Photon Detector

. .Solid-StatePhoton DetectorOperators Manual.B.t:.1 GN % P- CLS-OCII I .. GMX SeriesGAMMA-X@ HPGe(High-Purity Germanium)Coaxial Photon Detector System

GMX SeriesGAMMA-X@ HPGe(High-Purity Germanium)Coaxial Photon Detector SystemPrinted in U.S.A.

FOR INSTRUCTIONS ON THE USE OF THE NDR OPTION PLEASE CALL GREG MARTINAT ORTEC Phone!! (615)483-2190.

CONTENTSPageQUALITY ASSURANCE DATA SHEET............................ i1 . GETTING STARTED WITH A NEW DETECTOR SYSTEM .................. 12 . DETECTOR SYSTEMS GENERAL INFORMATION ...................... 32.1. The Basic Detector Element ............................. 32.2. EG&G ORTEC Photon Detectors .......................... 42.3. Cryostat-Dewar Configurations ........................... 52.4. System Electronics ................................. 82.5. Basic Operation .................................. 113 . RECEIVING AND INSPECTION .............................. 123.1. General ...................................... 123.2. Unpacking Instructions ............................... 123.3. Shipping Damage .................................. 124 . FILLING WITH LIQUID NITROGEN (LNz) ......................... 134.1. Bucket Dewars (CFG-SL. .SD. or -SJ) ........................ 134.2. Dipstick Cryostat Models (CFG.VT. .HZ. .SV. or -SH) .................. 134.3. Filling With LN2 While Operating ........................... 135 . SAFETY PRECAUTIONS ................................. 145.1. High Voltage .................................... 145.2. Liquid Nitrogen Safety ............................... 145.3. Beryllium Windows and Internal Cryostat Pressure .................. 145.4. Well Detector Systems ............................... 156 . FACTORY SERVICE ................................... 166.1. Warranty Statement and Return InstructionsTemperature Cyclable Detector Systems 17......................7 . SYSTEM OPERATION .................................. 187.1. Assembling an Energy Spectroscopy System ..................... 187.2. Cable Termination ................................. 187.3. Shaping Time, Noise. and Pulse Pileup ........................ 197.4. Pole-Zero and DC Output Level Adjustments ..................... 197.5. Common Setup Problems: Microphonics. Ground Loops. and Pickup ......... 217.6. The Initial Application of Bias ............................ 218 . GMX SERIES DETECTOR PERFORMANCE MEASUREMENTS .............. 238.1. Detector Resolution ................................ 238.2. Peak Width Ratios ................................ 248.3. Noise ...................................... 258.4. Peak-to-Compton ................................. 258.5. RelativeEfficiency ................................ 25 '8.6. Marinelli Beaker Standard Source Efficiency .................... 269 . MAINTENANCE AND tROUBLESHOOTlNG ....................... 289.1. Liquid Nitrogen Maintenance and Warm-up Protection ............... 289.2. Neutron Damage - Identification and Treatment .................. 289.3. Neutron Damage Resistance of the GAMMA-X Detector .............. 299.4. Troubleshooting .................................. 29

CONTENTS (contlnued)Fig . 1.1.Fig . 2.1.Fig . 2.2.Fig . 2.3.Fig . 2.4.Fig . 2.5.Fig . 2.6.Fig . 4.1.Fig . 5.1.Fig . 7.1.Fig . 7.2.Fig . 7.3.Fig . 8.1.Fig . 8.2.Fig . 9.1.ILLUSTRATIONSEnergy Spectroscopy System ............................ 1EG&G ORTEC Photon Detectors (cross section) ................... 4Comparison of Efficiency as a Function of Energy for the GAMMA.X. HPGe Coax. andLEPS Detectors .................................. 4Standard Configuration Options ........................... 6Standard Dewar Options .............................. 8Streamline Detector System Preamplifier ...................... 9The Full Width at Half Maximum (FWHM) of a Spectral Peak ............ 11Tubing Arrangement for Liquid Nitrogen Transfer .................. 13Simplified Isometric View of HPGe Well Detector Cryostat (GWL Series) ....... 15Connection of Components in a Typical Energy Spectroscopy System I........ 18Pole-Zero Adjustment as Seen With an Oscilloscope ................. 20Circuit Diagram for Clamp Network Used to Prevent Overloading the Input to theOscilloscope Vertical Amplifier ........................... 20Marinelli Beaker Geometry ............................. 26Well Detector (EG&G ORTEC Style) Geometry .................... 27Tailing on the Low-Energy Side of 1.17- and 1.33-MeV Photopeaks Due toRadiation Damage ................................. 28

1. GETTINGSTARTED WITH ANEW DETECTORSYSTEMIn order to become familiar with the operation of your newdetector system, we suggest that you install it as part of anenergy spectroscopy system, and measure its basic performancespecifications. This procedure verifies thewarranted specifications and establishes that there wasno shipping damage. Also, if you establish the detector'sperformance using your own spectroscopy system, anyfuture change in system performance may be accuratelyevaluated.Unpack and examine the detector system for any obviousshipping damage. Unpacking instructions are attachedto the outside of the shipping container and may also befound in Section 3 of this manual.Before the instrument is connected to external electronics,the detector element must be properly cooled byfilling the dewar with liquid nitrogen. A waiting period ofat least six hours is normally required before the detectoris placed in operation, unless EG&G ORTEC providesother information for specific detector systems (e.g., theGamma Gage Tu ). Otherwise, damage such as failure of theinput field effect transistor (FET) may render the systeminoperable. Extended overfilling or any splashing of liquidnitrogen over the cryostat flanges may damage vacuumseals and must be avoided. Complete filling instructionsmay be found in Section 4 of this manual (or in supplementaryliterature in thecaseof the Gamma Gageor otherunique systems). General safety precautions for the useof liquid nitrogen are given in Section 5.2.The fully cooled detector system may be connected to theother components of an energy spectroscopy system(Fig. 1.1). The illustrated connections are made withappropriate coaxial cable (see Section 7.1). Consult theQuality Assurance Data Sheet in the front of this manualto find the proper bias polarity and voltage. Be sure thatthe high voltage power supply is adjusted for the properpolarity and initially set at zero. After all other systemcomponents are operating, bias may be applied to thedetector. On the initial application of bias, it is particularlyimportant to observe the amplifier output on an oscilloscopewhile the bias voltage is slowly increased to therecommended value. The characteristically high noiselevel of an unbiased detector should decrease smoothlyas the applied voltage causes the detector to deplete ofcharge carriers. The baseline should be carefullyexamined for any spurious oscillations. (See Section 7.5for a description of common set-up problems and how toeliminatethem.)*Radiation SourceUnipolaroutput, ,--Input ,.-PREAMPLIFIERAMPLIFIERPULSEHEIGHT' 1 FILTERI1HlGHVOLTAGE,-- InputI Attenuated I OutputI1 1 PULSEI1GENERATORHIGH VOLTAGEBIAS SUPPLY'0-5 kVOutput'Bias supply should beoff and set at zerountil system is readyto use.Fig.l.1. Energy Spectroscopy System.

When your detector is properly biased, expose it to aradioactive source while watching the oscilloscope. Asthe source is brought closer to the detector, look forintense pulses of one or more specific heights. Thesecorrespond to characteristic x-ray or ?-ray lines of specificenergies and will produce spectral peaks on a pulseheight analyzer. You will notice many pulses of randomheight. These correspond to the observed spectral backgroundwhich results primarily from Compton scatteredphotons that do not deposit all of their energy within thedetector.The amplifier gain should be adjusted until one or moresignal pulses, representing energies of interest, have aheight somewhere within the range of the pulse heightanalyzer input. An appropriate shaping time should beselected (Section 7.3). A good initial choice is theshapingtime which is listed on the Quality Assurance Data Sheetas having been used for resolution measurements atEG&G ORTEC. Amplifier pole-zero and dc output leveladjustments should be made as described in Section 7.4.Now a spectrum can be accumulated on the pulse heightanalyzer and its basic characteristics examined.The spectral peaks should be nearly symmetrical,approaching a Gaussian shape. No "tail" should be seenon the higher energy side of any peak, and tailing on thelower energy side should be minimal. Peak asymmetryand tailing are especially prominent on asemilogarithmicdisplay of pulse height (counts per channel) versuschannel number with an expanded horizontal scale.Excessive tailing observed during the initial use of adetector system is often caused by improperly adjustedpole-zero (Section 7.4) or by an incorrect bias voltage.The detector system's performance specifications can bemeasured. Begin by measuring the spectral resolution atthose energies for which it is warranted. Section 8.1explains how to determine the full width at half maximum(FWHM) of a spectral peak as a measure of system resolution.The detector system resolution is energy dependent,and results obtained at different energies cannot bedirectly compared. Resolution is especially sensitive toany problems within the spectroscopy system which mayresult in excess electronic noise. However, neutrondamage will result in worsening resolution, especially athigher energies, without any increase in system noise.(Do not warm up a neutron damaged detector. See Section9.2 for details.)Spectral peak width may also be determined at specificheights other than the half maximum. The full width attenth maximum (FWTM) and full width at fiftieth maximum(FWFM) are often measured, and the tenth-to-halfratio (FWTM/FWHM) and fiftieth-to-half ratio (FWFM/FWHM) are calculated. These ratios are good indicatorsof the quality of the peak shape and are sensitive to peakasymmetry and tailing.The measurement of system noise, as described in Section8.3, is useful for unfolding the components (electronicnoise and detector contribution) of the total spectralresolution. A knowledge of the noise performance ofyour detector when it is operating properly may aid in thediagnosis of system problems which may occur.All other warranted specifications of your detectorshouldbe measured according to the techniques described inSection 8. If any problems arise, first be sure that all componentsof the spectroscopy system are properlyadjusted as described in Sections 7 and 8and in the individualcomponent instruction manuals. Be sure that allmeasurements have been made correctly. If problemspersist, examine the output signal on the oscilloscope andlook for spurious oscillations, unusual pulses, or otheranomalies. Consult Section 9.3 for detailed troubleshootinginstructions.With the preliminary measurements completed, you maywish to acquire spectra from several radioactive sourcesand calculate energies and intensities. A calibration ofdetector efficiency as a function of energy is usuallydesirable. Measuring resolution, noise, and dead time atvarious time constants and count rates may be useful.You may contact the EG&G ORTEC Customer ServiceDepartment in Oak Ridge, Tennessee at (615) 482-441 1,or your local EG&G Instruments Office if outside theU.S.A., to report any problems or to obtain needed assistance.If you are returning a detector system, you mustcontact the factory so that a Return Authorization Numbermay be assigned to it. Under no circumstancesshouIda detector with suspected radioactive contamination bereturned to EG&G ORTEC (Section 6).

