Model 672 Spectroscopy Amplifier Operating and Service Manual
Contents
1. ede 3 2 2 CONTROLS AND 6 2 4 2 3 eu E eu xeu eee eue ee epe up ed ODE 5 2 4 cM ELM 5 2 5 ELECTRICAL AND MECHANICAL 2 6 3 INSTAEDATION s mi ett e E ne n e E e enn 7 POWER CONNEC HON e e ele tee ee hel v o i et 7 3 2 PREAMPLIFIER CONNECTION 4 7 3 3 PULSED RESET PREAMPLIFIERS AND INHIBIT IN 7 3 4 CONNECTION OF TEST PULSE GENERATOR 7 3 5 SHAPING CONSIDERATIONS 00 8 3 6 LINEAR OUTPUT CONNECTIONS 8 3 7 PILE UP REJECTION USING PUR 9 3 8 LIVETIME CORRECTION USING BUSY OUTPUT 9 3 9 INPUT COUNT RATE USING CRM OUTPUT 9 4 OPERATING INSTRUCTIONS 10 4 1 INITIAL TESTING AND OBSERVATION OF PULSE WAVEFORMS 10 4 2 STANDARD SETUP PROCEDURES 0 0 0 eee 10 4 3 POLE ZERO ADJUSTMENTS FOR RESISTIVE FEEDBACK PREAMPLIFIER 11 4 4 BASELINE RESTORER BLR SETTING 12 42. rj exp 1 where r is the unpiled up output count rate is the input count rate and Ty is the deadtime or effective processing time of the amplifier The value of T is equal to the sum of the effective amplifier pulse Jenkins R L Gould and D A Gedcke Quantitative X Ray Spectroscopy Marcel and Dekker Inc New York 1980 16 exp n Tw Ty Tu Tw Tw TJ width T and the time to peak of the amplifier output pulse T The type of deadtime in the shaping amplifier is referred to as extending deadtime since a second event arriving before the end of the initial deadtime extends the deadtime by an additional amplifier output pulse width from the occurrence of the second pulse A normalized plot of Equation 1 is shown as the solid line in Fig 4 6 The maximum mean output rate equals 1 T exp 1 and occurs when the mean input rate equals 1 T At this maximum output rate the deadtime losses are 63 296 For input count rates exceeding 1 T the unpiled up output rate decreases When using a pile up inspection circuit the value of T is given either by the sum of T and or by the sum of T and the pile up inspection time whichever is larger Spectroscopy systems also have a deadtime that is caused by the digitizing time of the Analog to Digital Converter ADC This deadtime is a non extending deadtime since events arriving during the digitizing time are igno
3. Change the Shaping Time switch to 0 5 through 10 Us At each setting the PZ should be close to being correct A push of the Auto PZ button should complete the pole zero compensation e Decrease the Coarse Gain switch stepwise from 1K to 5 and ensure that the output amplitude changes by the appropriate amount for each step Return the Coarse Gain switch to 1 K f Decrease the Fine Gain control from 1 5 to 0 5 and check to see that the output amplitude decreases by a factor of 3 Return the Fine Gain control to maximum at 1 5 g With the Shaping Time switch set for 1 Us measure the time to the peak on the unipolar output pulse this should be 2 2 Us for 2 2T h Change the Shaping Time switchto 0 25 through 6 Us At each setting check to see that the time to the unipolar peak is 2 2T Return the switch to 1 Us OVERLOAD TESTS Start with maximum gain T 2Us and a 10 V output amplitude Increase the pulser output amplitude by X1000 and observe that the unipolar output returns to within 200 mV of the baseline within 27 Us after the application of a single pulse from the pulser It will probably be necessary to vary the PZ Adj control on the front panel in order to cancel the pulser pole and minimize the time required for return to the baseline WERTICAL Fig 5 1 Circuit Used to Measure Nonlinearity LINEARITY The integral nonlinearity of the 672 can be measured by the technique shown in Fig 6 1 In effect
4. LINEAR AMPLIFIER MICRO AMMETER DETECTOR 1 Fig 4 15 System for Detector Current and Voltage Measurements Normat Circuit BYPASS CAPACITOR A ORTEC 030 007 300 B ORTEC BA 025 050 100 C ORTEC 025 100 100 D ORTEC BA 030 200 100 E ORTEC 84 045 450 100 E Back Current 4A Bias Voltage Fig 4 16 Silicon Detector Back Current vs Bias Voltage LJ DETECTOR Pump mouta DETECTOR BIAS SUPPLY PULSE GENERATOR 4 11 OPERATION IN SPECTROSCOPY SYSTEMS HIGH RESOLUTION ALPHA PARTICLE SPECTROSCOPY SYSTEM The block diagram of a high resolution spectroscopy system for measuring natural alpha particle radiation is shown in Fig 4 17 Since natural alpha radiation occurs only above several MeV an ORTEC 444 Biased Amplifier is used to suppress the unused portion of the spectrum the same result can be obtained by using digital suppression on the MCA in many cases Alpha particle resolution is obtained in the following manner a Use appropriate amplifier gain and minimum biased amplifier gain and bias level Accumulate the alpha peak in the MCA b Slowly increase the bias level and biased amplifier gain until the alpha peak is spread over 5 to 10 channels and the minimum to maximum energy range desired corresponds to the first and last channels of the MCA c Calibrate the analyzer in keV per channel using the pulser and the known energy of the alp
5. Bins
6. Fig 2 2 The fast amplifier in the pile up rejector includes a gated baseline restorer with its own automatic noise discriminator A multicolor pile up rejector LED on the front panel indicates the throughput efficiency of the amplifier At low counting rates the LED flashes green The LED turns yellow at moderate counting rates and red when pulse pile up losses are gt 70 When long connecting cables are used between the detector preamplifier output and the amplifier input noise induced in the cable by the environment can be a problem The Model 672 provides two low to moderate solutions For interference 25 After AUTO PZ FWHM Resolution keV 2 0 Triangular 6 Shaping Time Betore AUTO PZ frequencies the differential input mode can be used with paired cables from the preamplifier to suppress the induced noise At high frequencies a common mode rejection transformer built into the 672 input reduces noise pick up The transformer is particularly effective in eliminating interference from the display raster generators in personal computers All toggle switches on the front panel lock to prevent accidental changes in the desired settings 3873 10 20 40 Input Count Rate to Amplifier 103 counts s Fig 1 1 Effectiveness of the Automatic Pole Zero Feature a An uncancelled pole produces an undershoot on the amplifier output pulse which in turn causes b premature degradation of the resolu
7. RATEMETER PULSE HEIGHT ANALYZER Unipolar or GI OSCILLO GERMANIUM DETECTOR and CRYOSTAT Fig 4 18 System for High Resolution Gamma Spectroscopy 23 HIGH VOLTAGE POWER SUPPL Y SCINTIL LATION MULTICHANNEL PULSE HEIGHT ANALYZER LINEAR COUNTER AMPLIFIER PREAMPLIFIER SCINTILLATION COUNTER PULSE GENERATOR Fig 4 19 Scintillation Counter Gamma Spectroscopy System SCINTILLATION COUNTER GAMMA SPECTROSCOPY SYSTEMS The ORTEC 672 can be used in scintillation counter spectroscopy systems as shown in Fig 4 19 The amplifier shaping time constants should be selected in the region of 0 5 to 1 us for Nal or plastic scintillators For scintillators having longer decay times longer time constants should be selected X RAY SPECTROSCOPY USING PROPORTIONAL COUNTERS Space charge effects in proportional counters operated at high gas amplification tend to degrade the resolution capabilities drastically at x ray energies even at relatively low counting rates By using a high gain low noise amplifying system and lower gas amplification these effects can be reduced and a considerable improvement in resolution can be obtained The block diagram in Fig 4 20 shows a system of this type Analysis can be accomplished by simultaneous acquisition of all data on a multichannel analyzer or counting a region of interest in a single channel analyzer window with a counter and timer or countin
8. times the charge collection time of the detector Use the detector manufacturer s suggested shaping times as a starting point and adjust the shaping as your needs for resolution versus count rate vary GERMANIUM DETECTORS Shaping times for high purity germanium HPGe detectors will vary from 1 to 6 Us using the Unipolar output depending on the size configuration and charge collection time of the specific detector and preamplifier Coaxial detectors have significant variations in charge collection times due to their large volumes Compromises must often be made since the shaping time that will give the best resolution will usually be longer than the optimum time needed for the best throughput at high counting rates Planar detectors require shaping times in the range of 3to 10 Us for optimum resolution Lithium drifted silicon detectors Si Li have similar shaping time requirements SILICON CHARGED PARTICLE DETECTORS These detectors have very fast risetimes on the order of 10 ns or less A Unipolar output and a 0 5 to 2 Us shaping time will generally provide optimum resolution SCINTILLATION DETECTORS energy resolution of scintillation counters depends largely onthe scintillator and photomultiplier and therefore a shaping time of five times the decay time constant of the scintillator is a reasonable choice For Nal detectors that have a decay time constant of about 230 ns the optimum shaping time is 1 Us The Bipolar out
