Model 671Spectroscopy Amplifier

Operating and Service Manual

Printed in U.S.A. ORTEC® Part No. 736840 1202Manual Revision G

Advanced Measurement Technology, Inc.

a/k/a/ ORTEC®, a subsidiary of AMETEK®, Inc.

WARRANTYORTEC* warrants that the items will be delivered free from defects in material or workmanship. ORTEC makesno other warranties, express or implied, and specifically NO WARRANTY OF MERCHANTABILITY ORFITNESS FOR A PARTICULAR PURPOSE.

ORTEC’s exclusive liability is limited to repairing or replacing at ORTEC’s option, items found by ORTEC tobe defective in workmanship or materials within one year from the date of delivery. ORTEC’s liability on anyclaim of any kind, including negligence, loss, or damages arising out of, connected with, or from the performanceor breach thereof, or from the manufacture, sale, delivery, resale, repair, or use of any item or services coveredby this agreement or purchase order, shall in no case exceed the price allocable to the item or service furnishedor any part thereof that gives rise to the claim. In the event ORTEC fails to manufacture or deliver items calledfor in this agreement or purchase order, ORTEC’s exclusive liability and buyer’s exclusive remedy shall be releaseof the buyer from the obligation to pay the purchase price. In no event shall ORTEC be liable for special orconsequential damages.

Quality ControlBefore being approved for shipment, each ORTEC instrument must pass a stringent set of quality control testsdesigned to expose any flaws in materials or workmanship. Permanent records of these tests are maintained foruse in warranty repair and as a source of statistical information for design improvements.

Repair ServiceIf it becomes necessary to return this instrument for repair, it is essential that Customer Services be contacted inadvance of its return so that a Return Authorization Number can be assigned to the unit. Also, ORTEC must beinformed, either in writing, by telephone [(865) 482-4411] or by facsimile transmission [(865) 483-2133], of thenature 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 standardprocedure requires that instruments returned for repair pass the same quality control tests that are used fornew-production instruments. Instruments that are returned should be packed so that they will withstand normaltransit handling and must be shipped PREPAID via Air Parcel Post or United Parcel Service to the designatedORTEC repair center. The address label and the package should include the Return Authorization Numberassigned. Instruments being returned that are damaged in transit due to inadequate packing will be repaired at thesender's expense, and it will be the sender's responsibility to make claim with the shipper. Instruments not inwarranty should follow the same procedure and ORTEC will provide a quotation.

Damage in TransitShipments should be examined immediately upon receipt for evidence of external or concealed damage. The carriermaking delivery should be notified immediately of any such damage, since the carrier is normally liable for damagein shipment. Packing materials, waybills, and other such documentation should be preserved in order to establishclaims. After such notification to the carrier, please notify ORTEC of the circumstances so that assistance can beprovided in making damage claims and in providing replacement equipment, if necessary.

Copyright © 2002, Advanced Measurement Technology, Inc. All rights reserved.

*ORTEC® is a registered trademark of Advanced Measurement Technology, Inc. All other trademarks usedherein are the property of their respective owners.

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CONTENTS

WARRANTY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

SAFETY INSTRUCTIONS AND SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

SAFETY WARNINGS AND CLEANING INSTRUCTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

1. DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1. GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1. PERFORMANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2. CONTROLS AND INDICATORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3. INPUTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.4. OUTPUTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.5. ELECTRICAL AND MECHANICAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3. INSTALLATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.1. POWER CONNECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2. PREAMPLIFIER CONNECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3. PULSED RESET PREAMPLIFIERS AND INHIBIT IN CONNECTION . . . . . . . . . . . . . . . . . . . . 63.4. CONNECTION OF TEST PULSE GENERATOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.5. SHAPING CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.6. LINEAR OUTPUT CONNECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.7. PILE-UP REJECTION USING PUR OUTPUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.8. LIVETIME CORRECTION USING BUSY OUTPUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.9. INPUT COUNT RATE USING CRM OUTPUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4. OPERATING INSTRUCTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.1. INITIAL TESTING AND OBSERVATION OF PULSE WAVEFORMS . . . . . . . . . . . . . . . . . . . . 94.2. STANDARD SETUP PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.3. POLE-ZERO ADJUSTMENTS FOR RESISTIVE-FEEDBACK PREAMPLIFIER . . . . . . . . . . . 104.4. BASELINE RESTORER (BLR) SETTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.5. INTERNAL CONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.6. DIFFERENTIAL INPUT MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.7. SYSTEM THROUGHPUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.8. CHARGE COLLECTION OR BALLISTIC DEFICIT EFFECTS . . . . . . . . . . . . . . . . . . . . . . . . 154.9. PILE-UP REJECTOR (PUR) AND LIVETIME CORRECTOR . . . . . . . . . . . . . . . . . . . . . . . . . 164.10. OPERATION WITH SEMICONDUCTOR DETECTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.11. OPERATION IN SPECTROSCOPY SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.12. OTHER EXPERIMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5. MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.1. TEST EQUIPMENT REQUIRED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.2. PULSER TEST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.3. SUGGESTIONS FOR TROUBLESHOOTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.4. FACTORY REPAIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.5. TABULATED TEST POINT VOLTAGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

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SAFETY INSTRUCTIONS AND SYMBOLS

This manual contains up to three levels of safety instructions that must be observed in order to avoidpersonal 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 notobserved.

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 notobserved.

Please read all safety instructions carefully and make sure you understand them fully before attempting touse 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 touse this product.

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DANGER Opening the cover of this instrument is likely to expose dangerous voltages. Disconnect theinstrument from all voltage sources while it is being opened.

WARNING Using this instrument in a manner not specified by the manufacturer may impair theprotection provided by the instrument.

CAUTION To prevent moisture inside of the instrument during external cleaning, use only enough liquidto dampen the cloth or applicator.

SAFETY WARNINGS AND CLEANING INSTRUCTIONS

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.

! Allow the instrument to dry completely before reconnecting it to the power source.

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Fig. 1.1. Gaussian Triangular, and Bipolar Pulse Shapesfor a 2-:s Shaping Time. Vertical scale, 5 V per

division; horizontal scale, 2 :s per division.

Fig. 1.2. (a) Resolution and (b) Peak Position Stability as a Function of Counting Rate. See specifications for spectrum broadening and spectrum shift.

ORTEC MODEL 671SPECTROSCOPY AMPLIFIER

1. DESCRIPTION

1.1. GENERAL

The ORTEC Model 671 high-performance, energyspectroscopy amplifier is ideally suited for use withgermanium, silicon surface-barrier, and Si(Li)detectors. It can also be used with scintillationdetectors and proportional counters. The Model 671input accepts either positive or negative polaritysignals from a detector preamplifier and provides apositive 0 to 10-V output signal suitable for use withsingle- or multichannel pulse height analyzers. Itsgain is continuously variable from 2.5 to 1500.

Automation of critical adjustments makes the 671easy to set up with any detector, while minimizingthe required operator expertise.

A front-panel switch on the Model 671 provides thechoice of either a triangular or a Gaussian pulseshape on the UNIPOLAR output connector.(Fig. 1.1) The noise performance of the triangularpulse shape is equivalent to a Gaussian pulseshape having a 17% longer shaping time constant.In applications where the series noise component isdominant, and the pile-up rejector is utilized, thetriangular shape will generally offer the samedeadtime and slightly lower noise than theGaussian pulse shape. A front-panel switch permitsselection of the optimum shaping time constant foreach detector and application. Six time constants in

the range of 0.5 to 10 :s, and the TRI/GAUSSswitch combine to offer 12 different shaping times.A bipolar output is also provided for measurementsrequiring zero cross-over timing.

To minimize spectrum distortion at medium andhigh counting rates (Fig. 1.2), the unipolar outputincorporates a high-performance, gated, baselinerestorer with several levels of automation.Automatic positive and negative noisediscriminators ensure that the baseline restorer

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Fig. 1.3. Demonstration of the Effectiveness of thePile-Up Rejector in Suppressing the Pile-up Spectrum.