2. DETECTOR SYSTEMS GENERAL INFORMATIONEG&G ORTEC offers a complete line of semicoriductorphoton detectors for nuclear spectroscopy in the energyrange of 1 keV to 10 MeV. All EG&G ORTEC photon detec- .tors operate at near liquid nitrogen temperature in orderto lower the leakage current and hence the noise of thesystem. Each photon detector system consists of (1) asemicor,ductor detector element, (2) a cryostat whichmaintains the detector element in a vacuum at cryogenictemperature, (3) a liquid nitrogen dewar, and (4) electronicsconsisting of a low noise charge-sensitive preamplifierand a high-voltage filter.2.1. THE BASIC DETECTOR ELEMENTThe semiconductor detector element is a single crystal ofgermanium or silicon that has been made into a diodecapable of withstanding high reverse biasvoltageat cryogenictemperature. Under these conditions, electron-holepairs produced by the absorption of an x-ray or gammarayphoton are swept to opposite contacts by an electricfield. The resulting induced current pulse is integrated bya charge-sensitive preamplifier producing an output voltagepulse with height proportional to the incident photonenergy.Material, geometry, and contact structure are especially) important for characterizing detectors and determiningtheir suitability for partic'ular applications (Table 2.1).Germanium, with its higher atomic number and largerphoton absorption cross section, is more suitable for thedetection of higher energy photons, and yet remainsuseful at energies as low as 3 keV. Silicon detectors areuseful as low as 1 keV and have less background attributableto higher energy photon scattering and to escapepeaks.The basic detector element configurations are shown inFig. 2.1. Principal detector geometries are planar andcoaxial. The planar geometry is useful for constructingsmall diameter, low capacitance devices which have lownoise, an essential factor for obtaining high resolution atlow energies. In addition, large diameter planar detectorscan be constructed to achieve large front surface areaswith thin entrance windows. This is an efficient structurefor detecting lower energy photons which are mainlyabsorbed at relatively shallow depths.In the closed-end coaxial configuration, the outercontactend is closed, blending into the outside cylinder surface.The coaxial geometry makes possible detectors withlarge volumes that are especially efficient for detectinghigher energy gamma rays for which absorption crosssections are small. Because of their low capacitance,closed-end detectors have usually had low noise andsuperior resolution performance. Now, EG&G ORTEChas developed a closed-end geometry, based on anexceptionally deep center contact together with otherrefinements, which has excellent charge collection andtiming performance while maintaining the superior resolutionof a closed-end detector. This geometry is used forall EG&G ORTEC high-purity germanium (HPGe) coaxialdetectors, both p-type and n-type.Table 2.1. Characteristics of EG&G ORTEC Photon Detectors.(See Fig.2.1 for Functional Drawings.)DetectorElementDetector Window UsefulDetector Semiconductor Type and Thickness EnergySeries Material Geometry (Microns) RangeGEMGMXGWLLO-AXGLPSLPP-TY peHigh-PurityGermaniumn-TypeHigh-PurityGermaniumP-TY peHigh-PurityGermaniumn-TypeHigh-PurityGermaniumP-TY PeHigh-PurityGermaniumLithium-DriftedSiliconClosed-End 500-800 40 keV-10 MeVHPGe CoaxialClosed-End 0.3 3 keV-10 MeVHPGe CoaxialHPGe Well 0.3 10 keV-10 MeVClosed-End 0.3 3 keV-1 MeVHPGe CoaxialPlanar/LEPS 0.3 3 keV-1 MeV

2.2. EG&G ORTEC PHOTON DETECTORSa) GEM SeriesP-typeHPGe (High-PurityGermanium) Coaxial Detectorb) GMX Series (GAMMA-X)1 n-typeHPGe Coaxial DetectorIIc) LO-AX Seriesn-typeHPGe Coaxial Detectord) GWL SeriesHPGe Well DetectorAluminum EndcapAll EG&G ORTEC photon detectors can be repeatedlycycled to room temperature without performance degradation.The GEM Series HPGe (High-Purity Germanium) CoaxialDetector [Fig. 2.1 (a)] uses p-type material with a lithiumdiffused outer contact. The inner contact is ionimplantedfor the ultimate in reliability. The EG&GORTEC closed-end geometry is especially designed tomaximize charge collection and timing performance.The GMX Series GAMMA-X@ Detector [Fig. 2.1 (b)] is atotally new type of coaxial HPGe detector developedin our R&D laboratories. This detector is made of n-type high-purity germanium which permits the entireouter contact to be ion-implanted. The use of a thin,rugged ion-implanted contact as a wrap-around entrancewindow allows the GAMMA-X detector to be used effectivelyat energies as low as 3 keV. It is also useful at highenergies (up to -10 MeV) like other coaxial germaniumdetectors.The GAMMA-X detector is the only semiconductor spectrometerwith high precision and efficiency for bothgamma- and x-ray spectroscopy. This is well illustratedby a comparison of the relative efficiencies of a 10°h GMXSeries GAMMA-X detectorwith an 11% GEM Series HPGeCoaxial Detector and an 8 cm2 x 1-cm deep GLP Series,HPGe LEPS Planar Detector (Fig. 2.2). In order to allowfull benefit of these capabilities, the GAMMA-X detector issupplied with a large beryllium window.@Registered in United States Patent and Trademark Office.II10% RELATIVE EFFICIENCYGAMMA-X DETECTORe) GLP Series and SLP SeriesHPGe LEPS (Low EnergyPhoton Spectrometer) andSilicon LEPS Planar Detectors11% RELATIVE- EFFICIENCY -HPGe COAXIALDETECTOR- -8 crn 2 1-cm DEEPHPGe LEPSPLANAR DETECTORThick Contact (e.g., lithium diffusion)500-1000 micronsThin Contact (e.g., ion implantation) - - - - - -