9. 5 INTERNAL CONTROLS EROR URGE UN ROUND OUR ee ee 13 4 6 DIFFERENTIAE INPUT MODE 22 pop RR RI pv UY Ups 14 4 7 SYSTEM THROUGHPUT 16 4 8 CHARGE COLLECTION OR BALLISTIC DEFICIT EFFECTS 17 4 9 PILE UP REJECTOR PUR AND LIVETIME CORRECTOR 18 4 10 OPERATION WITH SEMICONDUCTOR DETECTORS 20 4 11 OPERATION IN SPECTROSCOPY SYSTEMS 22 4 12 OTHER EXPERIMENTS TUS 23 eere UE 27 5 1 TEST EQUIPMENT REQUIRED 2244 weee reko ogbe nekeneke 27 5 2 PULSER TEST sete ates ace UU UE E NEP PME UNE EM 27 5 3 SUGGESTIONS FOR TROUBLESHOOTING 28 FACTORY REPAIR cab tee Pies sagte aA Ena SPIRAT SIRE NIRE RI 28 5 5 TABULATED TEST POINT 29 SAFETY INSTRUCTIONS AND SYMBOLS This manual contains up to three levels of safety instructions that must be observed in order to avoid personal injury and or damage to equipment or other property These are DANGER Indicates a hazard that could result in death or serious bodily harm if the safety instruction is no
10. Two position locking toggle switch on the front panel selects either GAUSSIAN or TRIANGLE pulse shaping for the UNIPOLAR output INPUT Front panel four position switch accommodates either or input polarities and selects the differential DIFF or normal NORM input modes In the NORM mode only the NORM input connector is used In the DIFF mode the preamplifier signal cable is connected to the NORM input and a cable having its center conductor connected to the preamplifier ground through an impedance matching resistor is connected to the DIFF REF input The impedance matching resistor must match the output impedance of the preamplifier DIFFERENTIAL INPUT GAIN BALANCE 20 turn potentiometer mounted on the PC board inside the module allows the gains of NORM and DIFF REF inputs to be matched for maximum common mode noise rejection in DIFF mode PZ AUTO MAN SWITCH Locking toggle switch selects either the AUTO automatic or MAN manual pole zero cancellation adjustment mode Both modes permit PZ cancellation for preamplifier exponential decay time constants from 40 Us to AUTO PZ BUTTON With PZ switch in AUTO PZ position momentarily pressing AUTO PZ button turns on the BUSY LED and initiates automatic adjustment of the PZ cancellation circuit BUSY LED turns off when adjustment is complete Once completed the PZ adjustment is held until the button is pushed again Memory of the last PZ adjustment is protected against unfore
11. a rear panel BNC connector Provides a 5 V NIM standard logic pulse when pulse pile up is detected Output also present for a pulsed reset preamplifier during reset and reset overload recovery Output pulse is selectable as active high or active low by means of a printed circuit board jumper Output impedance is 50 Used with an associated ADC multichannel analyzer to prevent analysis of distorted pulses Fig 2 2 PREAMP Rear panel standard ORTEC connector Amphenol 17 10090 provides power for the associated preamplifier Mates with power cords on all standard ORTEC preamplifiers Registration Offset for Visibility Without Pile Up Rejection COUNTS With Pile Up Rejection 50 000 counts s 0 650 0 1300 0 1950 0 2600 0 ENERGY Fig 2 2 Demonstration of the Effectiveness of the Pile Up Rejector in Suppressing the Pile Up Spectrum 2 5 ELECTRICAL AND MECHANICAL POWER REQUIRED The Model 672 derives its power from a NIM Bin supplying 24 V and 12 V such as the ORTEC Model 4001A 4002A Bin Power Supply The power required is 24 V at 90 mA 24 V at 170 mA 12 V at 330 mA and 12 V at 190 mA WEIGHT Net 2 3 kg 5 0 Ib Shipping 3 6 kg 8 0 Ib DIMENSIONS NIM standard double width module 6 90 X 22 13 cm 2 70 X 8 714 in per DOE ER 0457T 3 INSTALLATION 3 1 POWER CONNECTION The 672 operates on power that must be provided by a NIM standard bin and power supply such as
12. and hence the selected shaping time is 9 times the front panel shaping time T High count rate for the PUR is when the normalized count R T 20 5 where is the amplifier input rate see Fig 4 6 For example for 6 Us shaping R is 9 kHz and for 2 Us shaping R is 28 kHz Amplifier throughput for this condition using Equation 1 in Section 4 7 is 60 of the input rate A multicolor pile up rejector LED is included on the front panel to indicate the throughput efficiency of the amplifier At low counting rates pulse pile up losses lt 40 the LED flashes with a green color At moderate counting rates the color changes to yellow The color changes to red at high counting rates when the pulse pile up losses are gt 70 The fast amplifier in the pile up rejector includes a gated baseline restorer with its own automatic noise discriminator to eliminate the need for any operator 19 FWHM 24 o2 os 10 20 40 6 0 10 0 Fig 4 10 Energy Resolution FWHM as a Function of Amplifier Shaping Time Constant for a 10 HPGe Detector and 28 HPGe Detector for the 122 keV Co Line and the 1 33 MeV Line adjustments This function is also protected against negative overloads from pulsed reset preamplifiers The PUR pile up reject output logic pulse can be used at the gate or reject input of a multichannel Go Source HPGe DETECTOR PREAMP HIGH VOLTAGE SUPPLY inhibi
13. at the energy of the alpha particles striking the detector e g set the dial at 547 divisions for a 5 47 MeV alpha particle energy e Turn on the pulser and use its Normalize control and attenuators to set the output due to the pulser for the same pulse height as the pulse obtained in step c Lock the Normalize control and do not move it again until recalibration is required The pulser is now calibrated the Pulse Height dial reads directly in MeV if the number of dial divisions is divided by 100 LINEAR AMPLIFIER OSCILLO SCOPE DETECTOR CAPACITOR PULSE GENERATOR RMS VOLTMETER 100182 Fig 4 12 System for Measuring Amplifier and Detector Noise Resolution 20 AMPLIFIER NOISE AND RESOLUTION MEASUREMENTS As shown in Fig 4 12 a preamplifier amplifier pulse generator oscilloscope and wide band rms voltmeter such as the Hewlett Packard 3400A are required for this measurement Connect a suitable capacitor to the input to simulate the detector capacitance desired To obtain the resolution spread due to amplifier noise a Measure the rms noise voltage Ems at the amplifier output b Turn on the 419 precision pulse generator and adjust the pulser output to any convenient readable voltage E as determined by the oscilloscope The full width at half maximum FWHM resolution spread due to amplifier noise is then 2 35 Ems N FWHM E where is the puls
14. in Section 4 10 Detector Noise Resolution Measurements The detector noise will vary with detector size and bias conditions and possibly with ambient conditions CURRENT VOLTAGE MEASUREMENTS FOR Si AND Ge DETECTORS The amplifier system is not directly involved in semiconductor detector current voltage measurements but the amplifier serves to permit noise monitoring during the setup The detector noise measurement is a more sensitive method of determining the maximum detector voltage than a current measurement and should be used because the noise increases more rapidly than the reverse current at the onset of detector breakdown Make this measurement in the absence of a source Figure 4 15 shows the setup required for current voltage measurements ORTEC 428 Bias Supply is used as the voltage source Bias voltage should be applied slowly and reduced when noise increases rapidly as a function of applied bias Figure 4 16 shows several typical current voltage curves for ORTEC silicon surface barrier detectors Whenitis possible to float the microammeter at the detector bias voltage the method of detector current measurement shown by the dashed lines in Fig 4 15 is preferable The detector is grounded as in normal operation and the microammeter is connected to the current monitoring jack on the 428 detector bias supply IT x 4 DETECTOR micro BIAS SUPPLY AMMETER i 4 Uana VOLTMETER
15. in terms of simple pulse generator impulses at the input The 672 must be disconnected from its position in any system and routine diagnostic analysis performed with a test pulse generator and an oscilloscope It is imperative that testing not be performed with a source and detector until the amplifier performs satisfactorily with the test pulse generator The testing instructions in Section 5 2 should provide assistance in locating the region of trouble and repairing the malfunction The two side plates can be completely removed from the module to enable oscilloscope and voltmeter observations 5 4 FACTORY REPAIR This instrument can be returned to the ORTEC factory for service and repair at a nominal cost Our standard procedure for repair ensures the same quality control and checkout that are used for a new instrument Always contact Customer Services at ORTEC 865 483 2231 before sending in an instrument for repair to obtain shipping instructions and so that the required Return Authorization Number can be assigned to the unit This number should be marked on the address label and on the package to ensure prompt attention when the unit reaches the factory 5 5 TABULATED TEST POINT VOLTAGES The voltages given in Table 5 1 are intended to indicate typical dc levels that can be measured on the printed circuit board In some cases the circuit will perform satisfactorily even though due to component tolerances there may be some