See Pulse Pile-Up Rejector specifications.

operates only on the true baseline between pulsesin spite of changes in the noise level. No operatoradjustment of the baseline restorer is needed whenchanges are made in the gain, the shaping timeconstant, or the detector characteristics. Negativeoverload recovery from the reset pulses generatedby transistor reset preamplifiers and pulsed opticalfeedback preamplif iers is also handledautomatically. A monitor circuit gates off thebaseline restorer and provides a reject signal for amultichannel analyzer until the baseline has safelyrecovered from the overload.

Several operating modes are selectable for thebaseline restorer. For making either a manual orautomatic PZ adjustment, the PZ position isselected. This position can also be used where theslowest baseline restorer rate is desired. Forsituations where low frequency noise interference isa problem, the HIGH rate can be chosen. Ondetectors where perfect PZ cancellation isimpossible, the AUTO baseline restorer rateprovides the optimum performance at both low andhigh counting rates.

A front-panel limit (LIM) push button is included withthe unipolar output to facilitate monitoring theaccuracy of the PZ adjustment on an oscilloscope.When pressed, this button inserts a diode limiter inseries with the unipolar output connector. Thisprevents overload distortions in the oscilloscopewhen using the more sensitive amplitude scalesrequired for observing the PZ adjustment.

An efficient pile-up rejector is incorporated in the671 Spectroscopy Amplifier. It provides an outputlogic pulse for the associated multichannel analyzerto suppress the spectral distortion caused by pulsespiling up on each other at high counting rates(Fig. 1.3). The fast amplifier in the pile-up rejectorincludes a gated baseline restorer with its own

automatic noise discriminator. A multicolor pile-uprejector LED on the front panel indicates thethroughput efficiency of the amplifier. At lowcounting rates the LED flashes green. The LEDturns yellow at moderate counting rates and redwhen pulse pile-up losses are >70%.

When long connecting cables are used between thedetector preamplifier output and the amplifier input,noise induced in the cable by the environment canbe a problem. The Model 671 provides twosolutions. For low to moderate interferencefrequencies the differential input mode can be usedwith paired cables from the preamplifier to suppressthe induced noise. At high frequencies a commonmode rejection transformer built into the 671 inputreduces noise pick-up. The transformer isparticularly effective in eliminating interferencefrom the display raster generators in personalcomputers.

All toggle switches on the front panel lock toprevent accidental changes in the desired settings.

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1 Specifications subject to change without notice.† Results 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. SPECIFICATIONS1

2.1. PERFORMANCE

Note: Unless otherwise stated, performancespecifications are measured on the unipolar outputwith 2-:s Gaussian shaping, the manual PZ mode,and the AUTO BLR mode.

GAIN RANGE Continuously adjustable from 2.5 to1500. Gain is the product of the COARSE and FINEGAIN controls.

UNIPOLAR PULSE SHAPES Switch selection ofa nearly triangular pulse shape or a nearlyGaussian pulse shape at the UNI output (Fig. 1.1,Table 2.1).

BIPOLAR OUTPUT PULSE SHAPE Rise of thebipolar output pulse from 0.1% to maximumamplitude is 1.65 times selected SHAPING TIME.Zero cross-over of the bipolar output pulse delayedfrom the maximum amplitude of GaussianUNIPOLAR output by 0.33 times selectedSHAPING TIME.

INTEGRAL NONLINEARITY (UNIPOLAR Output)<±0.025% from 0 to +10 V.

NOISE Equivalent input noise <5.0 :V rms forgains >100, and <4.5 :V rms for gains >1000.

TEMPERATURE COEFFICIENT (0 to 50°C)

UnipolarOutput <±0.005%/°C for gain, and <±7.5:V/°C for dc level. Bipolar Output <±0.007%/°C for gain, and <±30:V/°C for dc level.

WALK Bipolar zero cross-over walk is <±3 ns overa 50:1 dynamic range.

OVERLOAD RECOVERY Unipolar and bipolaroutputs recover to within 2% of the rated outputfrom a X 1000 overload in 2.5 non-overloadedpulse widths using maximum gain.

SPECTRUM BROADENING† (Fig. 1.2.) Typically<8% broadening of the FWHM for counting rates upto 100,000 counts per second (counts/s), and <15%broadening for counting rates up to 200,000counts/s. Measured on the 1.33-MeV gamma-rayline from a 60Co radioactive source under thefollowing conditions: 10% efficiency ORTECGAMMA-X PLUS detector, 8.5-V amplitude for the1.33-MeV gamma-ray on the unipolar output.

SPECTRUM SHIFT† (Fig. 1.2) Peak positiontypically shifts <±0.018% for counting rates up to100,000 counts/s, and <±0.05% for counting ratesup to 200,000 counts/s. Measured on the 1.33-MeVline under conditions specified for SPECTRUMBROADENING.

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DIFFERENTIAL INPUT Differential nonlinearity<±0.012% from -9 V to +9 V. Maximum input ±10 V(dc plus signal). Common mode rejection ratio>1000.

PULSE PILE-UP REJECTOR Threshold Automatically set just above noise levelon fast amplifier signal. Independent of slowamplifier BLR threshold. Minimum Detectable Signal Limited by detectorand preamplifier noise characteristics. Pulse Pair Resolution Typically 500 ns. Measuredusing the 60CO 1.33-MeV gamma-ray under thefollowing conditions: 10% efficiency germaniumdetector, 4-V amplitude for the 1.33-MeV gamma-ray at the unipolar output, 50,000 counts/s.

2.2. CONTROLS AND INDICATORS

FINE GAIN Front-panel, 10-turn precisionpotentiometer with locking, graduated dial providescontinuously variable, direct reading, gain factorfrom 0.5 to 1.5.

COARSE GAIN Front-panel, eight-position switchselects gain factors of 5, 10, 20, 100, 200, 500, and1000.

SHAPING TIME Six-position switch on the frontpanel selects shaping times of 0.5,1, 2, 3, 6, and 10:s for the pulse-shaping filter network.

MODE Two-position locking toggle switch on thefront panel selects either GAUSS (Gaussian) or TRI(Triangular) pulse shaping for the UNI (unipolar)output.

INPUT POS/NEG Front-panel, two-positionlocking toggle switch accommodates either positiveor negative input polarities.

NORM/DIFF Two-position slide switch mounted onthe printed circuit board selects the normal (NORM)or differential (DIFF) input modes. In the NORMposition, both front- and rear-panel INPUTconnectors function as the same normal input forthe preamplifier signal cable. In the DIFF mode,the rear-panel INPUT connector becomes adifferential ground reference input, and the front-panel INPUT remains the normal input for thepreamplifier signal cable. In the DIFF mode, thepreamplifier signal cable is connected to the front-

panel INPUT and a cable having its centerconductor connected to the preamplifier groundthrough an impedance matching resistor isconnected to the rear-panel INPUT. The impedancematching resistor must match the output impedanceof the preamplifier.

BAL (Differential Input Gain Balance) A 20-turnpotentiometer mounted on the PC board inside themodule allows the gains of normal and differentialreference inputs to be matched for maximumcommon mode noise rejection in DIFF mode.

PZ ADJUSTMENT 20-turn potentiometer on thefront panel permits screwdriver adjustment of thePZ cancellation. The adjustment coverspreamplifier exponential decay time constants from40 :s to 4. For transistor reset preamplifiers orpulsed optical feedback preamplifiers, set the PZadjustment fully counterclockwise.

LIMIT PUSHBUTTON Inserts a diode limiter inseries with the front-panel UNI output connector.Prevents overload distortions in the oscilloscopewhen observing accuracy of the PZ adjustment onthe more sensitive oscilloscope ranges.