'The characteristics of the GAMMA-X photon detectorgive it additional advantages. For a given O/O relative efficiency(=OCo at 25 cm), it has the highest Marinelli beakerefficiency (see Section 8.6) of any type of germaniumdetector. Also, its thin outer dead layer enhances theeffectiveness of its performance when used with a Comptonsuppression system.Recently published data shows that a GAMMA-X detectoris at least 25 times more resistant to neutron radiationdamage than a conventional germanium detector of thesame size.lv2 The GAMMA-X detector provides maximumreliability and exceptional value for use in environmentswhere neutron damage is probable.The GWL Series HPGe (High-Purity Germanium) .WellDetector [Fig. 2.1 (d)] has a well in the center of the endcapwhich extends deeply into the germanium crystal.Thus radioactive samples may actually be placed in themiddle of the detector element. Unlike other well detectorswhich have a hole all the way through thegermaniumcrystal, EG&G ORTEC's well detector has a blind holewith at least 5 mm of active germanium at the bottom ofthe hole. This geometry (nearly 47r) provides the maximumcounting efficiency available with a germaniumdetector,ness of the beryllium window. Within this energy range, asilicon detector is often preferable to a germanium LEPSdetector because the germanium detector is not usablebelow approximately 3 keV, and at higher energies Geescape peaks are present.EG&G ORTEC detectors have high-resolution at both lowand high count rates. The limiting factor is often not thecount rate, but rathertheenergy rate. Therefore detectorsfor high count rate applications should be specifiedaccording to the intended energy range. Special EG&GORTEC SLP Series detectors function usefully at ratesabove 500,000 cps at 60 keV. Special GLP Series detectorsfunction usefully above300,000cps at 122 keV. StandardGEM and GMX Series detectors are capable of effectiveperformance at 1.0 MeV with rates up to 180,000 cpsfor the GEM and 140,000 cps for the GMX.2.3. CRYOSTAT-DEWAR CONFIGURATIONSThe cryostat-dewar system maintains the detectorelement in a high vacuum at close to liquid nitrogen temperature.The dewar serves as a reservoir of liquid nitrogenwhile the cryostat provides a path for heat transferfrom the detector element to the liquid nitrogen reservoir.Both the dewar and the cryostat rely upon a vacuum toThe large (1.O- or 1.45-cm diameter, 4.0-cm useful length) insulate cold inner parts from the outer surfaces.standard accommodates a wide range Of sample The cryostat provides an outer envelope which can besizes. The well is surrounded by an ion-implanted centermaintained internally at high vacuum. In addition, thecontact, with an entrance-window thickness of only 0.3cryostat contains a mount for the detector element andmicrons of germanium. The EG&G ORTEC Well Detec- associated electronic components. vacuum feedtorprovides the user with the widest range (lo keV throughs in the cryostat allow electrical cbnnectionsto beto MeV) and the efficient counting geometrymade from the detector to the preamplifier and high-any well detector available.The LO-AX Series Detector [Fig. Z.l(c)] is an HPGe(high-purity germanium) detector in the coaxial configurationwith a thin,ion-implanted front contact. It isuseful at any energy between 3 keV and 1 Mev where largesurface area and superior resolution are important. It isparticularly useful in actinide x-ray spectroscopy.The GLP Series Low-Energy Photon Spectrometer(LEPS) [Fig. 2.1 (e)] is an HPGe (high-purity germanium)detector in the planar configuration with a thin, ionimplantedfront contact. The cryostat endcap has a berylliumwindow to maximize its low-energy efficiency. TheGLP Series LEPS is useful over the approximate energyrange of 3 keV to 300 keV.The SLP Series Lithium-Drifted Silicon Detector [Fig. 2.1(e)] is a planar (LEPS) x-ray detector with a useful energyrange from less than 1 keV to approximately 30 i.cV,depending on the resolution of thedetector and the thick-voltage filter.All EG&G ORTEC cryostats contain a cryogenic vacuumpump. This consists of one or more reservoirs of rnolecularsieve - a pelletized material with an extremely largesurface-to-volume ratio which absorbs large volumes ofgas when cooled to liquid nitrogen temperature. Thismethod of cryosorption pumping provides a good cryostatvacuum for continuous operation over a period ofapproximately five years. The sealed system requires noexternal pumping, and the only maintenance requir~c' :,-;periodic filling with liquid nitrogen while in use.EG&G ORTEC produces a wide range of cryostat-dewarconfigurations as standard options (Fig. 2.3). All configurationsare shown equipped with 30-liter dewars exceptfor the hand-held Gamma Gage model. Standard optiondewars (Fig. 2.4) may be appropriately combined with thevarious cryostats. Other configurations are available onrequest, and custom configurations can be supplied tomeet your special requirements.(''R.H. Pehl, "Germanium Gamma-Ray Detectors," Physics Today(November 1979).2R. H. Pehl, N. W. Madden, J. H.Elliot,T.W.Raudorf,R.C.Trammell,andL.S. Darken,Jr.lEEE Trans. Nucl. Sci. NS-26N1,322(1979).

(2.750 in.)l7.5 cm(2.950 in.)21cm(B1H in )12cm(4-518 in.)(E-114 in.)88 cm(34-112 in.)62 cm,MODELCFG-SV(27-7116 in.)62 cm(24-112 in.).iMODELCFG-SHL(lz;2n.) 4VERTICAL STREAMLINE MODEL CFG-SV(Supplied as standard on GEM and GMX Serles)HORIZONTAL STREAMLINEMODEL CFG-SHCFG-GGCFG-GGbaf13 25 t .12 in.)-MODEL CFG-GGWITH DEWAR MODEL DWR-0.4G(8-Hour Holding Time)MODEL CFG-GGWITH DEWAR MODEL DWR-1.2G(24-Hour Holding Time)Coax. Large Diam.125O10 Coax 228% LEPSa 6.98 + .08 8.26 rt .08 6.98 f .08(2.75 f .03) (3.25 f .03) (2.75 + .03)b 7.49 f .08 8.76 + .08 7.49 -+ .08(2.95 + .03) (3.45 f .03) (2.95 f .03)c 11.76 =t .30 11.76 f .30 6.99 1 .30(4.63f .12) (4.63f .12) (2.751 .12)d 20.65 f .51 21.92k .51 15.88 + .51(8.13 f .20) (8.63 f .20) (6.25 1 .20)e 18.34 + .51 19.61 + .5i 13.56 -+ .51(7.22 f .20) (7.72 + .20) (5.34 1 ,201f 48.49 f 1.27 49.76 rt 1.27 43.74 -+ 1.27(19.09 f .50) (19.59 f .50) (17.22 1 .50)Coax Large Diam.525% 228% LEPSFig. 2.3.Standard Configuration Options.''Systems shown are for coaxial detectors. GLP and SLP Series streamline systems have endcaps 5-cm shorter than those shown.

7.5 cm12 cm(4-518 in.)62 cm(8-114 in.)CFG-SLSIDE LOOKINGMODEL CFG-SL7.5 cm(27-7116 in.) .ll62 cm(24-112 in.)MODELCFG-SHDWR-30 OP/23 cm(9-3116 in.)L ( l ~ i ? ~ . ) - -STREAMLINED HORIZONTALIN OFFSET PORT DEWARMODEL CFG-SH; DWR-30 OP(3 In.)J STREAMLINE MODEL CFG-SJ749f 008~mMODELCFG-N-SH(Same dimensions as CFG-SH except as noted)MODEL CFG-SHP and MODEL CFG-AMFlg. 2.3.(Contlnued).

(16 In I140.3 f 2.51 cm30-LITER DEWAR(Supplied standard except as noted)MODEL DWR-308 cm(3-114 in.) 12 cm ., (4-34 n, t-T- 1ji--I62 cm(24-112 tn.)53 crn '(21-114 in.)42.9 cm(16-7/8 in.)(Hsighl dependenl on type- ,3:;:" ,-pi30-LITER STORAGE/FILL DEWAR FOR THEGAMMA GAGE PORTABLE SPECTROMETERMODEL DWR-SIFL7(1 7-3/4 cm in.)9.1 cm/'(3.62 in.)I 1.7.6cm(3 in.)~ ( 1 7 % ? " . , 430-LITER OFFSET PORT DEWARMODEL DWR-30 OPJ6.9 cm 30.7 cm-.(2-314 in.) ---- '-(12.1 in.) I15 -LITER STAINLESS STEELBUCKET DEWARMODEL DWR-15sFig. 2.4. Standard Dewar Options.22.2 cm -I+-/r&314 in3' -'" (2.7 cm in.)7.5-LITER BUCKET DEWARMODEL DWR-7.5B2.4. SYSTEM ELECTRONICS The high-rate (or preamplifier over-range) indicator andhigh-voltage shutoff options are described in followingsections.Absorption of a photon by the detector produces a currentpulse at the preamplifier input (Fig. 2.5): Each currentpulse is integrated by the charge-sensitive loop which isessentially an FET input operational amplifier with capacitivefeedback. A feedback resistor Rr in parallel withthe feedback capacitor Cr removes charge stored by theinput capacitance. This results in the exponential decay,with time constant T = RrCr, of ea$h charge-loop outputpulse. The pole-zero cancellation network differentiatesthe charge-loop output to give a shorter decay time constantand is adjusted to eliminate ~ulse undershoot followingdifferentiation. The output driver stage is capableof driving at least 500 feet of properly terminated cable.The advanced hybrid- electronics manufacturing capabilityof EG&G ORTEC has allowed the incorporation ofthe detector element, preamplifier, and high-voltage filterinto a compact streamline system. These three componentsare contained in a cylinder less than three inches indiameter. Since there are no vulnerable, obstructing electronicmodules, the streamline system is ideal for applicationswhere active or passive shielding is required.The hybrid electronics preamplifiers are more reliablethan the older discrete-component type. The epoxy-

inside CryostatI/ Charge-Sensitive Loop \+ l III DETECTOR PZII - ~Pole-Zero \ Output Drtver \CancellationNetworkSignal OutputA --II 8- High-Rate(or PreamplifierOver-Range)- lndtcator3tTemperature SensorH.V. FILTER .laAutomaticHigh-VoltageShutollFig. 2.5. Streamilne Detector System Preamplifier.Simplified Schematic with High-Rate.and AutomaticHigh-Voltage Shutoff Options.potted high-voltage filter used with a streamline systemis also trouble free. Because both of these units arethoroughly tested under standard operating conditionsbefore le&ing the factory, the probability is very low thatyour detector system electronics will ever need servicing.If the need to service the electronics of your streamlinesystem should arise, we strongly recommend that you donot open the closed electronics shield unless you areacting under explicit instructions from an authorizedrepresentative of EG&G ORTEC. If your system is notworking properly, get prompt assistance by calling EG&GORTEC Customer Service (615) 482-441 1, or by contactingyour local EG&G Instruments Office if outside theU.S.A.The hybrid electronics and potted high-voltage filter arenot difficult to service in the field. Most often, all that isneeded is to replace a small plug-in hybrid module. However,these are only obtainable through EG&G .ORTECand must be carefully installed by a trained person.The following specifications are for the EG&G ORTECStreamline Cryostat Configuration Hybrid Preamplifiers,and High-Voltage Filters. For systems containing thediscrete-compone'nt 117-Series Preamplifier or 120 Preamplifierand 119 High-Voltage Filter, consult the separateOperating and Service Manuals for information.STREAMLINE CRYOSTAT CONFIGURATIONHYBRID PREAMPLIFIER ANDHIGH-VOLTAGE FILTER SPECIFICATIONSSince essentially all of the electronic specificationsdepend to some extent on the compone'nts within thecryostat, the following should be interpreted as resultantsystem specifications. Furthermore, due to the variabilityin these components, these specifications should beunderstood as typical values.GEM and GWL Series137CP2 (Preamplifier) and 138 (H.V. Filter) CombinationTEST INPUT One 18-in. RG174 coax cable with femaleBNC connector.HIGH-VOLTAGE BIAS INPUT One 18-in. RG59 coaxcable with female SHV connector.OUTPUTS Two 18-in. RG!74 coax cables with femaleBNC connectors.CABLE DRIVE CAPABILITY AND TERMINATION Testinput terminated in 93R; Outputs are series terminated in93n and may drive terminated or unterminated 93R coaxcables (RG62 recommended). Termination recommendedfor cable lengths greater than 50 ft.