16. particular preamplifier being used DIRECTLY INTO THE 672 The ORTEC test pulse generators are designed for direct connection When any one of these units is used it should be terminated with a 100 O terminator at amplifier input or be used with at least one of the output attenuators set at In SPECIAL CONSIDERATIONS FOR POLE ZERO CANCELLATION When tail pulser is connected directly to the amplifier input the Pole Zero should be adjusted See Section 4 3 for the pole zero adjustment If a preamplifier is used and a tail pulser is connected to the preamplifier test input it is not possible to adjust the pole zero for both the preamplifier pole and the pole from the pulser tail 3 5 SHAPING CONSIDERATIONS The Shaping Time switch on the front panel of the 672 can be set to select time constants in steps of 0 5 1 2 3 6 and 10 Us Choice of triangular and Gaussian filters doubles the time constants available for optimum resolution Triangular shaping will usually give better results The choice of the proper shaping time is generally a compromise between operating at a shorter time constant for accommodation of high counting rates and operating with a longer time constant for a better signal to noise ratio Since the full amplitude of the preamplifier output pulse must be preserved the peaking time measurement time must be large compared to preamplifier output pulse risetime The amplifier shaping time should be greater than five
17. stabilize A block diagram of a typical ORTEC gamma ray spectroscopy system is shown in Figure 4 2 HiGH OSCILLO VOLTAGE SCOPE SUPPLY Fig 4 2 Typical Gamma Ray Spectroscopy System b Set the 672 controls initially as follows Shaping 3 or 6 Us Unipolar Mode Triangle Coarse Gain 20 Fine Gain 1 00 BLR Rate PZ Polarity Norm Match preamplifier output polarity PZ Auto c Use a calibration source set about 25 cm from the active face of the detector The unipolar output pulse from the 672 should be about 8 V using a detector that has a preamp with a conversion gain of 300 mV MeV d Readjust the Gain control so that the higher peak from the source 1 33 MeV provides amplifier output at about 9 V 4 3 POLE ZERO ADJUSTMENTS FOR RESISTIVE FEEDBACK PREAMPLIFIER The pole zero adjustment is critical for good performance at high count rates in unipolar operation and for correct operation of the BLR circuit This adjustment should be checked carefully for the best possible results Whenever the shaping time is changed the pole zero must be adjusted The bipolar output resolution is not as sensitive to misadjusted PZ but it is important for recovery from very large overload pulses When using a transistor reset type preamplifier the Manual PZ Mode should be used and should be set to full counterclockwise AUTO POLE ZERO MODE This mode should give good results for most detectors for low to med
18. the negative pulser output is subtracted from the positive amplifier output to cause a null point that can be measured with excellent sensitivity The pulser output must be varied between 0 and 10V which usually requires an external control source for the pulser The amplifier gain and the pulser attenuator must be adjusted to measure 0 V at the null point when the pulser output is 10 V The variation in the null point as the pulser is reduced gradually from 10 V to 0 V is a measure of the nonlinearity Since the subtraction network also acts as a voltage divider this variation must be less than 10 V full scale x 0 025 maximum nonlinearity 1 2 for divider network 1 25 V for the maximum null point variation 28 OUTPUT LOADING Use the test setup of Fig 5 1 Adjust the amplifier output to 10 V and observe the null point when the front panel output is terminated 1009 The change should be 5 mV NOISE Measure the noise at the amplifier Unipolar output with maximum amplifier gain and 2 Lis shaping time Using a true rms voltmeter the noise should be less than 5 UV x 1500 gain or 7 5 mV For an average responding voltmeter the noise reading would have to be multiplied by 1 13 to calculate the rms noise The input must be terminated in 1000 during the noise measurements 5 3 SUGGESTIONS FOR TROUBLESHOOTING In situations where the 672 is suspected of a malfunction it is essential to verify such malfunction
19. voltage measurements that differ slightly from the listed values Therefore the tabulated values should not be interpreted as absolute voltages but are intended to serve as an aid in troubleshooting 29 Table 5 1 2 TPS Ty pice de Voltageut 25 mW 25 mY 10 10 my 25 mW 50 my 124 31 2 V 005 te 0 2 V Hic Logic D HC Logic 1 5 008 ta 0 2 V 1T 3V 45 10 All vollages measured wih nma moul agral with input berm nated n 10011 and contrals set chock wide a maximum 30 Bin Module Connector Pin Assignments For Standard Nuclear Instrument Modules per DOE ER 0457T 2 5 Function 3 V 3V Spare bus Reserved bus Coaxial Coaxial Coaxial 200 V dc Spare 6 V 6V Reserved bus Spare Spare Reserved 12 V 12V Spare bus Reserved bus Spare 21 Spare Reserved 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Function Reserved Reserved Reserved Spare Spare 24 V 24 Spare bus Spare Spare 117 V ac hot Power return ground Reset Scaler Gate Reset Auxiliary Coaxial Coaxial Coaxial 117 V ac neutral High quality ground Ground guide pin Pins marked are installed and wired in ORTEC s 4001A and 4001C Modular System
20. 3 Hewlett Packard 3400A RMS Voltmeter 5 2 PULSER TEST Coarse Gain 1K Gain 1 5 Input Polarity Positive Shaping Time Constant 2 us BLR PZ UNI Shaping Gaussian PZ MAN a Connect a positive pulser output to the 672 input and adjust the pulser to obtain 10 V at the 672 Unipolar output This should require an input pulse of 6 6 mV using a 1000 terminator at the input Switch Unipolar Mode to Triangle This should also be 10 V b Measure the positive lobe of the Bipolar output This should also be 10 V c Change the Input polarity switch to Neg and then back to Pos while monitoring the outputs for a polarity inversion The negative output should clamp at 1 V d Auto PZ Check Adjust a 448 pulser or equivalent pulser with the decay time set 5015 See IEEE Standards No 301 1976 to obtain a positive 9 V pulse at the Unipolar output A 419 pulser with its longer decay time can also be used but the time needed for the Auto PZ process may be quite long at the 60 or 70 Hz repetition rate Select Auto PZ and push the button The red LED will go on and then turn off when the unipolar output pulse has been pole zero compensated The quality of the PZ can be observed by increasing the scope input sensitivity to 10 or 20 mV cm with the LIMIT button pushed The LIMIT push button switch inserts a diode limiter in series with the front panel Unipolar Output connector to prevent overloading the input of the oscilloscope
21. DETECTOR TIMING FILTER AMPLIFIER LINEAR AMPLIFIER PREAMP DELAY AMPLIFIER Fig 4 22 Gamma Ray Charged Particle Coincidence Experiment Linear Output to Multichannet Analyzer 26 SCINTIL LATION PREAMP LINEAR AMPLIFIER TIMING SCA FAST COINCIDENCE UNIT GATED BIASED AMPLIFIER Linear Output to Multichannel Analyzer Source CFT OISCRIMI GERMANIUM NATOR DETECTOR 9 ATE amp DELAY GENERATOR TIMING FILTER AMPLIFIER LINEAR PREAMP AMPLIFIER AMPLIFIER 200329 Fig 4 24 Gamma Gamma Coincidence Experiment SCINTIL LATION PREAMP TIMING SCA LINEAR AMPLIFIER CFT DISCRIMI NATOR FAST COINCIDENCE UNIT ATE amp DELAY GENERATOR GERMANIUM DETECTOR TIMING FILTER AMPLIFIER 8 DELAY GATED Linear Output PREAMP AMPLIFIER BIASED to Multichannet P M AMPLIFIER Analyzer TUBE amp BASE SCINTIL LATION PREAMP TIMING SCA LINEAR AMPLIFIER 12003208 Fig 4 23 Gamma Ray Pair Spectroscopy 27 5 MAINTENANCE 5 1 TEST EQUIPMENT REQUIRED The following test equipment should be utilized to adequately test the specifications of the 672 Spectroscopy Amplifier 1 ORTEC 419 Precision Pulse Generator or 448 Research Pulser 2 Tektronix 465 475 or 485 Series Oscilloscope or equivalent
22. Model 672 Spectroscopy Amplifier Operating and Service Manual U S A Patent No 4 866 400 Printed in U S A ORTEC Part No 740370 1202 Manual Revision H Advanced Measurement Technology Inc a k a ORTEC a subsidiary of AMETEK Inc WARRANTY ORTEC warrants that the items will be delivered free from defects in material or workmanship ORTEC makes no other warranties express or implied and specifically NO WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE ORTEC s exclusive liability is limited to repairing or replacing at ORTEC s option items found by ORTEC to be defective in workmanship or materials within one year from the date of delivery ORTEC s liability on any claim of any kind including negligence loss or damages arising out of connected with or from the performance or breach thereof or from the manufacture sale delivery resale repair or use of any item or services covered by this agreement or purchase order shall in no case exceed the price allocable to the item or service furnished or any part thereof that gives rise to the claim In the event ORTEC fails to manufacture or deliver items called for in this agreement or purchase order ORTEC s exclusive liability and buyer s exclusive remedy shall be release of the buyer from the obligation to pay the purchase price In no event shall ORTEC be liable for special or consequential damages Quality Control Before being approved for shipment each