BLR A front-panel, three-position, locking, toggleswitch selects the baseline restorer rate. PZ positionoffers lowest fixed rate, for adjusting PZcancellation. AUTO position matches the rate of thePZ position at low counting rates, but increases therestoration rate as the counting rate rises. HIGHrate position is provided for suppressing lowfrequency interference.

PUR ACCEPT/REJECT LED Multicolor LEDindicates percentage of pulses rejected because ofpulse pile-up. LED appears green for 0-40%, yellowfor 40-70%, and red for >70% rejection.

2.3. INPUTS

INPUT (Front Panel) Front-panel, BNC connectoraccepts preamplifier signals of either polarity withrisetimes less than the selected SHAPING TIME,and exponential decay time constants from 40 :s to4. For the NEG INPUT switch setting, the inputimpedance is 1000 S on a coarse gain of 5, and465 S at coarse gain settings $10. For the POSINPUT switch setting, the input impedance is2000 S for a coarse gain of 5, and 1460 S forcoarse gains $10. Input is dc-coupled, andprotected to ±25 V.

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INPUT (Rear Panel) BNC connector, identical tof ront-panel INPUT when PWB-mountedNORM/DIFF slide switch is in the NORM position.When operating in the differential input mode withthe slide switch set to DIFF, the rear-panel INPUTis used for the preamplifier ground referenceconnection. For the DIFF and POS INPUT switchsetting, the input impedance is 1000 S on a coarsegain of 5, and 465 S at coarse gain settings $10.For the DIFF and NEG INPUT switch setting, theinput impedance is 2000 S for a coarse gain of 5,and 1460 S for coarse gains $10. Input is dc-coupled, and protected to ±25 V.

INH IN Rear-panel BNC input connector acceptsreset signals from transistor reset preamplifiers orpulsed optical feedback preamplifiers. Positive NIMstandard logic pulses or TTL levels can be used.Logic is selectable as active high or active low viaprinted circuit board jumpers. Inhibit input initiatesthe protection against distortions caused by thepreamplifier reset. This includes turning off thebaseline restorers, monitoring the negativeoverload recovery at the unipolar output, andgenerating PUR (reject) and BUSY signals for theduration of the overload. The PUR and BUSY logicpulses are used to prevent analysis and correct forthe reset deadtime in the associated ADC ormultichannel analyzer.

2.4. OUTPUTSUNI Front- and rear-panel BNC connectorsprovide positive, unipolar, shaped pulses with alinear output range of 0 to +10 V. Front-paneloutput impedance <1 S. Rear-panel outputimpedance selectable for either <1 S or 93 S usinga printed circuit board jumper. Outputs are dc-restored to 0 ± 5 mV and short-circuit protected.

Bi Front- and rear-panel BNC connectors providebipolar shaped pulses with the positive lobe leading.The linear output range is 0 to ±10 V. Front- paneloutput impedance <1 S. Rear-panel outputimpedance selectable for either <1 S or 93 S usinga printed circuit board jumper. Baseline betweenpulses has a dc level of 0 ± 10 mV. Short-circuitprotected.

CRM The Count Rate Meter output has a rear-panelBNC connector and provides a 250-ns-wide, +5-Vlogic signal for every linear input pulse that exceedsthe pile-up inspector threshold. Output impedanceis 50 S.

BUSY Rear-panel BNC connector provides a +5-Vlogic pulse for the duration that the linear signalsexceed the positive or negative baseline restorerthresholds, or the pile-up inspector threshold, or forthe duration of the INH IN input signal. Useful fordeadtime corrections with an associated ADC ormultichannel analyzer. Positive NIM standard logicpulse is selectable as active high or active low viaa printed circuit board jumper. Output impedance is50 S.

PUR Pile-Up Reject output is a rear-panel, BNCconnector. Provides a +5-V NIM standard logicpulse when pulse pile-up is detected. Output alsopresent for a pulsed reset preamplifier during reset,and reset overload recovery. Output pulse isselectable as active high or active low by means ofa printed circuit board jumper. Output impedance is50 S. Used with an associated ADC ormultichannel analyzer to prevent analysis ofdistorted pulses.

PREAMP Rear-panel standard ORTEC connector(Amphenol 17-10090) provides power for theassociated preamplifier. Mates with power cords onall standard ORTEC preamplifiers.

2.5. ELECTRICAL AND MECHANICAL

POWER REQUIRED The Model 671 derives itspower from a NIM Bin supplying ±24 V and ±12 V,such as the ORTEC Model 4001A/4002A Bin/PowerSupply. The power required is +24 V at 100 mA, -24V at 200 mA, +12 V at 325 mA, and -12 V at180 mA.

WEIGHTNet 1.5 kg (3.3 lb). Shipping 3.1 kg (7.0 lb).

DIMENSIONS Standard single-width module, 3.45X 22.13 cm (1.35 X 8.714 in.) Front panel perDOE/ER- 0457T.

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3. INSTALLATION

3.1. POWER CONNECTION

The 671 operates on power that must be providedby a NIM-standard bin and power supply such asthe ORTEC 4001/4002 series. Convenient testpoints on the power supply control panel should beused to check that the dc voltage levels are notoverloaded. The bin and power supply is designedfor relay rack mounting. If the equipment is rackmounted, be sure that there is adequate ventilationto prevent any localized heating of the componentsthat are used in the 671. The temperature of theequipment mounted in racks can easily exceed themaximum limit of 50°C unless precautions aretaken.

3.2. PREAMPLIFIER CONNECTION

The Preamp connector of this amplifier is directlycompatible with ORTEC preamplifiers as well aswith standard Aptec, Canberra, PGT, and Tennelec(serial numbers greater than 2000) preamplifiers.Preamplifier power at +24 V, -24 V, +12 V, and -12V is available through the Preamp connector on therear panel.

When a BNC cable longer than ten feet is used toconnect the preamplifier output to the amplifierinput, the characteristic impedance of the cableshould match the impedance of the preamplifieroutput. All ORTEC preamplifiers contain seriesterminations that are either 93 S or variable;coaxial cable type RG-62/U or RG-71/U isrecommended.

3.3. PULSED RESET PREAMPLIFIERSAND INHIBIT IN CONNECTION

The 671 Amplifier is directly compatible with mostpulsed reset preamplifiers such as the ORTEC TRP(Transistor Reset Preamplifier) Series. Theamplifier automatically senses preamplifier resetsand gates off the amplifier's baseline restorer.Preamplifier inhibit signals are not required for

proper amplifier operation; however, since thepreamplifier resetting process is nonlinear bynature, spurious phantom peaks may show up inthe spectra if the inhibit signal from the preamplifieris not used.

INHIBIT IN CONNECTION Connection of thePREAMPLIFIER INHIBIT OUT signal to the rear-panel INHIBIT IN connector will result in the systembeing disabled during the reset period and thusavoid spurious peaks in the spectra. Preamplifierswith an Inhibit time switch such as ORTEC PLUSDetector with series 132 Preamplifier can be set toposition "1", which is the shortest preamp inhibitblocking time.

PZ SETTING The Amplifier's PZ control should beset fully counterclockwise (CCW) when used with apulsed reset preamplifier.

3.4. CONNECTION OF TEST PULSEGENERATOR

THROUGH A PREAMPLIFIER The satisfactoryconnection of a test pulse generator such as theORTEC 419 or 448 Pulse Generator or equivalentdepends primarily on two considerations: thepreamplifier must be properly connected to the 671as discussed in Sections 3.2 and 3.3, and theproper signal simulation must be applied to thepreamplifier. To ensure proper input signalsimulation, refer to the instruction manual for theparticular preamplifier being used.

DIRECTLY INTO THE 671 The ORTEC test pulsegenerators are designed for direct connection.When any one of these units is used, it should beterminated with a 100-S terminator at amplifierinput or be used with at least one of the outputattenuators set at In.