CONVERSION GAlN Nominally 400 mV/MeV (Ge),positive output pulse signal.RlSETlME Pulser risetime typically 25 ns; actual risetimeto nuclear event depends on detector characteristics.MAXIMUM OUTPUT Maximum pulse output to a singleevent is +10 V.MAXIMUM ENERGY RATE With standard 2000 MRfeedback resistor, maximum average energy rate untilpreamp saturation is 180,000 MeV/%NONI,INEARITIES Integral and differential,

if the detector temperature becomes too high. Although noalarm is provided, the bias supply meter will indicate zerovoltage, and system noise will greatly Increase after shutoffoccurs. For the unit to be operational, preamplifier powermust be provided through the power cable. After the highvoltage has been automatically shut off, the bias supplyswitch must be turned off until the system has been filledwith liquid nitrogen and completely cooled for the recommendedperiod (see Quality Assurance Data Sheet in thefront of this manual or label on detector system). Accidentalapplication of hlgh voltage to a detector which is not fullycold can cause serious damage and void your warranty.The automatic shutoff should be placed in operation beforeattempting to apply bias to the detector. ~ hus the circuit willalso prevent the accidental application of bias to a detectorwhich has not yet reached operating temperature. This is asignificant advantage over the liquid nitrogen level monitorwhich is ba:ed on a temperature sensing probe in thedewar. However, the EG&G ORTEC Model 729A LiquidNitrogen Level Monitor provides an alarm feature which isnot practical with the automatic shutoff located within thepreamplifier. Also, a liquid nitrogen level monitor providesan earlier warning allowing the addition of liquid nitrogenbefore the detector temperature is affected.High-Rate IndicatorFor any dc-coupled charge-sensitive preamplifier, if theenergy rate (count rate X energy product) exceeds a givenlevel (value dependent on the particular system), the preamplifierwill shut off. As the energy rate approaches this level,the detector system may suffer from excessive resolutiondegradation and peak shift. If the energy rate hovers aroundthe shutoff level, the preamplifier may turn on and offintermittently. Obviously, data collected under such conditionsare not reliable and corrective action should betaken.K ~ D LGDA hybrid circuit within the preamplifier monitors the chargeloop output voltage. When a condition of excessively highrate exists, an output is provided which is suitable forlighting an LED located in the preamplifier shield.----.-A- .-- --/Thegamma ray or X-ray and produces a current pulse whoseintegral is proportional to the absorbed energy. This pulsedetector element absorbs the energy from an incident- 2.4 dcCU bqsf'-is integrated, converted to a voltage pulse, and shaped by Ga ;-the preamplifier. Additional amplification and pulse shaping ,is provided by the spectroscopy or shaping amplifier. ' I-Output pulses from this amplifier go to the pulse height '&

3. RECEIVING AND INSPECTION3.1. GENERALEach EG&G ORTEC detector system is shipped in either acustom-built wooden crate or a heavy cardboard box inwhich the system is protected by foam. On the outside ofeach container is a set of unpacking instrtrctions. Instructionsfor unpacking detector systems are given here forreference. Read these instructions before unpacking yourdetector.High-purity detectors of all kinds are normally shipped atroom temperature (without LNz). There arecertain exceptionsto this such as returned detectors which might 'beneutron damaged. High-purity detectors should beun~acked before filling with liquid nitrogen. IT ISIMPERATIVE THAT ANY DETECTOR SYSTEM PACKEDIN FOAM BE COMPLETE'LY UNPACKED BEFOREFILLING. If the detector is filled while in the foam, theinsulating properties of the foam, together with the tendencyof any spilled liquid nitrogen to become trappedbetween the detector cryostat-dewar and the foam, maycause serious damage to the system. A COLD DETEC-TOR MUST NOT BE ENCLOSED IN THERMALLY INSU-LATING MATERIALS AT ANY TIME.3.2. UNPACKING INSTRUCTIONSIf your detector system is packed in a cardboard box,simply remove external strapping and open the box.Remove the top layer of foam, then lift out your detector'system. DO NOT ATTEMPT TO FILL A DETECTOR SYS-TEM THAT IS STILL SITTING IN FOAM. Immediatelyinspect the detector system for physical damage. Ifdamage is evident, see Section 3.3.If your detector system is packed in a wooden crate, thefollowing instructions should be carried out.-1. Remove external strapping from around the crate. Ifthe crate has a plywood top, remove the six wing nutswhich are accessible through the crate sides near thefloor. When the top cover is free from the pallet base, carefullylift it straight up to uncover the detector system.Some custom crates may require extra steps to removethe top.2. Cut and remove the strapping that secures the cryostatand preamplifier to the dewar handles, Inspect theassembly for physical damage that might have occurredduring shipment. If damage is evident, see Section 3.3.3. Cut the straps that hold the dewar to the pallet baseand remove the complete detector system from the crate.The pad under the dewar should be used for shockmounting of the system during operation. Store the cratefor possible re-use at a later date.4. The dewar of a high-purity detector may be filled withliquid nitrogen at this time. Afterfilling iscomplete, wait atleast six hours before applying bias unless you havereceived supplementary filling instructions with yoursystems which specify otherwise (e.g., Gamma Gage).3.3. SHIPPING DAMAGEIf a detector arrives with externally visible damage, DONOT UNPACK IT. Notify the carrier and make arrangementsto file a damage claim. IN ALL CASES OF SHIP-PING DAMAGE, IT IS THE CUSTOMER'S RESPONSI-BILITY TO FILE A DAMAGE CLAIM.If during unpacking, concealed damage is noted, notifythe carrier and file a claim. Packing materials, waybillsand other such documentation should be preserved toestablish claims.Contact EG&G ORTEC Customer Service Department,(615) 482-4411, for further instructions. Outside theU.S.A., contact your local EG&G Instruments Office.INOTICEBefore opening crate, Inspect carefully for shippingdamage. l'f damage is evldent, see Sectlon 3.3.I

4. FILLING WlTH LIQUID NITROGEN (LN2)Filling the dewar of a detector system with liquid nitrogen One common filling method uses a standard 30-litermay be accomplished easily and safely. However, proper dewar of liquid nitrogen as a supply dewar (Fig. 4.1). Aprocedures must be followed to avoid personal injury or gas-tight fixture, which holds a metal outlet tube and a gasdetector system damage. Please read Section 5.2 on inlet for pressurization, is attached to the supply dewarliquid nitrogen safety as well as the following filling in- flange. The metal outlet tube is connected to a shortstructions.length of plastic tubing which serves as the supply hose.In all cases, it is necessary to prevent the electronics andThis tube carries liquid Out the dewarthe outside of the cryostat from getting excessively cold.bottom. Transfer is e f fected by pressurizing the dewar atAvoid spilling liquid nitrogen on the cryostat or elec- 3 to 5 psi with gas. The supply dewar or thetronics, and vent cold gas away from the system being gas inlet tube must have a pressure relief valve set at 5 psifilled. A detector may also be damaged by filling it with (see Safety Precautions in Section 5). Terminate thelihuid nitrogen while it is enclosed in thermally insulating liquid nitrogen transfer by relieving the pressure in thematerials (such as packing materials). NEVER FILL A supply dewar. This transfer requires only a few minutes,DETECTOR BEFORE REMOVING IT FROM ANY and it should be monitored continuously. Allow supplyINSULATING PACKING MATERIALS, ESPECIALLY IF and exhaust hoses to thaw completely before removingIT IS PACKED IN FOAM. However, a detector may be them the fill tubesfilledwhile still strapped to its shipping pallet. Even if a Alternatively, asimple LN2fillsystemcan becreated usingcold system is later packed in insulating materials, a funnel and a short length of plastic tube attached toonedamage is likely to occur. A COLD DETECTOR MUST of the fill tubes. LN2 is then poured continuously into theNOT BE ENCLOSED IN THERMALLY INSULATING fill line until it overflows the exhaust tube line. Care mustMATERIALS AT ANY TIME. Following these precautions be taken not to spill LN2 on the cryostat, endcap, or elecwillprevent damage to vacuum seals or to system elec- tronics section of the detector system.tronics which could occur from excessive chilling.4.1. BUCKET DEWARS (CFG-SL, -SD, or -SJ)Systems with bucket dewars (see CFG-SL, -SD, or -SJ inFig. 2.3) are filled by simply removing the dewar fill capand pouring in liquid nitrogen from the top. Care must betaken not to spill liquid nitrogen on the cryostat. If liquidnitrogen contacts a flange which contains a vacuum seal,it is possible that the seal may be breached. To preventthis, it is strongly recommended that such systems befilled by carefully pouring in the liquid nitrogen through alarge metal funnel.' ,'4.2. DIPSTICK CRYOSTAT MODELS(CFG-SV OR -SH)The dipstick enters the dewar through a white RTV siliconerubber collar which forms a gas-tight seal againstthe dipstick and the dewar flange. The silicone collar containstwo'stainless Steel tubes which are used for fillingand gas exhaust. These tubes extend about 6 inchesdown into the dewar neck. In filling, liquid nitrogen entersthrough either tube, and the other exhaust tube will preventthe liquid level from rising within 6 inches of.thedewar flange, if there are no leaks at the collar. Thiskeeps the vacuum seal at the dewar flange from gettingtoo cold. IT IS IMPORTANT NOT TO DAMAGE THESILICONE RUBBER COLLAR. Do not useexcessive forceto attach or remove a hose from the fill tubes.Prepare for filling the dewar by connecting the supplyhose and an exhaust hose to the fill tubes in the siliconerubber collar. The exhaust hose is a &foot length ofplastic tubing which carries cold gas and liquid overflowaway from the cryostat and electronics. The connectionto your liquid nitrogen supply hose is made by a shortlength of plastic tube.Fig. 4.1. Tubing Arrangement for Liquid Nitrogen Transfer.4.3. ' FILLING WITH LN2 WHILE OPERATINGIt is possible that cold gas or liquid nitrogen may causetemporary moisture condensation within the electronicshousing during filling. Therefore, we strongly recommendthat all power.be removed from the detector electronicswhile the dewar is being filled with liquid nitrogen.However, if this presents serious operational problems,the procedures of Sections 4.1 and 4.2 may be used whileoperating if care is taken to assure that cold gas or liquiddoes not come in contact with the electronics housing orelectronic modules. Use a long plastic exhaust tube andcover the electronics with plastic sheets as needed toaccomplish this. Monitor the filling process very carefullyand stop immediately when the dewar is full.