23. ORTEC instrument must pass a stringent set of quality control tests designed to expose any flaws in materials or workmanship Permanent records of these tests are maintained for use in warranty repair and as a source of statistical information for design improvements Repair Service If it becomes necessary to return this instrument for repair it is essential that Customer Services be contacted in advance of its return so that a Return Authorization Number can be assigned to the unit Also ORTEC must be informed either in writing by telephone 865 482 4411 or by facsimile transmission 865 483 2133 of the nature of the fault of the instrument being returned and of the model serial and revision Rev on rear panel numbers Failure to do so may cause unnecessary delays in getting the unit repaired The ORTEC standard procedure requires that instruments returned for repair pass the same quality control tests that are used for new production instruments Instruments that are returned should be packed so that they will withstand normal transit handling and must be shipped PREPAID via Air Parcel Post or United Parcel Service to the designated ORTEC repair center The address label and the package should include the Return Authorization Number assigned Instruments being returned that are damaged in transit due to inadequate packing will be repaired at the sender s expense and it will be the sender s responsibility to make claim with the shipper Inst
24. Unipolar and bipolar outputs recover to within 296 of the rated output from a X 1000 overload in 2 5 non overloaded pulse widths using maximum gain Table 2 1 Unipolar Pulse Shape Parameters for the Triangular and Gaussian Pulse Shapes Time interval From start of input pulse to maximum amplitude of unipolar output pulse Rise of output pulse from 0 196 to maximum amplitude Width of output pulse at 5096 of maximum amplitude Width of output pulse at 196 of maximum amplitude Width of output pulse at 0 196 of maximum amplitude Shaping Time Multiplier Triangular Gaussian 2 6 2 8 2 4 2 0 2 5 2 0 5 6 5 0 6 9 6 3 Time interval equals the selected front panel SHAPING TIME multiplied by the Shaping Time Multiplier 1 Specifications subject to change without notice FWHM Resolution keV Input Counting Rate cps Peak Shitt keV Triangular 2 us Shaping Time Fig 2 1 a Resolution and b Peak Position Stability as a Function of Counting Rate SPECTRUM Fig 2 1 Typically 896 broadening of the FWHM for counting rates up to 100 000 counts per second counts s and 1596 broadening for counting rates up to 200 000 counts s Measured on the 1 33 MeV gamma ray line from radioactive source under the following conditions 1096 efficiency ORTEC GAMMA X PLUS detector 8 5 V amplitude for the 1 33 MeV gamma ray on the unipolar output SPECTRUM 5 Fig 2 1 Peak positio
25. ain of 5 and 465 at coarse gain settings 210 For the INPUT switch setting the input impedance is 2000 for a coarse gain of 5 and 1460 for coarse gains 210 Input is dc coupled and protected to 25 V LINEAR Rear panel connector Identical to NORM input DIFF REF Front panel BNC connector is used for the preamplifier ground reference connection when operating in the differential input mode Operative only with the INPUT switch in the DIFF mode For the DIFF INPUT switch setting the input impedance is 1000 Q on a coarse gain of 5 and 465 at coarse gain settings gt 10 For the DIFF INPUT switch setting the input impedance is 2000 Q for a coarse gain of 5 and 1460 Q for coarse gains gt 10 Input dc coupled protected to 25 V INHIBIT Rear panel BNC input connector accepts reset signals from transistor reset preamplifiers or pulsed optical feedback preamplifiers Positive NIM standard logic pulses or TTL levels can be used Logic is selectable as active high or active low via printed circuit board jumpers INHIBIT input initiates the protection against distortions caused by the pre amplifier reset This includes turning off the baseline restorers monitoring the negative overload recovery at the unipolar output and generating PUR reject and BUSY signals for the duration of the overload The PUR and BUSY logic pulses are used to prevent analysis and correct for the reset deadtime in the associated ADC or
26. aseline restorer with several levels of automation Automatic positive and negative noise discriminators ensure that the baseline restorer operates only on the true baseline between pulses in spite of changes in the noise level No operator adjustment of the baseline restorer is needed when changes are made in the gain the shaping time constant or the detector characteristics Negative overload recovery from the reset pulses generated by transistor reset preamplifiers and pulsed optical feedback preamplifiers is also handled automatically A monitor circuit gates off the baseline restorer and provides a reject signal for a multichannel analyzer until the baseline has safely recovered from the overload Several operating modes are selectable for the base line restorer For making either a manual or automatic PZ adjustment the PZ position is selected This position can also be used where the slowest baseline restorer rate is desired For situations where low frequency noise interference is a problem the HIGH rate can be chosen On detectors where perfect PZ cancellation is impossible the AUTO baseline restorer rate provides the optimum performance at both low and high counting rates An efficient pile up rejector is incorporated in the 672 Spectroscopy Amplifier It provides an output logic pulse for the associated multichannel analyzer to suppress the spectral distortion caused by pulses piling up on each other at high counting rates
27. before reconnecting it to the power source vi E ET M SPECTROSCOPY AMPLIFIER FINE GAIN COARSE BATH 0 5 1 5 50 60 UAT ze REJECT D n H DIFF REF INPUTE ORTEC MODEL 672 SPECTROSCOPY AMPLIFIER 1 DESCRIPTION 1 1 GENERAL The ORTEC Model 672 high performance energy spectroscopy amplifier is ideally suited for use with germanium silicon surface barrier and Si Li detectors It can also be used with scintillation detectors and proportional counters The Model 672 input accepts either positive or negative polarity signals from a detector preamplifier and provides a positive 010 10 V output signal suitable for use with single or multichannel pulse height analyzers Its gain is continuously variable from 2 5 to 1500 Automation of all the critical adjustments makes the 672 easy to set up with any detector and provides a performance that is nearly independent of operator expertise The Automatic Pole Zero Adjustment feature significantly simplifies the tuning of the amplifier to compensate for the decay time of the preamplifier pulse This minimizes the operator skill and effort needed to achieve good energy resolution and peak position stability at moderate to high counting rates Fig 1 1 When changing time constants or detectors an accurate pole zero PZ adjustment is achieved by simply pushing the AUTO PZ button and waiting a few seconds f
28. e baseline can be used for cross over timing or may be preferred for spectroscopy when operating into ac coupled systems at high counting rates Typical system block diagrams for a variety of experiments are described in Section 4 3 7 PILE UP REJECTION USING PUR OUTPUT The PUR Pile Up Reject output on the rear panel is used at the gate or pile up reject input of a multichannel analyzer to suppress pile up in the recorded spectrum The fast amplifier in the pile up rejector includes a gated baseline restorer with an automatic noise discriminator to eliminate the need for any operator adjustments When pileup occurs a logic true pulse is generated which lasts until the unipolar output returns to the baseline normally a width of six times the shaping time If used with a pulsed reset preamplifier this output also includes a reject during the reset and recovery interval 3 8 LIVETIME CORRECTION USING BUSY OUTPUT The signal from the rear panel Busy output connector provides a nominally 5 V logic pulse for the duration that the Unipolar output pulse exceeds the baseline restorer threshold or pile up inspector threshold or when the external INHIBIT IN is true For livetime correction Busy should be connected to the Busy In connector on the MCA For optimal livetime correction with ORTEC analyzers like the ADCAMS an internal jumper in the amplifier should be setto match the unipolar triangular or Gaussian mode The output is inter