SPECIAL CONSIDERATIONS FOR POLE-ZEROCANCELLATION When a tail pulser is connecteddirectly to the amplifier input, the Pole-Zero shouldbe adjusted. See Section 4.3 for the pole-zero

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adjustment. If a preamplifier is used and a tailpulser is connected to the preamplifier test input, itis not possible to adjust the pole-zero for both thepreamplifier pole and the pole from the pulser tail.

3.5. SHAPING CONSIDERATIONS

The Shaping Time switch on the front panel of the671 can be set to select time constants in steps of0.5, 1, 2, 3, 6, and 10 :s. Choice of triangular andGaussian filters doubles the time constantsavailable for optimum resolution. Triangularshaping will usually give better results. The choiceof the proper shaping time is generally acompromise between operating at a shorter timeconstant for accommodation of high counting ratesand operating with a longer time constant for abetter signal-to-noise ratio. Since the full amplitudeof the preamplifier output pulse must be preserved,the peaking time (measurement time) must be largecompared to preamplifier output pulse risetime. Theamplifier shaping time should be greater than fivetimes the charge collection time of the detector.Use the detector manufacturer's suggested shapingtimes as a starting point and adjust the shaping asyour needs for resolution versus count rate vary.

GERMANIUM DETECTORS Shaping times forhigh-purity germanium (HPGe) detectors will varyfrom 1 to 6 :s using the unipolar output, dependingon the size, configuration, and charge collectiontime of the specific detector and preamplifier.Coaxial detectors have significant variations incharge collection times due to their large volumes.Compromises must often be made since theshaping time that will give the best resolution willusually be longer than the optimum time needed forthe best throughput at high counting rates.

Planar detectors require shaping times in the rangeof 3 to 10 :s for optimum resolution. Lithium-driftedsilicon detectors, Si(Li), have similar shaping timerequirements.

SILICON CHARGED PARTICLE DETECTORSThese detectors have very fast risetimes on theorder of 10 ns or less. A unipolar output and a 0.5-to 2-:s shaping time will generally provide optimumresolution.

SCINTILLATION DETECTORS The energyresolution of scintillation counters depends largelyon the scintillator and photomultiplier, and thereforea shaping time of five times the decay-timeconstant of the scintillator is a reasonable choice.For Nal detectors that have a decay time constantof about 230 ns, the optimum shaping time is l :s.The bipolar output can be used to reduce overloadeffects and microphonics without sacrificingresolution.

GAS PROPORTIONAL COUNTERS Proportionalcounters have both short and long components intheir charge collection times. The componentstypically fall in the 0.5- to 5-:s range, and lead tovariable amounts of preamplifier output signal beinglost as the amplifier shaping time constant ischanged. Selection of longer shaping times (>2 :s)helps to minimize the problem caused by longrisetimes. Due to the multiple components in thecharge collection time, the correct pole-zerocancellation is not possible. This will often cause anundershoot if the Unipolar output is used. Bipolarshaping can be used to reduce this effect with littlechange in the resolution.

3.6. LINEAR OUTPUT CONNECTIONS

Since the 671 unipolar output is normally used forspectroscopy, the 671 is designed with a greatamount of flexibility for the pulse to be interfacedwith an analyzer. To minimize spectrum distortionat medium and high counting rates, the unipolaroutput incorporates a high-performance, gatedbaseline restorer with automatic setup. Automaticpositive and negative noise discriminators ensurethat the baseline restorer operates only on the truebaseline between pulses in spite of changes in thenoise level. For pulse-height analysis, the unipolaroutput must be directly connected to the input of amultichannel analyzer.

The bipolar output, with its symmetry about thebaseline, can be used for cross-over timing or maybe preferred for spectroscopy when operating intoac-coupled systems at high counting rates. Typicalsystem block diagrams for a variety of experimentsare described in Section 4.

8

3.7. PILE-UP REJECTION USING PUROUTPUT

The PUR (Pile-Up Reject) output on the rear panelis used at the gate or pile-up reject input of amultichannel analyzer to suppress pile-up in therecorded spectrum. The fast amplifier in the pile-uprejector includes a gated baseline restorer with anautomatic noise discriminator to eliminate the needfor any operator adjustments. When pileup occurs,a logic true pulse is generated which lasts until theunipolar output returns to the baseline, normally awidth of six times the shaping time. If used with apulsed reset preamplifier, this output also includesa reject during the reset and recovery interval.

3.8. LIVETIME CORRECTION USINGBUSY OUTPUT

The signal from the rear-panel Busy outputconnector provides a nominally +5 V logic pulse forthe duration that the Unipolar output pulse exceeds

the baseline restorer threshold or pile-up inspectorthreshold or when the external INH IN is true. Forlivetime correction, Busy should be connected tothe Busy In connector on the MCA. For optimallivetime correction with ORTEC analyzers like theADCAM®, an internal jumper in the amplifier shouldbe set to match the unipolar, triangular, or Gaussianmode. The output is internally jumper selectable asactive low or active high. It is shipped as activehigh.

3.9. INPUT COUNT RATE USING CRMOUTPUT

A positive logic pulse is generated for each 671input pulse that exceeds the pile-up inspectorthreshold level. The pulses are available throughthe CRM (Count Rate Meter) output on the rearpanel and are intended for use in a count rate meteror counter to monitor the true input count rate intothe amplifier.

9

Fig. 4.1. Typical Effects of Shaping-Time Selectionon Gaussian, Triangular, and Bipolar Output

Waveforms. Fig. 4.2. Typical Gamma-Ray Spectroscopy System.

4. OPERATING INSTRUCTIONS

4.1. INITIAL TESTING AND OBSERVATIONOF PULSE WAVEFORMS

Refer to Section 6 for information on testingperformance and observing waveforms using apulser. Figure 4.1 shows some typical unipolarGaussian, unipolar triangular, and bipolar outputwaveforms.

4.2. STANDARD SETUP PROCEDURES

a. Connect the detector, preamplifier, high-voltagepower supply, and amplifier into a basic system andconnect the amplifier unipolar output to anoscilloscope. Connect the preamplifier power cableto the Preamp power connector on the rear panel ofthe 671. Turn on power in the bin and power supplyand allow the electronics of the system to warm upand stabilize.

A block diagram of a typical ORTEC gamma-rayspectroscopy system is shown in Figure 4.2.

b. Set the 671 controls initially as follows:

Shaping Time 3 or 6 :s Mode Triangle Coarse Gain 20Fine Gain 1.00 BLR PZ Polarity Match preamplifier output

polarity

c. Use a 60Co calibration source; set about 25 cmfrom the active face of the detector. The unipolaroutput pulse from the 671 should be about 8 V,using a detector that has a preamp with aconversion gain of 300 mV/MeV.

d. Readjust the Gain control so that the higher peakfrom the 60Co source (1.33 MeV) provides anamplifier output at about 9 V.

10

Fig. 4.3. Typical Waveforms Illustrating Pole-ZeroAdjustment Effects; Oscilloscope Trigger, Busy Output;

60Co Source with 1.33-MeV Peak Adjusted -9 V; CountRate, 3 kHz; Shaping Time Constant, 2 :s.

4.3. POLE-ZERO ADJUSTMENTS FORRESISTIVE-FEEDBACK PREAMPLIFIER

The pole-zero adjustment is critical for goodperformance at high count rates in unipolaroperation and for correct operation of the BLRcircuit. This adjustment should be checked carefullyfor the best possible results. Whenever the shapingtime is changed. the pole-zero must be adjusted.The bipolar output resolution is not as sensitive tomisadjusted PZ, but it is important for recoveryfrom very large overload pulses. When using atransistor reset-type preamplifier, the PZ should beset to full counterclockwise.

a. Adjust the radiation source spacing from thedetector to provide a count rate between 1 and 10kHz.

b. Observe the unipolar output with an oscilloscope.Increase the scope input sensitivity to 20-100 mVper vertical division. Depress the front-panel LIMpush-button to limit the voltage applied to theoscilloscope. Adjust the PZ adjust control so thatthe trailing edge of the pulses returns to thebaseline without overshoot or undershoot (Fig. 4.3).A slight bias toward an undershoot often gives thebest results.