5. SAFETY PRECAUTIONS5.1. HIGH VOLTAGE B. Personnel should avoid wearing anything capable oftrapping or holding spilled liquid nitrogen close to theirflesh. An impervious apron or coat, cuffless trousers, andhigh-topped shoes are recommended. Wear safetyglasses or, better yet, full-face protection. Remove allwatches, rings, bracelets, or other jewelry. When glovesare used to handle containers or cold metal parts, theyshould be impervious and sufficiently large to be easilytossed off the hand in case of a spill.Your detector system uses high voltage (up to 5000 V) tobias the detector element. The standard EG&G ORTECbias supplies made for use with photon detectors arecapable of delivering only very low current. However,your detector system has a high-voltage filter containingcapacitors capable of delivering a dangerously highcurrent for a brief time while being discharged (even if thebias supply has been disconnected). Such a discharge ispossible at the interface between the high-voltage filterand the cryostat feedthrough or within the filter modulebox (on non-streamline cryostat systems). These pointsare not accessible unless the closed electronics shield ormodule box is opened. Even when exposed, these pointsare sealed with wax or a rubber compound. However,such danger should be avoided by never opening theelectronics shield or module boxes except when followingthe explicit instructions of an authorized representativeof EG&G ORTEC.5.2. LIQUID NITROGEN SAFETYUsers of cooled detectors should be aware of the hazardsassociated with the cryogenic fluid being used. Threehazards in using liquid nitrogen are high pressure gas,contact with materials, and contact with personnel. Thelarge expansion ratio from liquid to gas (692 to 1) can producehigh pressures due to theevaporation of the liquid, ifthe container does not have adequate venting or pressurerelief provisions. Some materials become brittle andfracture when exposed to liquid nitrogen temperatures(77 K). For advice when selecting materials for use instoring and transferring liquid nitrogen, contact EG&GORTEC or your liquid nitrogen supplier.Other sources of information are safety manuals such asthe "CRC Handbook of Laboratory Safety," The ChemicalRubber Co., Cleveland, Ohio; and material codes such asthe American Society of Mechanical Engineers' "Boilerand Pressure Vessel Code, Section VIII." In addition topossible exposure to high pressure gas, another personnelhazard is burns similar to burns from high temperaturecontact. Eyes are especially vulnerable to thistype of exposure. It should be remembered, that althoughnitrogen gas is nontoxic, it is capable of causing asphyxiationby displacing air. TRANSFER LN2 ONLY IN AWELL-VENTILATED AREA.~&uid nitrogen is safely used everyday in factories suchas EG&G ORTEC and laboratories all over the world. Forsafety reasons, the following precautions should befollowed when working with liquid nitrogen.A. When using the filling procedures described inSection 4.2, use only dry nitrogen gas to pressurize thesupply dewar. Do not use air or oxygen because they maycontain moisture and oil which could freeze and causeblockage of the filling and/or vent tube. Use a pressurerelief of 5 psi on the supply dewar to avoid over-pressurizationin the event of ice blockage.C. Piping or transfer lines should always beconstructedso as to avoid trapping liquid nitrogen in the line. Evaporationcan result in pressure build-up and eventualexplosion of the line. If it is not possible to empty all lines,install safety relief valves and rupture discs.D. Vent storage containers to a well ventilated area ortothe outside to avoid build-up of nitrogen gas in the workarea.5.3. BERYLLIUM WINDOWSAND INTERNAL CRYOSTAT PRESSUREIf your detector cryostat is equipped with a berylliumwindow, an accidental rupture of the window will severelydamage the system. Thin beryllium windows, such asthose found on GLP and SLP Series systems, are especiallyfragile and can sometimes be ruptured by a lighttouch. The thicker beryllium windows found on GMXSeries detectors are somewhat tougher, but still relativelyfragile. You should avoid trouble by never allowing anythingto touch a beryllium window.If a beryllium window should rupture under normalcircumstances, it will implodeand personnel will normallynot be exposed to flying fragments and possible injury.Avoid injury by not handling any fragments of berylliumwith bare hands (tweezers are recommended).A more dangerous situation might result if the berylliumwindow were to rupture outwards because of a build up incryostat pressure. This can happen if your cryostatdevelops a leak while cold and then warms up after themolecular sieve has absorbed a large quantity of gas. Ifyour cold cryostat shows evidence of poor vacuum,' besure that theoriginal plastic cover is placed overthe berylliumwindow. If your window cover has a hole in it, coverthis hole with tape. Disconnect the system from externalelectronics. Do not warm up such a cryostat or take additionalaction except on lnstructlon from'EG&G ORTEC.Immediately contact EG&G ORTEC Customer Service oryour local EG&G Instruments Office If outside the U.S.A.Every cryostat made by EG&G ORTEC has a pressuresafety valve which should prevent beryllium windowsfrom rupturing outward. However, it is remotely possiblethat this safeguard could fail. Therefore, you should beaware of this possibility and act with appropriate caution.'Symptoms of a poor vacuum include an unusually cold cryostator endcap, moisture condensation or "sweating," or an outwardbulge to the end window on warm-up (extreme case).

5.4. WELL DETECTOR SYSTEMSStandard EG&G ORTEC Well Detector Systems (GWLSeries) have an aluminum well in theendcap. Surroundedby the detector element, this well allows high efficiencycounting of a source which,is placed inside. To reduceattenuation at lower energies, the aluminum well is relativelythin. Penetration of this well would cause loss ofvacuum resulting in ice condensation on the sensitivedetector surfaces and electronic components. Thus careshould be taken not to insert sharp objects or force oversizedsamples into the well. If reasonable care is exercisedwhen placing samples into the well, no problems shouldresult.Spilling corrosive chemicals or solvents into the wellshould also be avoided. To minimize the possibility ofcorrosion, the well should be kept clean and dry at alltimes.Fig. 5.1. Simplified Isometric View ofHPGe Well Detector Cryostat (GWL Series).Htgh-Purlly GerrnanlurnCryslat (Note the acllveGermanturn a1 thebollorn of Ihe well).'

6. FACTORY SERVICEEG&G ORTEC stands firmly behind all of its products. Wewill repair or replace any detector system shown to bedefective in accordance with the Detectorwarranty Statements(Sections 6.1 and 6.2). Also, EG&G ORTEC willrepair an older detector system for a reasonable price.Unless the detector element has been severely damaged,older detector systems can frequently be repaired toessentially new condition for only a fraction of the cost ofa new system.Your satisfaction is important to us. If you have a problemor need technical assistance with your detector system,please call us and ask for Customer Service. Outside theU.S.A., contact your local EG&G Instruments Office.If it becomes necessary to return a detector for repair,please consult the Return Instructions in Section 6.1 or6.2. It is essential that EG&G ORTEC be informed, eitherin writing or by telephone, of the nature of the problemand of the model and serial number. Failure to do so maycause unnecessary delays in getting the unit repaired. Inthe U.S.A. you may notify us of your need to return adetector by calling Customer Service, EG&G ORTEC,(615) 482-4411. Outside of the U.S.A., contact the nearestEG&G Instruments Office.The EG&G ORTEC standard procedure requires thatdetectors returned for repair be inspected immediately.They undergo the same seriesof final quality control testsused for new detectors. These tests establish the conditionand warranty status of the detector.Under no circumstances should a detector with radioactivecontamination be returned to EG&G ORTEC. If adetector is suspected of being contaminated, have itsurveyed before attempting to ship it. If a contaminateddetector needs to be returned to EG&G ORTEC, it must bedecontaminated and certified clean by the customer'sRadiation Safety Officer before shipping. Any costs orliability incurred by EG&G ORTEC as a result of receivinga detector with radioactive contamination will be theresponsibility of the customer who returned the detector.