29. er dial reading in MeV and 2 35 is factor for rms to FWHM For average responding voltmeters such as the Hewlett Packard 400D the measured noise must be multiplied by 1 13 to calculate the rms noise The resolution spread will depend on the total input capacitance since the capacitance degrades the signal to noise ratio much faster than the noise DETECTOR NOISE RESOLUTION MEASUREMENTS The measurement just described can be made with a biased detector instead of the external capacitor that would be used to simulate detector capacitance The resolution spread will be larger because the detector contributes both noise and capacitance to the input The detector noise resolution spread can be isolated from the amplifier noise spread if the detector capacity is known since where Nita is the total resolution spread and Namp is the amplifier resolution spread when the detector is replaced by its equivalent capacitance ORTEC BA 030 007 300 ORTEC BA 025 050 100 ORTEC BA 025 100 100 ORTEC BA 030 200 100 OHTEC BA 045 450 100 RMS Noise V 0 25 50 75 100 Bias Voitage Fig 4 13 Noise as a Function of Bias Voltage The detector noise tends to increase with bias voltage but the detector capacitance decreases thus reducing the resolution spread The overall resolution spread will depend upon which effect is dominant Figure 4 13 shows curves of typical noise resolution spread versus bias volta
30. g ratemeter 4 12 OTHER EXPERIMENTS Block diagrams illustrating how the 672 and other ORTEC modules can be used for experimental setups for various other applications are shown in Figs 4 21 through 4 24 To Counting Ratemeter SINGLE or Counter and Timer CHANNEL ANALYZER PROM PORTIONAL COUNTER PREAMPLIFIER IM ULTICHANNE t PULSE HEIGHT ANALYZER LINEAR AMPLIFIER PROPORTIONAL COUNTER PULSE GENERATOR Fig 4 20 High Resolution X Ray Energy Analysis System Using a Proportional Counter Energy Gated HIGH VOLTAGE POWER SUPPLY LINEAR GATE PREANP LINEAR on AMPLIFIER GATED BIASED AMPLIFIER Unipolar or GI DETECTOR MULTI SCA FOR ENERGY CHANNEL and or PULSE HEIGHT TIME PICKOFF ANALYZER 24 B Time Gated HIGH VOLTAGE POWER SUPPLY LINEAR GATE LINEAR PREAMP AMPLIFIER BIASED AMPLIFIER Unipolar or GI DETECTOR MULTI CHANNEL PULSE HEIGHT ANALYZER SCA FOR ENERGY andlor TIME PICKOFF Other Gating Signals Fig 4 21 General System Arrangement for Gating Control 25 LINEAR AMPLIFIER PREAMP TIMING SCA FAST GATED COINCIDENCE BIASED SILICON SURFACE UNIT AMPLIFIER BARRIER DETECTOR DISCRIMI NATOR ATE amp DELAY GENERATOR GERMANIUM
31. ge using data from several ORTEC silicon surface barrier semiconductor radiation detectors AMPLIFIER NOISE RESOLUTION MEASUREMENTS USING MCA Probably the most convenient method of making resolution measurements is with a pulse height analyzer as shown by the setup illustrated in Fig 4 14 The amplifier noise resolution spread can be measured directly with a pulse height analyzer and the mercury pulser as follows a Select the energy of interest with an ORTEC 419 Precision Pulse Generator Set the amplifier and biased amplifier gain and bias level controls so that the energy is in a convenient channel of the analyzer b Calibrate the analyzer in keV per channel using the pulser full scale on the pulser dial is 10 MeV when calibrated as described above c Obtain the amplifier noise resolution spread by measuring the FWHM of the pulser peak in the spectrum 21 BIASED AMPLIFIER LINEAR AMPLIFIER L DETECTOR oR CAPACITOR GATED BIAS AMPLIFIER 1 PULSE STRETCHER PULSE GENERATOR MULTICHANNEL PULSE HEIGHT ANALYZER Fig 4 14 System for Measuring Resolution with a Pulse Height Analyzer The detector noise resolution spread for a given detector bias can be determined in the same manner by connecting a detector to the preamplifier input The amplifier noise resolution spread must be subtracted as described
32. ha peak see Section 4 10 Calibration of Test Pulser or two known energy alpha peaks d Calculate the resolution by measuring the number of channels at the FWHM level in the peak and converting this to keV MULTI CHANNEL PULSE HEIGHT ANALYZER OSCILLO SCOPE Fig 4 17 System for High Resolution Alpha Particle Spectroscopy HIGH RESOLUTION GAMMA RAY SPECTROSCOPY SYSTEM high resolution spectroscopy system block diagram is shown in Fig 4 18 Although a biased amplifier is not shown an analyzer with more channels being preferred it can be used if the only analyzer available has fewer channels and only higher energies are of interest When germanium detectors that are cooled by a liquid nitrogen cryostat are used it is possible to obtain resolutions from about 1 keV FWHM up to 4 keV depending on the energy of the incident radiation and the size and quality of the detector Reasonable care is required to obtain such results Some guidelines for obtaining optimum resolution are a Keep interconnection capacities between the detector and preamplifier to an absolute minimum no long cables b Keep humidity low near the detector preamplifier junction Operate the amplifier with the shaping time that provides the best signal to noise ratio d Operate at the highest allowable detector bias to keep the input capacity low HIGH VOLTAGE BIAS SUPPLY
33. ier s baseline restorer Preamplifier inhibit signals are not required for proper amplifier operation however since the preamplifier resetting process is nonlinear by nature spurious phantom peaks may show up in the spectra if the inhibit signal from the preamplifier is not used INHIBIT IN CONNECTION Connection of the PREAMPLIFIER INHIBIT OUT signal to the rear panel INHIBIT IN connector will result in the system being disabled during the reset period and thus avoid spurious peaks in the spectra Preamplifiers with an Inhibit time switch such as ORTEC PLUS Detector with series 132 Preamplifier can be set to position 1 which is the shortest preamp inhibit blocking time PZ SETTING The Amplifier s Manual PZ control should be set fully counterclockwise CCW when used with a pulsed reset preamplifier The front panel control is marked with an arrow labeled TRP to indicate the direction to turn the control The Auto PZ should not be used with Pulsed Reset Preamplifiers 3 4 CONNECTION OF TEST PULSE GENERATOR THROUGH A PREAMPLIFIER The satisfactory connection of a test pulse generator such as the 419 or 448 Pulse Generator or equivalent depends primarily on two considerations the preamplifier must be properly connected to the 672 as discussed in Sections 3 2 and 3 3 and the proper signal simulation must be applied to the preamplifier To ensure proper input signal simulation refer to the instruction manual for the
34. ium count rates For very high count rates when the count rate times the dead time per pulse T is greater than 0 8 resolution may sometimes be improved with some detector preamp combinations by manually adjusting the PZ The dead time for the 672 is about nine times the front panel shaping time Memory of the last PZ adjustment is protected against unforeseen power outages a Using the system detector and a radioactive source as an input adjust the source distance for an input count rate of between 1 and 10 kHz b Select Auto PZ depress the push button and wait a few seconds The red LED will go on and then turn off when the unipolar output pulse has been pole zero compensated No oscilloscope is required for this procedure However the quality of the PZ can be observed with an oscilloscope by increasing the scope input 11 sensitivity to 10 or 20 mV cm with the LIMIT button pushed The LIMIT push button switch inserts a diode limiter in series with the front panel Unipolar Output connector to prevent overloading the input of the oscilloscope NOTE The Auto PZ must be reset after power down MANUAL POLE ZERO MODE a Adjust the radiation source spacing from the detector to provide a count rate between 1 and 10 kHz b Observe the unipolar output with an oscilloscope Increase the scope input sensitivity to 20 100 mV per vertical division Depress the front panel LIMIT push button to limit the voltage applied to
35. multichannel analyzer 2 4 OUTPUTS UNIPOLAR UNI Front and rear panel BNC connectors provide positive unipolar shaped pulses with a linear output range of 0 to 10 V Front panel output impedance 1 Rear panel output impedance selectable for either 1 or 93 O using a printed circuit board jumper Outputs are dc restored to 0 5 mV and short circuit protected BIPOLAR Bi Front and rear panel BNC connectors provide bipolar shaped pulses with the positive lobe leading The linear output range is O to 10 V Front panel output impedance 1 Rear panel output impedance selectable for either 1 O or 93 O using a printed circuit board jumper Baseline between pulses has a dc level of 0 10 mV Short circuit protected CRM The Count Rate Meter output has a rear panel BNC connector and provides a 250 ns wide 5 logic signal for every linear input pulse that exceeds the pile up inspector threshold Output impedance is 50 BUSY Rear panel BNC connector provides a 5 logic pulse for the duration that the linear signals exceed the positive or negative baseline restorer thresholds or the pile up inspector threshold or for the duration of the INHIBIT input signal Useful for deadtime corrections with an associated ADC or multichannel analyzer Positive NIM standard logic pulse is selectable as active high or active low via a printed circuit board jumper Output impedance is 50 PUR Pile Up Reject output is
