usually much slower in the order of 0 1 GB s depending on the particular system Advances system architectures may improve this data rate for example multiple hard drive arrays 42 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved internal clock Also it does not take into account the time required to send commands from the PC to the module RUN TIME The RUN TIME variable tracks the time during which the DSP on the Pixie module was switched on for data acquisition The usefulness of this variable is limited It may be less than the TOTAL TIME because it omits the time the SDRAM is full and waiting for readout during which the data acquisition is paused in all channels It is larger than the time an individual channel is ready to take data because it does n
80. s will be more appropriate In general the flat top needs to be wide enough to accommodate the longest typical 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 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 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
81. se height You can enter a known good value or click on Auto Find Tau to let the Pixie Viewer 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 50 5 973 50 50 50 Load C ave Extract Figure 3 3 The Energy tab of the PARAMETER SETUP Panel 1 173 1 173 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 500 Express User s Manual V3 20 XIA 2014 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 define VETO as 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 tim
82. ses acquisition and stops the live time counter 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 500 Express live time counter the last is counted separately Input signal N NA Out of range A Processing overload pa Veto FF Gate dead time a Live time AS LS 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 e g local buffer memory full or SDRAM not read out by host SFDT and FTDT are only counted when the live time is on The gate dead time is counted in 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 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 40 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved 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 ti
83. sult of standard XIA pulse shape analysis 1 2 3 A 5 7 10 11 12 15 Extended PSA Values 16 32 reserved PT The hit pattern is a bit mask which tells which channels were captured within the specified coincidence window plus some additional status information as listed in table 4 4 The channel 26 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved header may be followed by waveform data An offline analysis program can recognize this by reading the number of waveform blocks from the NumTraceBlks word The block size is defined in the file header Table 4 4 Event Pattern and Event Info bit description EvtPattern If set indicates that data for channel 0 3 have been recorded 4 7 4 Logic level of FRONT panel input 5 Result of LOCAL acceptance test 6 Logic level of backplane STATUS line 8 7 Logic level of backplane TOKEN line result of global coincidence test see section 7 If set indicates that channel 0 3 has been hit in this event EvtInfo o Y Coincidence testresut SSS 4 2 3 Reconstruction of List Mode Time Stamps In the Pixie 500 Express there is a 56 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 a rate of 125 MHz by 4 ticks so that the unit of the LSB is 2ns Hence the 56 bit word can span a time interval of over 800 days before rolling over The full 56 bit time
84. tered within the clover detector is binned in an additional addback spectrum 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 56 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved 9 Troubleshooting 9 1 Startup Problems 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 or the 64 bit equivalent 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 serial numbers entered in the Startup panel match the location of the modules OsciLLoscorE
85. tically disable it for all other modules The GATE signal is also distributed over the backplane using 4 PXI nearest neighbor lines Therefore a module to the left of a Pixie 500 Express can be used to input 4 GATE signals to the Pixie 500 Express 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 50 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 7 5 External Status Not yet implemented Besides Veto a second function for the Pixie 500 Express 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 record see 7 6 2 The fourth function for the Pixie 500 Express 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 m
86. ting point operations and includes it in the event header data sent to the SDRAM The captured waveform data is normally not touched by the DSP the DSP only enables a direct FPGA internal transfer from the channel processing block to the SDRAM interface block at a rate of 1GByte s The DSP also controls the overall operation of the Pixie 500 Express The host computer communicates with the DSP via the PCIe interface Reading and writing data to DSP memory does only temporarily pause its operation and can occur even while a measurement is underway The host sets variables in the DSP memory and if necessary calls DSP functions to apply them to the FPGA Through this mechanism all gain and offset DACs are set and the filter settings are applied to the FPGA The FPGA then processes the data without support from the DSP once it has received the filter settings In this scheme the greatest processing power is located in the FPGA processing the incoming waveforms from the ADCs in real time and producing 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 Express Interface The PCI Express PCIe interface through which the host communicates with the Pixie 500 Express is implemented in a PCIe endpoint IC which is
87. tion 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 a Pixie module The peak detection and sampling in a Pixie module 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 Gs 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 36 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved ADC units ORO la A bu ina licencia co a moment Sec i ul SAN RP 56 58 60 62 64 66 68us Time Figure
88. to 16 events and 8K samples of waveforms are buffered in the FPGA For each event a complete set of time stamps energy filter sums pileup inspection flags coincidence information and waveforms are stored Waveforms from closely following events may overlap i e the same ADC data is stored once but read twice for subsequent events User defined acceptance settings specify if an event is considered valid e g only accept events without pileup The last processing element are a number of counters that keep run statistics such as live time filter dead time number of triggers and so on 29 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 5 3 Digital Signal Processor DSP and Event Building The pulse processing described above runs independently in every channel of the Pixie module On a module wide level additional logic is implemented to distribute triggers and apply a coincidence test See section 7 for details The result of the coincidence test is fed back to every processing channel The DSP manages the flow of channel data into the SDRAM buffer Whenever a channel has an event in its buffer the DSP will read the raw data from the FPGA and based on the event status flags determine if the event is to be recorded At this point there is also the option of executing customized user DSP code to modify results and the acceptance decision If the event is acceptable the DSP computes the pulse height in a few floa
89. to support xia com and search for Pixie release 2 2 Getting Started To start the Pixie Viewer double click on the file Pixie 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 N of Pixie modules in the system Then specify the serial numbers of the modules this allows addressing the modules from 0 N independent of the physical slot In the following SmaLL Caps are used for panel names italic font is used for buttons and controls 6 PIXIE 500 Express User s Manual V3 20 O XIA 2014 All rights reserved IL Pixied Start Up Panel 1000 00 5 10 E Figure 2 1 The Pixie Viewer Starr Ur panel above and Main Panel right 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 similar to Module 0 in slot 5 started up successfully in the IGOR history window the Pixie 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 After the system is initialized successfully you will see the Main control panel that serves as a shortcut to the most common actions
90. tton 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 Record 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 22 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 3 6 3 File Series Analysis iM File Series Results Cree oe Le FWHM 9 0 26 3400 3360 3320 Position Filter us 50 00 Tau ps 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 To analyze a series of mca files you can use the Fitz Series ResuLts 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 require
91. ulseShape Figure 3 5 The Fites Patus Panel 15 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved 3 2 4 Cuassis 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 500 Express s front panel input See sections 7 2 2 and 7 6 2 for details 3 3 Run Control Group The Run Control group in the Maw 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 lox FWHM 36 FWHM ab Peak Area Fit peaks between cursors 33095 980 65 4 1914 11 684 640 04 a 0 4 3000 4000 968 64 1 153 11 168 639 55 1 1 04 o o 1 973 2 1 1126 10 828 630 04 1 1 12 67 320 964 88 11993 11572 639 41 2 1142 7 1656 1506 3 13620 180 44 estos v Update 25
92. w and send the triggers to all of their channels In other words the backplane line 46 PIXIE 500 Express User s Manual V3 20 XIA 2014 All rights reserved carries a system wide trigger that essentially acts as a 5 input to the trigger OR in the central system logic 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 group 7 2 3 Trigger Distribution across PXI segment boundaries Not yet implemented In PXIe chassis with more than 8 slots the PXIe bussed backplane lines are divided into segments with not more than 8 slots 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 500 Express have to be operated in a chained OR mode where trigger signals are passed from module to module using the PXle 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

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