6.1. WARRANTY STATEMENT AND RETURN INSTRUCTIONSTemperature Cyclable Detector SystemsWarranty StatementEGBG ORTEC warrants its Temperature Cyclable Photon Detector Systems to be free from defects in material andworkmanship for a period of one year after shipment. EGBG ORTEC guarantees that the detectorsystem will operatewithin the warranted specifications regardless of the number of thermal cycles between liquid nitrogen temperatureand room temperature to which the system has been subjected and regardless of how long the system hasbeen stored at room temperature during the one-year period. This warranty is subject to the following Customerobllgatlons:1. The cryogenic FET has not been damaged.2. The system has not been physically or electrically abused.3. The detector has not been subjected to neutron damage.Should the detector system fail, through no fault of the customer, within the warranty period, and having at all timesbeen handled in accordance with the above described customer obligations, it will be repaired or replaced withoutcharge, at EGBG ORTEC's option, and will be fully warranted to within the original.specifications for an extended3-month period or the remainder of the warranty period, whichever is longer.EGdG ORTEC's liability on any claim of any kind, including negligence, loss or damages arising out of, connectedwith, or from the performance or breach thereof, or from the manufacture, sale, delivery, resale, repair, or use of anyitem or services covered by this agreement or purchase order, shall in no case exceed the price allocable to the itemor service furnished or any part thereof that gives rise to the claim. In no event shall EGBG ORTEC be liable forspecial or consequential damages. EG&G ORTEC makes no otherwarranties, expressed or implied, and specificallyNO WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.lnstructlons for Returnlng a Temperature Cyclable Photon Detector SystemIf for any reason it becomes necessary to return a Temperature Cyclable Photon Detector System for repair or replacement,please note the following instructions and precautions regarding shipment.1. Contact EG&G ORTEC Contact the nearest EG&G ORTEC Office [in the U.S. call Customer Service,(615) 482-441 11 for specific instructions and authorization for returning the detector. Any detector thatis returned without prior notification having been given to EG&G ORTEC could result in loss or damageand would be automatically considered out-of-warranty.2. Proper Packlng A detectorWreturns kit" must be utilizedfor return shipment. The kit contains the necessaryinstructions and material to ensure proper packing. If you are unfamiliar with packing a detector or do nothave a returns kit, please contact the nearest EGBG ORTEC representative for assistance. Poor packingcould result in shipment damage that would not be covered under the EG&G ORTECwarranty orfreight forwarder'sinsurance policy.3. Shlpment Coordlnatlon and Communlcatlons ~etectorsho"ld be returned through the freight forwarderprepaid. For out-of-warranty repairs, the reshipment charges from EG&G ORTEC will be billed to thecustomer.For in-warranty repairs the reshipment charges from the factory will be paid by EGBG ORTEC.Should a detector system returned to EGBG ORTEC be found operating within specifications, thecustomerwill be charged an inspection fee and be responsible for freight charges to and from the factory, whether inwarrantyor out-of-warranty. Weekend delays in transit should be avoided; thus an early weekday shipmentis recommended. EGBG ORTEC must be notified of specific details of the returning shipment (i.e., freightforwarder, air freight carrier, date, time, airway bill number, etc.) at the time of the shipment to ensure attentionupon receipt at the repair facility. Although ternperature cyclable detectors can obviously be shlpped atroom ternperature wlthout llquld nltrogen, It Is not recommended for those detectors whlch have been or mayhave been exposed to neutron damage. Consult your factory for further Instructlons.The instructions and precautions stated here are to ensure proper attention in handling shipment of an expensiveradiation detector. Do not be apprehensive because of these precautions. Detectors are successfully shipped allover the world when just a few precautions are followed. When in doubt about any of the above-mentioned precautionsor instructions, please contact the nearest EGBG ORTEC Office for assistance.Effective Date: May 1981This policy supersedes all previous statements regarding warranty and repair of Temperature Cyclable DetectorSystems. Policy and prices subject to change without notice.

7. SYSTEM OPERATION7.1. ASSEMBLING AN ENERGY select the type of coupling, be sure that dc-coupling isSPECTROSCOPY SYSTEMused. Also be sure that the powerto all equipment, includingthe oscilloscope, is furnished from the same ac-powerAfter the dewar has been filled, allow time to assuresource to help prevent ground loops.complete cooling of the detector element (6 hours, unlessotherwise specified). The detector is then connected aspart of a complete energy spectroscopy system (Fig. 7.1).7.2. CABLE TERMINATIONIncluded are the detector with its attached preamplifierand high-voltage filter, a main amplifier, a count-ratemeter, a biased amplifier (optional), a multichannel analyzer,a precision pulse generator, a detector bias supply,and an oscilloscope.Connections between the preamplifier and the detectorand between the high-voltage filter and the detector aremade through the cryostat vacuum feedthroughs withinthe electronics shield. Detector bias (operating voltage) isfurnished from the detector bias supply and must becabled to thedetector's high-voltage filter. Use RG-59A/Ucable, or the equivalent, with SHV connectors. Thecaptive power cable from the preamplifier is attached tothe preamp power connector on the main amplifier. Theremaining connections within the spectroscopy systemare made using RG-62A/U, or equivalent, with BNC connectors.In addition, connect the attenuated output fromthe precision pulse generator to the test input of the preamplifier,and connect the preamplifier output to a mainamplifier input.The unipolar output of the main amplifier should be connectedto both theoscilloscope and the mukichannel analyzeror biased amplifier (if used). A BNC Tee may beneeded or separate amplifier outputs may be used.The system should be dc-coupled all the way from thepreamplifier through the multichannel analyzer. If any ofthe modules have optional connectors or switches toConnecting your system with proper cable impedancetermination is important. Three general methods oftermination are used. The simplest of these is a shunttermination at the receiving end of the cable. A secondmethod is series termination at the sending end. The thirdmethod is a combination of series and shunt termination,where the cable impedance is matched both in series atthe sending end and in shunt at the receiving end.All EG&G ORTEC preamplifiers contain 93R series terminations.The preamplifier of your detector system iscapable of driving at least 500 feet of 93R cable. Whenconnected to an EG&G ORTEC amplifier (or most othermodern spectroscopy amplifiers), no additional terminationis required if the length of the interconnectingcable is less than 50 feet. For greater distances, terminatethe amplifier input with a 1000 termination to ground.The cable connecting the amplifier output to the input ofthe pulse height analyzer or the biased amplifier shouldalso be properly terminated. EG&G ORTEC amplifiershave front panel outputs of