36. n typically shifts lt 0 01 8 for counting rates up to 100 000 counts s and lt 0 05 for counting rates up to 200 000 counts s Measured on the 1 33 MeV line under conditions specified for SPECTRUM BROADENING DIFFERENTIAL INPUT Differential nonlinearity 10 012956 from 9 V to 9 V Maximum input 10 V dc plus signal Common mode rejection ratio gt 1000 PULSE PILE UP REJECTOR Threshold Automatically set just above noise level on fast amplifier signal Independent of slow amplifier BLR threshold Minimum Detectable Signal Limited by detector and preamplifier noise characteristics Pulse Pair Resolution Typically 500 ns Measured using the 9CO 1 33 MeV gamma ray under the following conditions 1096 efficiency germanium detector 4 V amplitude for the 1 33 MeV gamma ray at the unipolar output 50 000 counts s TResults may not be reproducible if measured with a detector producing a large number of slow risetime pulses or having quality inferior to the specified detector 2 2 CONTROLS AND INDICATORS FINE GAIN Front panel 10 turn precision potentio meter with locking graduated dial provides continuously variable direct reading gain factor from 0 5 to 1 5 COARSE GAIN Front panel eight position switch selects gain factors of 5 10 20 100 200 500 and 1000 SHAPING TIME Six position switch on the front panel selects shaping times of 0 5 1 2 3 6 and 10 Us for the pulse shaping filter network UNI SHAPING
37. nally jumper selectable as active low or active high It is shipped as active high 3 9 INPUT COUNT RATE USING CRM OUTPUT A positive logic pulse is generated for each 672 input pulse that exceeds the pile up inspector threshold level The pulses are available through the CRM Count Rate Meter output on the rear panel and are intended for use in a count rate meter or counter to monitor the true input count rate into the amplifier 4 OPERATING INSTRUCTIONS 4 1 INITIAL TESTING AND OBSERVATION OF PULSE WAVEFORMS Refer to Section 6 for information on testing performance and observing waveforms using a pulser Figure 4 1 shows some typical unipolar Gaussian unipolar triangular and bipolar output waveforms L emi Shaping Time esa LAT NS MN 222 Lese L 1 ping Time DECERN LES 2 m Fig 4 1 Typical Effects of Shaping Time Selection on Gaussian Triangular and Bipolar Output Waveforms 4 2 STANDARD SETUP PROCEDURES a Connect the detector preamplifier high voltage power supply and amplifier into a basic system and connect the amplifier unipolar output to an oscilloscope Connect the preamplifier power cable to the Preamp power connector on the rear panel of the 672 Turn on power in the bin and power supply and allow the electronics of the system to warm up and
38. needed when changes are made in the gain the shaping time constant or the detector characteristics Negative overload recovery from the reset pulses generated by transistor reset preamplifiers and pulsed optical feedback preamplifiers is also handled automatically to eliminate the need for operator adjustments A monitor circuit gates off the baseline restorer and provides a reject signal for a multichannel analyzer until the baseline has safely recovered from the overload BLR RATE For making either a manual or automatic pole zero adjustment the PZ position is selected This position can also be used where the slowest baseline restorer rate is desired 13 Fig 4 4 Pole Zero Adjustment Using a Square Wave Input to the Preamplifier a PZ properly adjusted slow trigger to separate pulses b Overcompensated fast trigger to superimpose pulses c Properly adjusted pulses superimposed d Undercompensated pulses superimposed With the BLR Rate set to AUTO the BLR is automatically set for optimum performance throughout the usable input range for the shaping selected The HIGH rate can be used for situations where low or medium frequency noise interference is present and is independent of the counting rate The HIGH rate setting is normally not used since there will be a small loss of resolution due to increased noise when used in high resolution systems 4 5 INTERNAL CONTROLS These controls are on the printed wiring boa
39. or the AUTO PZ BUSY LED to turn off No oscilloscope is required for this procedure The AUTO PZ memory is protected against unforeseen power failures In extreme situations where the preamplifier pulse shape is deformed from the normal exponential decay complete PZ cancellation is not possible In such cases a slight improvement in the high counting rate performance can sometimes be achieved using the manual PZ adjustment mode to arrive at a compromise solution A front panel switch on the Model 672 provides the choice of either a triangular or a Gaussian pulse shape on the UNIPOLAR output connector The noise performance of the triangular pulse shape is equivalent to a Gaussian pulse shape having a 1796 longer shaping time constant In applications where the series noise component is dominant short shaping time constants and the pile up rejector is utilized the triangular shape will generally offer the same deadtime and slightly lower noise than the Gaussian pulse shape A front panel switch permits selection of the optimum shaping time constant for each detector and application Six time constants in the range of 0 5 to 10 Us and the TRIANGULAR GAUSSIAN switch combine to offer 12 different shaping times A bipolar output is also provided for measurements requiring zero cross over timing To minimize spectrum distortion at medium and high counting rates Fig 2 1 the unipolar output incorporates a high performance gated b
40. ount rates is clearly shown in Fig 4 7 However shorter time constants also result in increased noise and increased charge collection time effects Under worst case conditions the noise increases inversely as the square root of the ratio of shaping time constants The increase in the total energy resolution is the noise contribution combined quadrature with the statistical contribution of the detector at the energy of interest Consequently the percentage of degradation in energy resolution can be much less than the percentage increase in noise 4 8 CHARGE COLLECTION OR BALLISTIC DEFICIT EFFECTS Charge collection distances in large volume HPGe detectors are often 3 cm or more resulting in 17 charge collection times exceeding 300 ns These charge collection times are due to the transit time of the holes and the electrons in germanium and are not due to defects in the detector Fig 4 8 a shows some typical current pulse waveforms from a 140 cm 28 efficient HPGe detector These current pulse waveforms were obtained using the simple differentiation circuit shown in Fig 4 8 b which has a 15 ns time constant The current pulses range in duration from 100 ns to greater than 350 ns Pulses having equivalent total charge but different durations produce different output pulse heights when processed by a charge sensitive preamplifier and a semi Gaussian filter amplifier This results in the distortion of the spectrum in direc
41. output signal If there is a problem in getting minimum noise repeat the initial procedure with the BNC T and the adjustment DIFF INPUT SIGNAL DIFF input signal or phantom is used only in the differential input mode The normal preamp output is connected to the NORM input with the amplifier input polarity set to match this signal A second output cable must be added to the preamplifier with its center signal pin connected to the preamplifier ground with the same value as the normal preamp output series resistor usually 93 1 or 51 Many preamplifiers have two Energy outputs each with 93 1 series resistor For differential operation one output is connected to the amplifier NORM input The second output is modified by connecting the preamplifier end of the series 93 1 resistor to ground within the preamp soldering may be necessary This second output should be properly marked and connected to the DIFF REF input Both cables should be the same length and be run next to each other 4 7 SYSTEM THROUGHPUT To achieve the desired results in high rate energy spectroscopy the experimenter must consider not only the input rate but also the unpiled up output rate The unpiled up output rate is determined by the processing time of the shaping amplifier the pile up inspection time and the input rate For semi Gaussian time invariant filter amplifiers the unpiled up output rate is theoretically given by
42. owing procedure a Remove all radioactive sources from the vicinity of the detector Set up the system as for normal operation including detector bias b Set the amplifier controls as for normal operations this includes gain shaping and input polarity 12 c Connect the source of 1 kHz square waves through an attenuator to the Test input of the preamplifier Adjust the attenuator so that the amplifier output amplitude is 8 to 10 volts d Observe the unipolar output of the amplifier with an oscilloscope triggered from the amplifier Busy output Adjust the PZ control for proper response according to Fig 4 4 Depress the LIMIT push button on the 672 while observing the adjustment on the oscilloscope display Figure 4 4A shows the amplifier output as a series of alternate positive and negative shaped pulses In B C and D of this figure the oscilloscope was triggered to show both positive and negative pulses simultaneously These pictures show more detail to aid in proper adjustment 4 4 BASELINE RESTORER BLR SETTING To minimize spectrum distortion at medium and high counting rates the unipolar output incorporates a high performance gated baseline restorer with several levels of automation Automatic positive and negative noise discriminators ensure that the baseline restorer operates only on the true baseline between pulses in spite of changes in the noise level No operator adjustment of the baseline restorer is