The oscilloscope used must be dc-coupled andmust not contribute distortion in the observedwaveforms. Oscilloscopes such as Tektronixmodels 465, 475, and 7904 will overload for a 10-Vsignal when the vertical sensitivity is <100 mV/Div.The LIM push-button switch inserts a diode limiterin series with the front-panel UNI output connectorto prevent overloading the input of the oscilloscope.

USING S Q UARE WAV E T HROUGHPREAMPLIFIER TEST INPUT

A more precise pole-zero adjustment of theamplifier can be obtained by using a square wavesignal as the input to the preamplifier. Manyoscilloscopes include a calibration output on thefront panel, and this is a good source of squarewave signals at a frequency of about 1 kHz. Theamplifier differentiates the signal from thepreamplifier so that it generates output signals of

alternate polarities on the leading and trailing edgesof the square wave input signal, and these can becompared as shown in Fig. 4.4 to achieve excellentpole-zero cancellation.

11

Fig. 4.4. Pole-Zero Adjustment Using a Square Wave Input to the Preamplifier. (a) PZ properly adjusted; slow trigger toseparate pulses. (b) Overcompensated; fast trigger to superimpose pulses. (c) Properly adjusted; pulses superimposed.

(d) Undercompensated; pulses superimposed.

Use the following procedure:

a. Remove all radioactive sources from the vicinityof the detector. Set up the system as for normaloperation, including detector bias.

b. Set the amplifier controls as for normaloperations; this includes gain, shaping, and inputpolarity.

c. Connect the source of 1 kHz square wavesthrough an attenuator to the Test input of thepreamplifier. Adjust the attenuator so that theamplifier output amplitude is 8 to 10 volts.

d. Observe the unipolar output of the amplifier withan oscilloscope triggered from the amplifier Busyoutput. Adjust the PZ control for proper responseaccording to Fig. 4.4. Depress the LIMIT push-button on the 671 while observing the adjustmenton the oscilloscope display.

Figure 4.4.(a) shows the amplifier output as a seriesof alternate positive and negative shaped pulses. InFig. 4.4.(b)-(c), the oscilloscope was triggered toshow both positive and negative pulsessimultaneously. These pictures show more detail toaid in proper adjustment.

12

Fig. 4.5. Position of Internal Controls.

4.4. BASELINE RESTORER (BLR)SETTING

To minimize spectrum distortion at medium andhigh counting rates, the unipolar output incorporatesa high-performance, gated, baseline restorer withseveral levels of automation. Automatic positiveand negative noise discriminators ensure that thebaseline restorer operates only on the true baselinebetween pulses in spite of changes in the noiselevel. No operator adjustment of the baselinerestorer is needed when changes are made in thegain, the shaping time constant, or the detectorcharacteristics. Negative overload recovery fromthe reset pulses generated by transistor resetpreamplifiers and pulsed optical feedbackpreamplifiers is also handled automatically toeliminate the need for operator adjustments. Amonitor circuit gates off the baseline restorer andprovides a reject signal for a multichannel analyzeruntil the baseline has safely recovered from theoverload.

BLR RATE For making pole-zero adjustments, thePZ position is selected. This position can also beused where the slowest baseline restorer rate isdesired.

With the BLR Rate set to AUTO, the BLR isautomatically set for optimum performancethroughout the usable input range for the shapingselected.

The HIGH rate can be used for situations where lowor medium frequency noise interference is presentand is independent of the counting rate. The HIGHrate setting is normally not used since there will bea small loss of resolution due to increased noisewhen used in high resolution systems.

4.5. INTERNAL CONTROLS

These controls are on the printed wiring board(PWB) and can be accessed by removing the rightside cover. Figure 4.5 shows the location of thesecontrols.

13

NORM-DIFF Internal PWB mounted, two-positionslide switch. NORM position selects single endedinputs from front-panel input or rear-panel inputconnectors. In the DIFF position, the front-panelinput is connected to the preamplifier signal cable,and a cable connected to the preamplifier groundthrough an impedance matching resistor isconnected to the rear-panel input.

BAL (DIFFERENTIAL INPUT GAIN BALANCE)Internal PWB 20-turn screwdriver potentiometerallows maximization of noise rejection when usingdifferential input. See Section 4.6.

UNI-OUT (UNIPOLAR ZOUT) Jumper plug, W1,provides ZOUT, #1 S or -93 S for the rear-panelUnipolar output. Shipped in the 93-S position.

BI-OUT (BIPOLAR ZOUT) Jumper plug, W2,provides ZOUT, #1 S or -93 S) for the rear-panelBipolar output. Shipped in the 93- S position. _____BUSY/BUSY Jumper plug, W3, allows the Busyoutput to be a positive true or negative true logicsignal. Shipped in BUSY (positive true) position. ___PUR/PUR Jumper plug, W5, allows the Pile-UpReject (PUR) output to be a positive true ornegative true logic signal. Shipped in PUR (positivetrue) position. ___INH/INH Jumper plug, W6, allows the INH IN inputto accept either positive true or negative true logicsignals. Shipped in INH (positive true) position.

TRI/GAUSS Jumper plug, W7, allows optimallivetime correction when used with ORTECanalyzers like the ADCAM® by connecting theBUSY output to the analyzer Busy In as describedin Section 3.8. The jumper should be set to matchthe Unipolar Mode, TRI for Triangle and GAUSS forGaussian. Shipped in TRI position.

4.6. DIFFERENTIAL INPUT MODE

When long connecting cables are used between thedetector and preamplifier input, noise induced in thecable by the environment can be a problem. Thedifferential input mode can be used with paired

cables from the preamplifier to suppress theinduced noise.

BAL (DIFFERENTTAL INPUT GAIN BALANCE)The BAL potentiometer is used to adjust the gainbalance between the positive and negative inputsand to adjust the balance between the front- andrear-panel inputs when the differential (DIFF) inputmode is used. The initial adjustment of GainBalance is made by providing the same input toboth the front- and rear-panel inputs. This can beaccomplished by using a BNC "T" connector to feedthe input signal on the front-panel input to the rear-panel 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 oscilloscopeshows minimum signal. Remove the BNC "T"connector when the adjustment is complete, andthe positive and negative gains will be matched foruse with NORM input.

If the differential input mode is being used, connectthe differential input cable to the BNC connector onthe rear panel. Adjust BAL potentiometer until thereis minimum noise around the baseline of the outputsignal. If there is a problem in getting minimumnoise, repeat the initial procedure with the BNC "T"and the adjustment.

DIFFERENTIAL INPUT SIGNAL The differentialinput signal or phantom is used only in thedifferential (DIFF) input mode. The normal preampoutput is connected to the front-panel input with theamplifier input polarity set to match this signal. Asecond output cable must be added to thepreamplifier with its center, signal pin connected tothe preamplifier ground with the same value as thenormal preamp output series resistor (usually 93.1or 51 S).

Many ORTEC preamplifiers have two Energyoutputs, each with a 93.1-S series resistor. Fordifferential operation, one output is connected tothe amplifier front-panel input. The second output ismodified by connecting the preamplifier end of theseries 93.1-S resistor to ground within the preamp(soldering may be necessary). This second output

14

Fig. 4.6. Plot of Normalized Output Rate as a Function ofNormalized Input Rate for Spectrometers with Simple

Deadtime.

should be properly marked and connected to therear-panel input. Both cables should be the samelength and be run next to each other.