)receiving instrument with 93R cable. For thecombinationof series and shunt termination, the 93R amplifier outputis used together with the 100R terminator at the input ofthe receiving instrument. The useof both seriesand shunttermination is most effective, but it is not always advisedbecause it reduces the signal amplitude by 50%. Similarconsiderations apply when connecting a biased amplifieroutput to the input of the pulse height analyzer.7.3. SHAPING TIME, NOISE,AND PULSE PILEUPThe selection of shaping time, 7, strongly affects theamount of system noise and the degree of pulse pileupexperienced. The proper shaping time should be chosento strike a balance between noise and pulse pileupeffectsaccording to the application requirements.Almost all of the noise of your spectroscopy system isattributable to the cryogenic electronic components ofthe input FET stage of the preamplifier and to the detectorelement itself. Special care in theselection of these componentsand cooling of the FET are important to minimizethis noise. However, this noise is viewed through theamplifier filter network so that the effective system noisedepends on the filter time constant or shaping time.The system noise has three primary components as referencedto the preamplifier input: series noise, parallelnoise, and l/f noise. Considering both the frequencyresponse and the bandwidth of the filter, it is found thatseries noise is proportional to l/fiparallel noise is proportionalto & and l/f noise is independent of r. As aresult, a graph showing noise as a function of time constantwill exhibit a minimum point with the noise increasingas the time constant either increases or decreasesfrom that point. The noise minimum or "noise corner" isthe ideal time constant for your system strictly from thepoint of view of electronic noise. Typical noise cornervalues are 4 to 8 ,us for coaxial germanium detectors(GEM, GMX, and GWL), 6 to lops for planar germaniumLEPS (GLP), and 10 to 14ps for planar Si(Li) detectors(SLP).Pulse pileup at the amplifier output depends strongly oncount rate and time constant. The larger the time constant,the longer the signal is away from the baseline for agiven pulse. Thus the larger the timeconstant, the greateris the probability that an additional pulse or pulses willoccur while the signal is away from the baseline due to aprevious pulse. The probability of pulse pileup alsoincreases with increasing count rate. That fraction ofpulses which will be piled-up is approximately 1 - e-R'd,where R is the count rate and rd is the dead time associatedwith each pulse. Measured with an oscilloscope atthe amplifier output, rd is equal to the pulse width justabove the baseline plus the pulse risetime.The effects of pulse pileup increase the system dead time,increase the spectral background (especially at higherenergies), produce shifts and fluctuations in the signalbaseline, and degrade the system's resol.ution. All of theseeffects, except for increased system dead time, may bereduced by the use of a baseline restorer and a pulsepileuprejector. At lower count rates, the major source ofsystem dead time is the analyzer processing time. Athigher count rates, pulse pileup at the amplifier outputoften becomes the major sourceof dead time. It should bepointed out that dead time due to pulse pileup is not routinelymeasured by the analyzer unless specific provisionsare made. With EG&G ORTEC electronics such provisionis simply made by connecting the main amplifierbusy output to the analyzer busy input.Modern amplifiers have active-shaping networks whichproduce smaller pulse widths for a given time constantthan previously obtainable. Thus for a given time constant,higher count rates are now feasible with lessdeleteriouseffects than were observed in the past. Conversely,for a given count rate, the use of longer time constants isoften permitted.Each spectroscopist must consider the benefits ofreduced pulse pileup at shorter time constants versusdecreased system noise at longer time constants. Theappropriate shaping time is then selected (often by trialand error) according to particular application requirements.At EG&G ORTEC, the selection of time constant used fordetector system performance testing is governed by IEEEStd. 325-1971 (Reaffirmed 1977), "Standard Test Proceduresfor Germanium Gamma-Ray Detectors." Section 4,Detector Energy Resolution, of this document states:"Pulse shaping methods and time constants suitable foroptimum performance of the detectorshould be used, buttheir nature must be clearly specified and recorded asdescribed in IEEE Std. 301-1969."Therefore, our low count rate (1000 cps) specifications ofenergy resolution are often made at relatively long timeconstants to achieve optimum performance by minimizingnoise. At such time constants, a spectroscopist usinga modern amplifier should be able to achieve good resultsup to several thousand counts per second. At highercount rates, shorter time constants (e.g., 2-3 ps) may providebetter results.7.4. POLE-ZERO AND DC OUTPUTLEVEL ADJUSTMENTSThe pole-zero adjustment must be made for the signal toreturn promptly to the baseline after each pulse. Failuretomake this adjustment properly will greatly increase pulsepileupeffects and result in low or high side tailing onspectral peaks, greatly degrading high-count-rate performance.To set the pole-zero adjustment of your main amplifierwhen using your GEM Series detector, bring a radioactivesource (60Co for coaxial detectors, '37Cs or S7Co for LEPSor Si(Li) detectors) near your detector. Adjust the sourcedistance to achieve a total rate between 500 and 3000counts per second. Observe the unipolar output with anoscilloscope with enough vertical gain to allow the baselineto be observed in detail. Fora pulse height of 8to IOV,

a vertical sensitivity of 50 mV/cm is usually adequate. (Seethe following comments on overloading). As amplifiergain is changed, the oscilloscope sensitivity is adjusted in .inverse proportion. For the main amplifier pole-zeroadjustment, the horizontal sweep time should be set toachieve a pulse width of 2 to 4 cm.The amplifier pole-zero adjustment should be made sothat the trailing edge of each pulse returns to the baselinewithout overshoot or undershoot (Fig. 7.2). Some olderamplifiers may produce a small hump or other artifact onthe trailing edge of each pulse that cannot be changed bypole-zero adjustment. In addition, some detector systemsintroduce a slight overshoot or other pulse tail defect thatcannot be eliminated. When such artifacts or defects arepresent, adjust the pole-zero in such a way as to minimizethe time that the signal is away from the baseline.The oscilloscope must be dc-coupled and must not contributedistortion in the observed waveforms. Many oscilloscopeswill overload for an 8- to 10-V signal when thevertical sensitivity is greater than 100 mV/cm. This producesan apparent pulse defect which greatly resembles apole-zero problem. The effect may be worse if theoscilloscopeis slightly out of adjustment. Oscilloscope overloadmay be prevented by clipping off the top of each pulse byusing the clamp circuit shown in Fig. 7.3 at the oscilloscopeinput.From AmplifierOutput1,-1 KTo hcilloscopelnput1-HPA 2800SCHOTTKY DIODES orFAST Ge DiODESFig. 7.2.Undercompensated Pole-Zero.Bottom Trace 2 V/cmIOvercompensated Pole-Zero.Bottom Trace 2 V/cmProperly Adjusted Pole-Zero.Pole-Zero Adjustment as Seen With an Oscilloscope.Fig. 7.3. Circuit Diagram for Clamp Network Used to PreventOverloading the lnput to the Oscilloscope Vertical Amplifler.Your detector system preamplifier also contains a polezeroadjustment. To check the preamplifier pole-zero,reduce the oscilloscope sweep time until the pulse is 1 to2mm wide. Then look for undershoot 'or overshoot followingeach pulse. If both pole-zero adjustments areproperly set, the baseline should appear smooth '(littleragged appearance). Again, amplifier or detector systemanomalies may produce minor defects in pulse shapewhich cannot be removed by pole-zero adjustment. Thepreampllfler pole-zero adjustment has been properly setat the factory and should not need readjustment. However,if some adjustment becomes needed in time, pleasecontact EG&G ORTEC Customer Service or your localrepresentative.If the methods described herein are carefully applied,your pole-zero adjustment can be made with high accuracy.However, another method using a square-wavepulser may allow a more precise adjustment. Please referto an EG&G ORTEC amplifier manual for the details ofthis method.The dc-output level of your main amplifier and biasedamplifier (if used) should be set to zero. To do this,observe the amplifier output with an oscilloscope set to asensitive vertical gain (at most, 50 mV/cm). Then groundthe input of your oscilloscope and adjust the verticalheight to align the trace with the center line on themeasurement grid. Again look at the amplifier signal andadjust the dc level (see your amplifier manual) until thebaseline of the output signal is also aligned with thecenter line on the oscilloscope measurement grid. Thedclevel is now set to zero. It is extremely important that theinput signal into the analog-to-digital converter (ADC) of t-the pulse height analyzer has a dc level close to zero.Failure to achieve this may result in a substantial loss of/' -

counts (often manifested as an unexplained drastic efficiencyloss) or in an inconveniently large energy slopeintercept for the resulting spectrum. These effectsdepend upon the characteristics of particular analyzers.7.5. COMMON SETUP PROBLEMS:MICROPHONICS, GROUND LOOPS,AND PICKUPAll detector systems will exhibit some signal response(microphonics) when subjected to excessive vibration,mechanical shock, or very loud noise. A detector systemthat exhibits such behavior to an excessive degree is saidto be microphonic.The preamplifier input at the gate of the input field-effecttransistor (FET) must be extraordinarily sensitive to smallcurrent pulses. If anything electrically connected to thegate lead moves on the order of angstroms with respect toany surfaces at the high-voltage bias potential, thechange in capacitance induces a current pulse thatresults in a substantial output signal. Vibrations of thehigh-voltage surfaces with respect to ground on the filterside of the detector can also produce microphonic response.Such a response is usually less significantbecause its magnitude is reduced by the large filtercapacitor and by coupling through the relatively smalldetector capacitance. 'The use of a cooled FET makesdetector systems much less sensitive to .microphonicsbecause it greatly reduces the length of the critical FETgate lead.There are several things that a spectroscopist should doto minimize microphonics. The detector system and itsdewar should never sit directly upon a hard concrete flooror similar hard surface through which vibrations frompumps and equipment are easily transmitted. Instead, thedewar should be shock-mounted in some way. Onerecommended method of doing this is to set the dewarona section of plywood which has a 1- to 2-in. piece of polyurethanefoam sandwiched between it and the floor. If thismuch elevation of the dewar is not permissible, even a 1/4-in. piece of foam placed between the dewar and the floorwill be of substantial benefit. When possible, all parts ofthe cryostat or dewar should be prevented from contactingvibrating objects or objects which might transmitvibrations. Loud noises in the general vicinity of an operatingdetector system should be avoided.If you must operate your system in an environment whichis highly conducive to rnicrophonics, several adjustmentsmay be made to minimize the resulting effects. The useofshorter shaping time constants (-2 ps) will often greatlyreduce microphonic signal response at the expense ofincreased system noise. Setting the amplifier baselinerestorer to "auto" or "high" is usually helpful.If your detector system exhibits excessive microphonicswhen operated under reasonable conditions, EG&G, -ORTEC will repair it in accordance with the WarrantyI Statement.Improper system grounding can cause ground roopswhich may induce apparent electrical oscillations.Ground loops can result when several different electronicsystem components are "grounded" in different placeswhich may be at slightly differing near-ground potentials.Make sure all components of your spectroscopy systemshare one effective common ground. If your building hasan electrical system which is not well grounded, consult acompetent electrician. Another common cause of groundloops is a poor grounding connection through a faulty coaxialcable or a loose BNC connector on a cable or panel.Trace down and eliminate such problems by trial anderror replacement while closely observing the resultingbaseline noise on your oscilloscope.Signal pickup can occur even in properly grounded systems.Radio frequency electromagnetic waves may bepicked up from a radio station, a particie accelerator, orother sources. Although your detector system is wellshielded, additional RF shielding may be useful underextreme circumstances.Pickup over the ac power line can also be a problem.YOUR SYSTEM SHOULD NOT BE OPERATED ONClRCLllTS COMMON TO ELECTRIC MOTORS ANDOTHER ELECTRICALLY NOISY EQUIPMENT. See anelectrician about supplying an isolated power lineorcontactEG&G ORTEC Customer Service for recommendationsabout obtaining an appropriate ac line conditioner.7.6. THE INITIAL APPLICATION OF BIASBefore applying bias to the detector, apply power to theother components in the system. Observe the unipolaroutput from the main amplifier on the oscilloscope. Forpreliminary testing, use a main amplifier gain of about 40for coaxial detectors and 100 for LEPS and Si(Li) detectors.Adjust the oscilloscope gain for a baseline width of 1to 2 cm and observe the system noise.A 60- or 120- Hz sine wave with superimposed noise indicatesa gross ground loop. This should be found andeliminated. If there is any ripple at these frequencies, itshould beverysmall in comparison to the amplitude of thewhite noise. A complete inspection for smaller spuriousoscillations may be made after the detector is under bias.Occasionally, the system may appear dead when no biashas been applied. If the application of a small amount ofbias (-30 V) results in appearance of the white noise,there is no problem.Apply about 200 V of bias of the correct polarity to thedetector. The noise amplitude should decrease becausethe detector capacitance decreases as the depletion ofcharge carriers begins. Application of the wrong biaspolarity can cause the noise amplitude to either increasegreatly or drop completely to zero and remain there. Thismight also occur if the detector has been damaged inshipment.If the detector bias polarity is correct and the reaction isnormal, increase the detector bias gradually in steps ofabout 200 V while observing the oscilloscope. The noiseamplitude should normally continue to decrease down toa minimum. With the prescribed gain, the final baselinewidth should be about 10 to 20 mV. It is normal for.the