43. performance of two HPGe detectors is compared in Fig 4 10 at two different energies When using the 122 keV line of CO the principal cause of resolution degradation with decreased shaping time constant is the increase in noise However when using the 1 33 MeV line of C0 the significant degradation in resolution is due to charge collection effects The calculated resolution for the 1096 detector at 1 33 MeV is shown as the dashed line in Fig 4 10 and indicates approximately 2 0 keV FWHM at a 0 5 Us shaping time constant The measured resolution under these test conditons was 7 2 keV indicating that charge collection effects dominate In Fig 4 10 charge collection effects begin to appear at time constants less than 3 Us 18 Det P 6658 2 us w 572 2K counts s 60 1 33 MeV 181 eV ch Ch 3360 Det 665 _ 0 5 us w 572 2K counts s 60Co 1 33 MeV 181 eV ch Fig 4 9 Charge Collection Effect Spectrum Logarithmic Display of Spectrum Taken with a 10 Efficient HPGe Detector for the 1 33 MeV Line 2 Us Shaping Time Constant and b 0 5 us Shaping Time Constant 4 9 PILE UP REJECTOR PUR AND LIVETIME CORRECTOR An efficient pile up rejector is incorporated in the amplifier to suppress the spectral distortion which is caused by pulses piling up on each other at high counting rates High counting rate for pile up is dependent on the dead time per pulse Tp
44. put can be used to reduce overload effects and microphonics without sacrificing resolution GAS PROPORTIONAL COUNTERS Proportional counters have both short and long components in their charge collection times The components typically fall in the 0 5 to 5 Us range and lead to variable amounts of preamplifier output signal being lost as the amplifier shaping time constant is changed Selection of longer shaping times gt 2 Us helps to minimize the problem caused by long risetimes Due to the multiple components in the charge collection time the correct pole zero cancellation is not possible This will often cause an undershoot if the Unipolar output issued Bipolar shaping can be used to reduce this effect with little change in the resolution 3 6 LINEAR OUTPUT CONNECTIONS Since the 672 Unipolar output is normally used for spectroscopy the 672 is designed with a great amount of flexibility for the pulse to be interfaced with an analyzer To minimize spectrum distortion at medium and high counting rates the unipolar output incorporates a high performance gated baseline restorer with automatic setup Automatic positive and negative noise discriminators ensure that the baseline restorer operates only on the true baseline between pulses in spite of changes in the noise level For pulse height analysis the Unipolar output must be directly connected to the input of a multichannel analyzer The bipolar output with its symmetry about th
45. rd PWB and can be accessed by removing the right side cover Figure 4 5 shows the location of these controls BAL DIFFERENTIAL INPUT GAIN BALANCE Internal PWB 20 turn screwdriver potentiometer allows maximization of noise rejection when using differential input See Section 4 6 UNI OUT UNIPOLAR Zu Jumper plug W1 provides Zour lt 1 or 93 for the rear panel Unipolar output Shipped in the 93 position BI OUT BIPOLAR Zu Jumper plug W2 provides Zour lt 1 or 93 for the rear panel Bipolar output Shipped in the 93 Q position BUSY BUSY Jumper plug W3 allows the Busy output to be a positive true or negative true logic signal Shipped in BUSY positive true position PUR PUR Jumper plug W5 allows the Pile Up Reject PUR output to be a positive true or negative true logic signal Shipped in PUR positive true position INH INH Jumper plug W6 allows the inhibit in input to accept either positive true or negative true logic signals Shipped in INH positive true position TRI GAUSS Jumper plug W7 allows optimal livetime correction when used with ORTEC analyzers like the ADCAM by connecting the BUSY output to the analyzer Busy In as described in Section 3 8 The jumper should be set to match the Unipolar Mode TRI for Triangle and GAUSS for Gaussian Shipped in TRI position 4 6 DIFFERENTIAL INPUT MODE When long connecting cables are used between the detector and preamplifier input noi
46. red For non extending deadtime the output rate is given by ri fo 1 rTo 2 where Tj is the digitizing time for the ADC and is designated in Equation 3 This relationship is shown as the dashed line in Fig 4 6 The maximum obtainable output count rate is 1 T and occurs at When the ADC is considered as part of the spectroscopy system the deadtimes of the amplifier and ADC are in series The combination of the extending deadtime of the amplifier followed by the non extending deadtime of the ADC is given by fi 3 NORMALIZED OUTPUT RATE NORMALIZED INPUT RATE 81288 Fig 4 6 Plot of Normalized Output Rate as a Function of Normalized Input Rate for Spectrometers with Simple Deadtime where U Ty Tw Tp is a unit step function that changes value from 0 to 1 when is greater than Tw Tp Equation 3 reduces to Equation 1 when is less than Tw Tp A plot of the unpiled up ampilifier output rate as a function of input rate for six values of shaping time is shown in Fig 4 7 The measured deadtime Tp is shown for each shaping time constant The maximum value of the unpiled up output rate increases with decreasing values of shaping time constant A set of throughput curves will remain nearly unchanged for a given amplifier for various energy ranges detector types and sizes The advantage of shorter shaping time constants to achieve higher output c
47. ruments not in warranty should follow the same procedure and ORTEC will provide a quotation Damage in Transit Shipments should be examined immediately upon receipt for evidence of external or concealed damage The carrier making delivery should be notified immediately of any such damage since the carrier is normally liable for damage in shipment Packing materials waybills and other such documentation should be preserved in order to establish claims After such notification to the carrier please notify ORTEC of the circumstances so that assistance can be provided in making damage claims and in providing replacement equipment if necessary Copyright 2002 Advanced Measurement Technology Inc rights reserved is a registered trademark of Advanced Measurement Technology Inc All other trademarks used herein are the property of their respective owners CONTENTS WARRANT Y EEG eR ii SAFETY INSTRUCTIONS AND SYMBOLS iv SAFETY WARNINGS AND CLEANING INSTRUCTIONS T DESCRIPTION dte eara seltene See e eae RR NE PENSA Eee 1 Ad cce eee ehe t at 1 2 SPECIFICATIONS 3 2 1 PERFORMANCE npe tenes ed esed tede dete
48. se induced in the 14 cable by the environment can be a problem The differential input mode can be used with paired cables from the preamplifier to suppress the induced noise BAL DIFFERENTIAL INPUT GAIN BALANCE The BAL potentiometer is used to adjust the gain balance between the positive and negative inputs and to adjust the balance between the NORM and DIFF REF inputs when the differential input mode is used The initial adjustment of Gain Balance is made by providing the same input to both the NORM and DIFF REF inputs This can be accomplished by using a BNC T connector to feed the input signal on the NORM input to the DIFF REF input Set the amplifier gain to maximum Connect an oscilloscope to the unipolar output While observing the signal on the oscilloscope used a small screwdriver to adjust the Gain Balance internal adjustment has been factory set Fig 4 5 potentiometer until the display on the oscilloscope shows minimum signal Remove the BNC T connector when the adjustment is complete and the positive and negative gains will be matched for use with NORM input RS um a Ses 13252 2 T sua Zor ATO 8 dz gt Fig 4 5 Position of Internal Controls If the differential input mode is being used connect the differential input cable to the DIFF BNC connector on the front panel Adjust BAL potentiometer until there is minimum noise around the baseline of the