4.7. SYSTEM THROUGHPUT

To achieve the desired results in high-rate energyspectroscopy, the experimenter must consider notonly the input rate, but also the unpiled-up outputrate. The unpiled-up output rate is determined bythe processing time of the shaping amplifier, thepile-up inspection time, and the input rate. Forsemi-Gaussian time-invariant filter amplifiers, theunpiled-up output rate is theoretically given by2

ro = ri exp (-TDri) (1)

where ro is the unpiled-up output count rate, ri is theinput count rate, and TD is the deadtime or effectiveprocessing time of the amplifier. The value of TD isequal to the sum of the effective amplifier pulsewidth, Tw, and the time-to-peak of the amplifieroutput pulse, Tp. The type of deadtime in theshaping amplifier is referred to as extendingdeadtime since a second event arriving before theend of the initial deadtime extends the deadtime byan additional amplifier output pulse width, Tw, fromthe occurrence of the second pulse.

A normalized plot of Equation (1) is shown as thesolid line in Fig. 4.6. The maximum mean outputrate equals 1 /TD exp (1) and occurs when the meaninput rate equals 1 /TD. At this maximum output ratethe deadtime losses are 63.2%. For input countrates exceeding 1 /TD the unpiled-up output ratedecreases. When using a pile-up inspection circuit,the value of TD is given either by the sum of Tw andTp, or by the sum of Tp and the pile-up inspectiontime, whichever is larger.

Spectroscopy systems also have a deadtime that iscaused 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 ignored. For non-extendingdeadtime the output rate is given by2

where TD is the digitizing time for the ADC and isdesignated TM in Equation (3). This relationship isshown as the dashed line in Fig. 4.6. The maximumobtainable output count rate is 1/TD) and occurs atri = 4.

When the ADC is considered as part of thespectroscopy system, the deadtimes of theamplifier and ADC are in series. The combination ofthe extending deadtime of the amplifier followed bythe non-extending deadtime of the ADC is given by2

where U[TM-(TW-TP)] is a unit step function thatchanges value from 0 to 1 when TM is greater than(TW-TP). Equation (3) reduces to Equation (1) whenTM is less than (TW-TP).

A plot of the unpiled-up ampilifier output rate as afunction of input rate for six values of shaping timeis shown in Fig.4.7. The measured deadtime, TD, isshown for each shaping time constant. Themaximum value of the unpiled-up output rateincreases with decreasing values of shaping time

2R. Jenkins, R.L. Gould, and D.A. Gedcke, Quantitative X-RaySpectroscopy, Marcel and Dekker, Inc., New York, (1980).

15

Fig. 4.7. Plot of the Unpiled-Up Amplifier Output Rateas a Function of Input Rate for Six Values of Shaping

Time Constants.

Fig. 4.8. Charge Collection Effect Waveforms. (a) Typicalcurrent Pulse Waveforrns for a 28% Efficient HPGe

Detector, and (b) the Simple Differentiation Circuit Usedto Obtain the Current Waveforms.

constant. A set of throughput curves will remainnearly unchanged for a given amplifier for variousenergy ranges, detector types, and sizes.

The advantage of shorter shaping time constants toachieve higher output count rates is clearly shownin Fig. 4.7. However, shorter time constants alsoresult in increased noise and increased chargecollection time effects. Under worst case conditions,the noise increases inversely as the square root ofthe ratio of shaping time constants. The increase inthe total energy resolution is the noise contributioncombined in quadrature with the statisticalcontribution of the detector at the energy of interest.Consequently, the percentage of degradation inenergy resolution can be much less than thepercentage increase in noise.

4.8. CHARGE COLLECTION ORBALLISTIC DEFICIT EFFECTS

Charge collection distances in large-volume HPGedetectors are often 3 cm or more, resulting incharge collection times exceeding 300 ns.3,4,5

These charge collection times are due to the transittime 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 waveformsfrom a 140-cm3 28% efficient HPGe detector.These current pulse waveforms were obtainedusing the simple differentiation circuit shown in Fig.4.8(b), which has a 15-ns time constant. Thecurrent pulses range in duration from 100 ns togreater than 350 ns. Pulses having equivalent totalcharge but different durations produce differentoutput pulse heights when processed by a charge-sensitive preamplifier and a semi-Gaussian filteramplifier. This results in the distortion of thespectrum in direct proportion to the pulse amplitudeor energy. This distortion is most pronounced atshort shaping time constants. Figure 4.9(a) showsa portion of a spectrum obtained with a 10%efficient HPGe detector at 2-:s shaping time, usingthe 1.33-MeV line of 60Co. An equivalent spectrumusing a 0.5-:s shaping time is shown in Fig. 4.9(b)and is significantly distorted.

3E. Sakai, "Charge Collection in Coaxial Ge(Li) Detectors," IEEETrans. Nuct. Sci., NS-1 5, 310, (1968).

4E. Sakai, T.A. McMath, and R.G. Franks, "Further ChargeCollection Studies in Coaxial Ge(Li) Detectors," IEEE Trans.Nucl. Sci., NS-16, 68, (1968).

5T.H. Becker, E.E. Gross, and R.C. Trammell, "Characteristics ofHigh-Rate Energy Spectroscopy Systems with Time-invariantFilters," IEEE Tran& Nucl. Sci., NS-28, 1, (1981).

16

Fig. 4.9. Charge Collection Effect Spectrum. LogarithmicDisplay of Spectrum Taken with a 10% Efficient HPGe

Detector for the 1.33 MeV 60Co Line. (a) A 2-:s ShapingTime Constant and (b) a 0.5-:s Shaping Time Constant.

Fig.4.10. Energy Resolution FWHM as a Function ofAmplifier Shaping Time Constant for a 10% HPGe

Detector and a 28% HPGe Detector for the 122-keV 57CoLine and the 1.33-MeV 60Co Line.

Charge collection time effects are of significantimportance when using large-volume Ge detectorsat high energy. The performance of two HPGedetectors is compared in Fig. 4.10 at two differentenergies. When using the 122-keV line of 57CO,the principal cause of resolution degradation withdecreased shaping time constant is the increase innoise. However, when using the 1.33-MeV line of60Co, the significant degradation in resolution is dueto charge collection effects. The calculatedresolution for the 10% detector at 1.33 MeV isshown as the dashed line in Fig. 4.10. and indicatesapproximately 2.0 keV FWHM at a 0.5-:s shapingtime constant. The measured resolution underthese test conditons was 7.2 keV, indicating thatcharge collection effects dominate. In Fig. 4.10,charge collection effects begin to appear at timeconstants less than 3 :s.

4.9. PILE-UP REJECTOR (PUR) ANDLIVETIME CORRECTOR

An efficient pile-up rejector is incorporated in theamplifier to suppress the spectral distortion which is

caused by pulses piling up on each other at highcounting rates. High counting rate for pile-up isdependent on the dead time per pulse, TD, andhence the selected shaping time. TD is 9 times thefront-panel shaping time, Tc. High count rate for thePUR is when the normalized count rate RiTD >0.5,where Ri is the amplifier input rate (see Fig. 4.6).For example, for 6-:s shaping Ri is 9 kHz and for 2-:s shaping, Ri is 28 kHz. Amplifier throughput forthis condition using Equation (1) in Section 4.7 is60% of the input rate. A multicolor pile-up rejectorLED is included on the front panel to indicate thethroughput efficiency of the amplifier. At lowcounting rates (pulse pile-up losses <40%) the LEDflashes with a green color. At moderate countingrates the color changes to yellow. The colorchanges to red at high counting rates when thepulse pile- up losses are >70%.

The fast amplifier in the pile-up rejector includes agated baseline restorer with its own automatic noisediscriminator to eliminate the need for any operatoradjustments. This function is also protected againstnegative overloads from pulsed reset preamplifiers.The PUR (pile-up reject) output logic pulse can beused at the gate or reject input of a multichannelanalyzer to suppress pile-up in the recordedspectrum.

17

Fig. 4.11. Block Diagram for a Gamma-Ray Spectroscopy System with Pile-Up Rejection and Livetime Correction.

The block diagram for a gamma-ray spectroscopysystem with pile-up rejection and live timecorrection is shown in Fig. 4.11.

FOR A RESISTIVE FEEDBACK PREAMP,CONNECT:

a. Inhibit pulse from PUR to ADC PUR or ADCanticoincidence input.

b. Livetime correction signal (Busy output) to theADC Busy In.

ADDITIONAL CONNECTION FOR TRP (TransistorReset Preamplifiers) Shown in dotted lines.

c. Inhibit Output from TRP to the amplifier inhibit In.

4.10. OPERATION WITHSEMICONDUCTOR DETECTORS

CALIBRATION OF TEST PULSER An ORTEC419 Precision Pulse Generator, or equivalent, iseasily calibrated so that the maximum pulse heightdial reading (1000 divisions) is equivalent to a 10-MeV loss in a silicon radiation detector. Theprocedure is as follows:

a. Connect. the detector to be used to thespectrometer system, that is, preamplifier, mainamplifier, 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 thebiased amplifier to give a suitable output pulse.

d. Set the pulser Pulse Height control at the energyof the alpha particles striking the detector (e.g., setthe dial at 547 divisions for a 5.47-MeV alphaparticle energy).

e. Turn on the pulser and use its Normalize controland attenuators to set the output due to the pulserfor the same pulse height as the pulse obtained instep c. Lock the Normalize control and do not moveit again until recalibration is required.

The pulser is now calibrated; the Pulse Height dialreads directly in MeV if the number of dial divisionsis divided by 100.

18

Fig. 4.12. System for Measuring Amplifier and DetectorNoise Resolution.

Fig. 4.13. Noise as a Function of Bias Voltage.

AMPLIFIER NOISE AND RESOLUTIONMEASUREMENTS As shown in Fig. 4.12, apreamplif ier, amplif ier, pulse generator,oscilloscope, and wide-band rms voltmeter such asthe Hewlett-Packard 3400A are required for thismeasurement. Connect a suitable capacitor to theinput to simulate the detector capacitance desired.To obtain the resolution spread due to amplifiernoise:

a. Measure the rms noise voltage (Erms) at theamplifier output.

b. Turn on the 419 precision pulse generator andadjust the pulser output to any convenient readablevoltage, Eo, as determined by the oscilloscope.

The full-width-at-half-maximum (FWHM) resolutionspread due to amplifier noise is then

where Edial is the pulser dial reading in MeV and2.35 is factor for rms to FWHM. For average-responding voltmeters such as the Hewlett-Packard400D, the measured noise must be multiplied by1.13 to calculate the rms noise.

The resolution spread will depend on the total inputcapacitance, since the capacitance degrades thesignal- to-noise ratio much faster than the noise.

D E T E C T O R N O I S E - R E S O L U T I O NMEASUREMENTS The measurement justdescribed can be made with a biased detectorinstead of the external capacitor that would be usedto simulate detector capacitance. The resolutionspread will be larger because the detectorcontributes both noise and capacitance to the input.The detector noise-resolution spread can beisolated from the amplifier noise spread if thedetector capacity is known, since

(Ndet)2 +(Namp)

2 = (Ntotal)2,

where Ntotal is the total resolution spread and Namp

is the amplifier resolution spread when the detectoris replaced by its equivalent capacitance.

The detector noise tends to increase with biasvoltage, but the detector capacitance decreases,thus reducing the resolution spread. The overallresolution spread will depend upon which effect isdominant. Figure 4.13 shows curves of typicalnoise-resolution spread versus bias voltage, usingdata from several ORTEC silicon surface-barriersemiconductor radiation detectors.

A M P L I F I E R N O I S E - R E S O L U T I O NMEASUREMENTS USING MCA Probably themost convenient method of making resolutionmeasurements is with a pulse height analyzer asshown by the setup illustrated in Fig. 4.14.

19

Fig. 4.14. System for Measuring Resolution with a PulseHeight Analyzer. Fig. 4.15. System for Detector Current and Voltage

Measurements.

Fig. 4.16. Silicon Detector Back Current vs Bias Voltage.

The amplifier noise-resolution spread can bemeasured directly with a pulse height analyzer andthe mercury pulser as follows:

a. Select the energy of interest with an ORTEC 419Precision Pulse Generator. Set the amplifier andbiased amplifier gain and bias level controls so thatthe energy is in a convenient channel of theanalyzer.

b. Calibrate the analyzer in keV per channel, usingthe pulser; full scale on the pulser dial is 10 MeVwhen calibrated as described above.

c. Obtain the amplifier noise-resolution spread bymeasuring the FWHM of the pulser peak in thespectrum.

The detector noise-resolution spread for a givendetector bias can be determined in the samemanner by connecting a detector to the preamplifierinput. The amplifier noise resolution spread must besubtracted as described in Section 4.10, "DetectorNoise-Resolution Measurements." The detectornoise will vary with detector size and biasconditions and possibly with ambient conditions.

CURRENT-VOLTAGE MEASUREMENTS FOR SiAND Ge DETECTORS The amplifier system is notdirectly involved in semiconductor detector current-voltage measurements, but the amplifier serves topermit noise monitoring during the setup. Thedetector noise measurement is a more sensitivemethod of determining the maximum detector

voltage than a current measurement and should beused because the noise increases more rapidly thanthe reverse current at the onset of detectorbreakdown. Make this measurement in the absenceof a source.

Figure 4.15 shows the setup required for current-voltage measurements. An ORTEC 428 BiasSupply is used as the voltage source. Bias voltageshould be applied slowly and reduced when noiseincreases rapidly as a function of applied bias.Figure 4.16 shows several typical current-voltagecurves for ORTEC silicon surface-barrier detectors.

When it is possible to float the microammeter at thedetector bias voltage, the method of detectorcurrent measurement shown by the dashed lines inFig. 4.15 is preferable. The detector is grounded asin normal operation, and the microammeter isconnected to the current monitoring jack on the 428detector bias supply.

20

Fig. 4.17. System for High-Resolution Alpha-Particle Spectroscopy.

4.11. OPERATION IN SPECTROSCOPYSYSTEMS

HIGH-RESOLUTION ALPHA-PARTICLESPECTROSCOPY SYSTEM The block diagram ofa high-resolution spectroscopy system formeasuring natural alpha particle radiation is shownin Fig. 4.17. Since natural alpha radiation occursonly above several MeV, an ORTEC 444 BiasedAmplifier is used to suppress the unused portion ofthe spectrum; the same result can be obtained byusing digital suppression on the MCA in manycases. Alpha-particle resolution is obtained in thefollowing manner:

a. Use appropriate amplifier gain and minimumbiased amplifier gain and bias level. Accumulatethe alpha peak in the MCA.

b. Slowly increase the bias level and biasedamplifier gain until the alpha peak is spread over 5to 10 channels and the minimum- to maximum-energy range desired corresponds to the first andlast channels of the MCA.

c. Calibrate the analyzer in keV per channel usingthe pulser and the known energy of the alpha peak(see Section 4.10, "Calibration of Test Pulser") ortwo known energy alpha peaks.

d. Calculate the resolution by measuring thenumber of channels at the FWHM level in the peakand converting this to keV.

21

Fig. 4.18. System for High-Resolution GammaSpectroscopy.

Fig. 4.19. Scintillation-Counter Gamma Spectroscopy System.

H I G H - R E S O L U T I O N G A M M A - R A YSPECTROSCOPY SYSTEM A high-resolutiongamma-ray spectroscopy system block diagram isshown in Fig. 4.18. Although a biased amplifier isnot shown (an analyzer with more channels beingpreferred), it can be used if the only analyzeravailable has fewer channels and only higherenergies are of interest.

When germanium detectors nitrogen cryostat areused, it is from about 1 keV FWHM up that arecooled by a liquid possible to obtain resolutions to4 keV (depending on the energy of the incidentradiation and the size and quality of the detector).Reasonable care is required to obtain such results.Some guidelines for obtaining optimum resolutionare:

a. Keep interconnection capacities between thedetector and preamplifier to an absolute minimum(no long cables).

b. Keep humidity low near the detector-preamplifierjunction.

c. Operate the amplifier with the shaping time thatprovides the best signal-to-noise ratio.

d. Operate at the highest allowable detector bias tokeep the input capacity low.