49. seen power outage MANUAL PZ ADJUSTMENT 20 turn potentiometer on the front panel permits screwdriver adjustment of the PZ cancellation The screwdriver adjusted value is effective whenever the PZ switch is in the MAN manual position For transistor reset preamplifiers or pulsed optical feedback preamplifiers use manual PZ adjustment set fully counterclockwise LIMIT PUSHBUTTON Inserts a diode limiter in series with the front panel UNIPOLAR output connector and test point Prevents overload distortions in the oscilloscope when observing accuracy of the PZ adjustment on the more sensitive oscilloscope ranges BLR RATE A front panel three position locking toggle switch selects the baseline restorer rate PZ position offers lowest fixed rate for adjusting PZ cancellation AUTO position matches the rate of the PZ position at low counting rates but increases the restoration rate as the counting rate rises HIGH rate position is provided for suppressing low frequency interference PUR ACCEPT REJECT LED Multicolor LED indicates percentage of pulses rejected because of pulse pile up LED appears green for 0 40 yellow for 40 70 and red for gt 70 rejection 2 3 INPUTS NORM Front panel BNC connector accepts preamplifier signals of either polarity with risetimes less than the selected SHAPING TIME and exponential decay time constants from 40 Us to For the INPUT switch setting the input impedance is 1000 on a coarse g
50. t observed WARNING Indicates a hazard that could result in bodily harm if the safety instruction is not observed CAUTION Indicates a hazard that could result in property damage if the safety instruction is not observed Please read all safety instructions carefully and make sure you understand them fully before attempting to use this product In addition the following symbol may appear on the product ATTENTION Refer to Manual DANGER High Voltage Please read all safety instructions carefully and make sure you understand them fully before attempting to use this product SAFETY WARNINGS AND CLEANING INSTRUCTIONS DANGER Opening the cover of this instrument is likely to expose dangerous voltages Disconnect the instrument from all voltage sources while it is being opened WARNING Using this instrument in a manner not specified by the manufacturer may impair the protection provided by the instrument Cleaning Instructions To clean the instrument exterior Unplug the instrument from the ac power supply Remove loose dust on the outside of the instrument with a lint free cloth Remove remaining dirt with a lint free cloth dampened in a general purpose detergent and water solution Do not use abrasive cleaners CAUTION To prevent moisture inside of the instrument during external cleaning use only enough liquid to dampen the cloth or applicator Allow the instrument to dry completely
51. t TRP MAR INH 7 Inhibit analyzer to suppress pile up in the recorded spectrum The block diagram for a gamma ray spectroscopy system with pile up rejection and live time correction is shown in Fig 4 11 FOR A RESISTIVE FEEDBACK PREAMP CONNECT a Inhibit pulse from PUR to ADC PUR or ADC anticoincidence input b Livetime correction signal Busy output to the ADC Busy In ADDITIONAL CONNECTION FOR TRP Transistor Reset Preamplifiers Shown in dotted lines c Inhibit Output from TRP to the amplifier inhibit In PUR AMPLIFIER Dotted connections only for Preamp ORTEC Plus Detectors COUNTER TIMER Fig 4 11 Block Diagram for a Gamma Ray Spectroscopy System with Pile Up Rejection and Livetime Correction 4 10 OPERATION WITH SEMICONDUCTOR DETECTORS CALIBRATION OF TEST PULSER An ORTEC 419 Precision Pulse Generator or equivalent is easily calibrated so that the maximum pulse height dial reading 1000 divisions is equivalent to a 10 MeV loss in a silicon radiation detector The procedure is as follows a Connect the detector to be used to the spectrometer system that is preamplifier main amplifier and biased amplifier b Allow excitation from a source of known energy for example alpha particles to fall on the detector c Adjust the amplifier gain and the bias level of the biased amplifier to give a suitable output pulse d Set the pulser Pulse Height control
52. t proportion to the pulse amplitude or energy This distortion is most pronounced at short shaping time constants Figure 4 9 a shows a portion of a spectrum obtained with a 1096 efficient HPGe detector at 2 Us shaping time using the 1 33 MeV line of An equivalent spectrum using a 0 5 Us shaping time is shown in Fig 4 9 b and is significantly distorted Output Count Rate R in Events s To 7 3r rz6 s Tp7 7 8r 10K 100K B 1000K Fig 4 7 Plot of the Unpiled Up Amplifier Output Rate as a Function of Input Rate for Six Values of Shaping Time Constants Sakai Charge Collection in Coaxial Ge Li Detectors IEEE Trans Nuct Sci NS 1 5 310 1968 E Sakai T A McMath and R G Franks Further Charge Collection Studies in Coaxial Ge Li Detectors EEE Trans Nucl Sci NS 16 68 1968 Becker Gross Trammell Characteristics of High Rate Energy Spectroscopy Systems with Time invariant Filters Tran amp Nucl Sci NS 28 1 1981 i Horiz 50 ns div Vert 5 mV div UN LII MANS XML LA NA To Scope b Fig 4 8 Charge Collection Effect Waveforms a Typical current Pulse Waveforrns for a 28 Efficient HPGe Detector and b the Simple Differentiation Circuit Used to Obtain the Current Waveforms Charge collection time effects are of significant importance when using large volume Ge detectors at high energy The
53. the oscilloscope Adjust the PZ adjust control so that the trailing edge of the pulses returns to the baseline without overshoot or undershoot Fig 4 3 A slight bias toward an undershoot often gives the best results The oscilloscope used must be dc coupled and must not contribute distortion in the observed waveforms Oscilloscopes such as Tektronix models 465 475 and 7904 will overload for a 10 V signal when the vertical sensitivity is 100 mV Div The LIMIT push button switch inserts a diode limiter in series with the front panel Unipolar Output connector to prevent overloading the input of the oscilloscope USING SQUARE WAVE THROUGH PREAMPLIFIER TEST INPUT A more precise manual pole zero adjustment of the amplifier can be obtained by using a squarewave signal as the input to the preamplifier Many oscilloscopes include a calibration output on the front panel and this is a good source of square wave signals at a frequency of about 1 kHz The amplifier differentiates the signal from the preamplifier so that it generates output signals of alternate polarities on the leading and trailing edges of the square wave input signal and these can be compared as shown in Fig 4 4 to achieve excellent pole zero cancellation Fig 4 3 Typical Waveforms Illustrating Pole Zero Adjustment Effects Oscilloscope Trigger Busy Output Co Source with 1 33 MeV Peak Adjusted 9 V Count Rate 3 kHz Shaping Time Constant 2 15 Use the foll
54. the ORTEC 4001 4002 series Convenient test points on the power supply control panel should be used to check that the dc voltage levels are not overloaded The bin and power supply is designed for relay rack mounting If the equipment is rack mounted be sure that there is adequate ventilation to prevent any localized heating of the components that are used in the 672 The temperature of the equipment mounted in racks can easily exceed the maximum limit of 50 unless precautions are taken 3 2 PREAMPLIFIER CONNECTION The Preamp connector of this amplifier is directly compatible with ORTEC preamplifiers as well as with standard Aptec Canberra PGT and Tennelec serial numbers greater than 2000 preamplifiers Preamplifier power at 24 V 24 V 12 V and 12 V is available through the Preamp connector on the rear panel When a BNC cable longer than ten feet is used to connect the preamplifier output to the amplifier input the characteristic impedance of the cable should match the impedance of the preamplifier output All ORTEC preamplifiers contain series terminations that are either 93 O or variable coaxial cable type RG 62 U or RG 71 U is recommended 3 3 PULSED RESET PREAMPLIFIERS AND INHIBIT IN CONNECTION The 672 Amplifier is directly compatible with most pulsed reset preamplifiers such as the ORTEC TRP Transistor Reset Preamplifier Series The amplifier automatically senses preamplifier resets and gates off the amplif
55. tion in the energy spectrum as the counting rate increases c After pushing the AUTO PZ button the undershoot is automatically removed resulting in d better resolution at high counting rates 2 SPECIFICATIONS 2 1 PERFORMANCE Note Unless otherwise stated performance speci fications are measured on the unipolar output with 2 Us Gaussian shaping the manual PZ mode and the AUTO BLR mode GAIN RANGE Continuously adjustable from 2 5 to 1500 Gain is the product of the COARSE and FINE GAIN controls UNIPOLAR PULSE SHAPES Switch selection of a nearly triangular pulse shape or a nearly Gaussian pulse shape at the UNIPOLAR output Table 2 1 BIPOLAR OUTPUT PULSE SHAPE Rise of the BIPOLAR output pulse from 0 1 to maximum amplitude is 1 65 times selected SHAPING TIME Zero cross over of the bipolar output pulse delayed from the maximum amplitude of Gaussian UNIPOLAR output by 0 33 times selected SHAPING TIME INTEGRAL NONLINEARITY UNIPOLAR Output lt 0 025 from 0 to 10 V NOISE Equivalent input noise lt 5 0 UV rms for gains gt 100 and lt 4 5 UV rms for gains gt 1000 in manual PZ mode or lt 6 0 UV for gains gt 100 in AUTO PZ mode TEMPERATURE COEFFICIENT 0 to 50 C UnipolarOutput lt 0 005 C for gain and lt 7 5 JJ V C for dc level Bipolar Output lt 0 007 C for gain and x30 J V C for dc level WALK Bipolar zero cross over walk is 3 ns over a 50 1 dynamic range OVERLOAD RECOVERY
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