S C I N T I L L AT I O N - C O U N T E R G AM M ASPECTROSCOPY SYSTEMS The ORTEC 671can be used in scintillation-counter spectroscopysystems as shown in Fig. 4.19. The amplifiershaping time constants should be selected in theregion of 0.5 to 1 :s for Nal or plastic scintillators.For scintillators having longer decay times, longertime constants should be selected.

X - R A Y S P E C T R O S C O P Y U S I N GPROPORTIONAL COUNTERS Space chargeeffects in proportional counters, operated at highgas amplification, tend to degrade the resolutioncapabilities drastically at x-ray energies, even atrelatively low counting rates. By using a high-gainlow-noise amplifying system and lower gasamplification, these effects can be reduced and aconsiderable improvement in resolution can beobtained. The block diagram in Fig. 4.20 shows asystem of this type. Analysis can be accomplishedby simultaneous acquisition of all data on amultichannel analyzer or counting a region ofinterest in a single-channel analyzer window with acounter and timer or counting ratemeter.

4.12. OTHER EXPERIMENTS

Block diagrams illustrating how the 671 and otherORTEC modules can be used for experimentalsetups for various other applications are shown inFigs. 4.21 through 4.24.

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Fig. 4.20. High-Resolution X-Ray Energy Analysis SystemUsing a Proportional Counter.

Fig. 4.21. General System Arrangement for Gating Control.

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Fig. 4.22. Gamma-Ray Charged-Particle Coincidence Experiment.

Fig. 4.23. Gamma-Ray Pair Spectroscopy.

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Fig. 4.24. Gamma-Gamma Coincidence Experiment.

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5. MAINTENANCE

5.1. TEST EQUIPMENT REQUIRED

The following test equipment should be utilized toadequately test the specifications of the 671Spectroscopy Amplifier:

1. ORTEC 419 Precision Pulse Generator or 448Research Pulser.

2. Tektronix 465, 475, or 485 Series Oscilloscopeor equivalent with bandwidth greater than 100 MHz.

3. Hewlett-Packard 3400A RMS Voltmeter.

5.2. PULSER TEST6

Coarse Gain 1K Fine Gain 1.5 Input Polarity Positive Shaping Time Constant 2 :s BLR Rate PZ UNI Shaping Gaussian

a. Connect a positive pulser output to the 671 inputand adjust the pulser to obtain +10 V at the 671Unipolar output. This should require an input pulseof 6.6 mV, using a 100-S terminator at the input.Switch Unipolar Mode to Triangle. This should alsobe 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 thenback to Pos while monitoring the outputs for apolarity inversion. The negative output shouldclamp at -1V.

d. Decrease the Coarse Gain switch stepwise from1K to 5 and ensure that the output amplitudechanges by the appropriate amount for each step.Return the Coarse Gain switch to 1 K.

e. Decrease the Fine Gain control from 1.5 to 0.5and check to see that the output amplitudedecreases by a factor of 3. Return the Fine Gaincontrol to maximum at 1.5.

f. With the Shaping Time switch set for 1 :s,measure the time to the peak on the unipolar outputpulse; this should be 2.2 :s for 2.2J.

g. Change the Shaping Time switch to 0.25 through6 :s. At each setting, check to see that the time tothe unipolar peak is 2.2J. Return the switch to 1 :s.

OVERLOAD TESTS Start with maximum gain,J=2:s, and a +10 V output amplitude. Increase thepulser output amplitude by X1000 and observe thatthe unipolar output returns to within 200 mV of thebaseline within 27 :s after the application of asingle pulse from the pulser. It will probably benecessary to vary the PZ Adj control on the frontpanel in order to cancel the pulser pole andminimize the time required for return to thebaseline.

LINEARITY The integral nonlinearity of the 671can be measured by the technique shown inFig. 6.1. In effect, the negative pulser output issubtracted from the positive amplifier output tocause a null point that can be measured withexcellent sensitivity. The pulser output must bevaried between 0 and 10V, which usually requiresan external control source for the pulser. Theamplifier gain and the pulser attenuator must beadjusted to measure 0 V at the null point when thepulser output is 10 V. The variation in the null pointas the pulser is reduced gradually from 10 V to 0 Vis a measure of the nonlinearity. Since thesubtraction network also acts as a voltage divider,this variation must be less than

(10 V full scale) x (±0.025% maximum nonlinearity) x (1/2 for divider network) = ±1.25 mV

for the maximum null-point variation.

6See IEEE Standards, No. 301-1976.

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Fig. 5.1. Circuit Used to Measure Nonlinearity.

OUTPUT LOADING Use the test set up of Fig. 5.1. Adjust the amplifier output to 10Vand observe the null point when the front paneloutput is terminated in 100S. The changeshould be <5 mV.

NOISE Measure the noise at the amplifierUnipolar output with maximum amplifier gain and2-:s shaping time. Using a true rms voltmeter,the noise should be less than 5 :V x 1500 (gain),or 7.5 mV.

For an average responding voltmeter, the noisereading would have to be multiplied by 1.13 tocalculate the rms noise. The input must beterminated in 100S during the noisemeasurements.

5.3. SUGGESTIONS FORTROUBLESHOOTING

In situations where the 671 is suspected of amalfunction, it is essential to verify suchmalfunction in terms of simple pulse generatorimpulses at the input. The 671 must bedisconnected from its position in any system, androutine diagnostic analysis performed with a testpulse generator and an oscilloscope. It isimperative that testing not be performed with asource and detector until the amplifier performssatisfactorily with the test pulse generator.

The testing instructions in Section 5.2 shouldprovide assistance in locating the region oftrouble and repairing the malfunction. The twoside plates can be completely removed from themodule to enable oscilloscope and voltmeterobservations.

5.4. FACTORY REPAIR

This instrument can be returned to the ORTECfactory for service and repair at a nominal cost.Our standard procedure for repair ensures thesame quality control and checkout that are usedfor a new instrument. Always contact CustomerServices at ORTEC, (865) 483-2231, beforesending in an instrument for repair to obtainshipping instructions and so that the requiredReturn Authorization Number can be assigned tothe unit. This number should be marked on theaddress label and on the package to ensureprompt attention when the unit reaches thefactory.

5.5. TABULATED TEST POINTVOLTAGES

The voltages given in Table 6.1 are intended toindicate typical dc levels that can be measured onthe printed circuit board. In some cases the circuitwill perform satisfactorily even though, due tocomponent tolerances, there may be somevoltage measurements that differ slightly from the

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listed values. Therefore the tabulated valuesshould not be interpreted as absolute voltages butare intended to serve as an aid introubleshooting.

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Pin Function Pin Function1 +3 V 23 Reserved2 - 3 V 24 Reserved3 Spare bus 25 Reserved4 Reserved bus 26 Spare5 Coaxial 27 Spare6 Coaxial *28 +24 V7 Coaxial *29 - 24 V8 200 V dc 30 Spare bus9 Spare 31 Spare

*10 +6 V 32 Spare*11 - 6 V *33 117 V ac (hot)12 Reserved bus *34 Power return ground13 Spare 35 Reset (Scaler)14 Spare 36 Gate15 Reserved 37 Reset (Auxiliary)*16 +12 V 38 Coaxial*17 - 12 V 39 Coaxial18 Spare bus 40 Coaxial19 Reserved bus *41 117 V ac (neutral)20 Spare *42 High-quality ground21 Spare G Ground guide pin22 Reserved

Pins marked (*) are installed and wired inORTEC’s 4001A and 4001C Modular SystemBins.

Bin/Module Connector Pin Assignments For Standard Nuclear Instrument Modules

per DOE/ER-0457T.