noise to completely disappear for several secondsfollowingchanges in bias voltage. During the stepwise voltageincreases, if the noise disappears and does not reappearwithin 15 seconds, reduce the bias immediately, 100V at atime, until the noise reappears. The detector may betested for resolution at this reduced bias level. However, adetector problem is indicated unless your high-voltagepower supply is significantly uncalibrated or the detectorhas not been allowed to cool completely.When .the bias voltage has been raised to that level specifiedin the Quality Assurance Data Sheet, the noiseampli-tude may have increased slightly above the minimum levelnoted above, but the increase should never exceed 1O0/0.At this level of noise, any spuriousoscillations or baselinedisturbances due to ground loops, pickup, or microphonicsshould be clearly visible. Eliminate such problemsbefore the detector system performance is evaluated.Before attempting to make definite performancemeasurements, allow the detector system to stabilize,with bias applied, for at least 30 minutes.

8. GMX SERIES DETECTOR PERFORMANCE MEASUREMENTSI'NOTE: The white plastic beryllium window cover shouldbe removed when making any of the measurements describedin this section.8.1. DETECTOR RESOLUTIONBecause of the versatility of the GAMMA-X detector,methods are described for making resolution measurementsat a variety of energies (5.9 keV to 1.33 MeV). Althoughyour detector is unlikely to have warranted resolutionspecifications at each of these energies, thesemeasurements may be useful to more fully characterizeyour instrument.1.33 MeV, =OCoFirst measure the "CO resolution. Start by placing a '"Cosource in front of the detector endcap. Initially set themain amplifier gain to achieve a pulse height of 8 to 10Vfor the 1.33-MeV line as measured on an oscilloscope.Make preliminary adjustments of the pole-zero and BLRthreshold settings of your main amplifier. Adjust thesource-to-detector distance to achieve a count rate ofapproximately 1000 counts per second.Adjust the gain of the main amplifier (and biased amplifier,if used) and the pulse height analyzer settings (conversiongain and digital offset, if available) until the 1.17-and 1.33-MeV peaks are separated by 800 to 1000 channels.After this gain adjustment, carefully adjust the pole-zeroand BLR threshold. Check to make sure that amplifierdc-output levels are zero and that the entire system isdc-coupled. Use the oscilloscope to verify that the outputsfrom both the main amplifier and the biased amplifier(if used) are within their specified linear ranges andhave no distortion. Inspect the baseline for problems suchas microphonics or oscillations due to ground loops orpickup, and correct such problems before proceeding.Accumulate a spectrum in the pulse height analyzer witha 1.33-MeV peak of at least 4000 counts in the peakchannel and a full width at half maximum (FWHM) of atleast 6 channels (10 to 12 channels is preferred). Thesystem energy slope calibration can now be made in keV/channel since both peaks are identified by channel numbers,and the energy difference between the peaks is159.3 keV. Determine the FWHM in channels interpolatingto the nearest 0.01 channel. For detailed instructions, see"Determining FWHM Expressed in Channels," whichfollows. Multiply the FWHM in channels by the energysl'ope calibration in keV/channel to find thesystem FWHMresolution in keV. Compare this measured resolution withthe Quality Assurance Data Sheet at the front of thismanual If there is a significant discrepancy, first be surethat the measurements were made at the same shapingtime constant.122 keV, 57CoThe resolution of your detector system at 122 keV mayalso be measured. Replace the 60Co source by a "COsource and increase the amplifier gain to achieve a pulseheight of about 8 V at the 122-keV line. Once again makepreliminary adjustments of the pole-zero and BLR thresholdsettings of your main amplifier. Adjust the source-todetectordistance for a count rate of approximately 1000counts per second.Adjust the amplifier gains and the pulse height analyzeruntil the 122- and 136-keV peaks are separated by 500to700 channels. Carefully adjust the pole-zero and BLRthreshold. Use the oscilloscope to verify that the outpdsfrom both the main amplifier and the biased amplifierarewithin their specified linear ranges. lnspect the baselinefor any spurious oscillations or other problems.Accumulate a spectrum in the pulse height analyzer sothat the 122-keV peak has at least 8000 counts in the peakchannel. For thesuggested peak separation, the FWHM ofthe peak should be about 30 to 50 channels. These conditionsare used at the EG&G ORTEC test facility. However,the IEEE recommended minimum conditions (4000counts in the peak channel and 6 channels FWHM) areadequate.Calculate the system energy slope in eV/channel on thebasis of a 14.4-keV separation between the 122- and 136-keV peaks. Multiply this value by the FWHM of the 122-keV peak (in channels) to obtain the FWHM energy resolutionin eV.Failure to obtain satisfactory FWHM resolution valuescan indicate any one of a variety of problems. In almostevery case of apparent detector problems, the troublewill be found to lie in some other element of the spectroscopysystem. Please refer to Section 9.4 of this manualfor troubleshooting suggestions.60 keV, Z41AmThe 60-keV line of "'Am is also useful. Even for very lownoise coaxial detector systems the resolution at 60 keVwill be mainly determined by the system noise. Althoughthe noise may be measured by itself with a pulse generator,a radioactive source of reasonable strength canusually provide a much higher counting rate. Enough datato give statistically meaningful results can usually becollected in a much shorter time with a radioactivesourcethan with a pulse generator. Thus making periodic 60-keVresolution measurements is a quick, easy way to checkyour systems' noise level.System gain may be calibrated by using the line to bemeasured at 59.537 keV (100 units of intensity) and thenext lower line of significant intensity at 26.345 keV(6 units of intensity). A minimum peak height of 4000counts should be achieved. A gain selected to provide aseparation of 1000 to 1800 channels between these twocalibration peaks is suggested. However, if the IEEEminimum standards of 4000 counts peak height and 6channels FWHM are met, the results should be satisfactory.

5.9 keV, 55Fe of a linear interpolation of the number of channelsat half-It may be of interest to measure the resolution of your maximum, these programs usually make a fit to a Gaus-GAMMA-X detector at the 5.9-keV line of " ~ e [Mn x rav). sian curve.The noise of any coaxial detector is such that this lineiiilnot be completely resolvable from the 6.5-keV peak.NOTE: Computerized FWHM determinations must bebased upon sufficient counting data. Do not accumulateTherefore, the FWHM obtained will be larger than thata few hundred counts and expect proper results. Alwaysfor a single lineof this energy. However, it isan interestingmake these determinations to within +3% (90% confimeasurementbecause it demonstrates the usefulness ofdence level).the GAMMA-X detector at such low energies. Previouslv,only planar detectors were useful at such low energies. 1. Accumulate sufficient data and halt the analyzer.If the two x-ray peaks were sufficiently resolvable, thesystem could be calibrated with the weighted averageenergies (5.898 and 6.492 keV) of the Ka and Kp Mn x-raylines. However, calibration probably will require a precisionpulse generator.First, using the " ~ source, e accumulate a spectrum witha peak height of 1000 or more counts at 5.9 keV. Thenremove the source. Set the linear attenuator of the pulsegenerator to the numerical value (or some multiple) ofthe energy (5.90 in this case). Next use the normalizationattenuator to adjust the output until the pulse generatorcounts are being accumulated exactly on top of the 5.9-keV peak. Now the pulse generator is calibrated in.energyunits, and the calibrated pulse generator can provide asecond calibration peak at whatever energy is desired.A pulse generator peak at 7.9 or 8.9 keV is suggested inaddition to the 5.9-keV peak for calibration. The gainshould be adjusted to achieve an energy slope of 5 to10 eV/channel. A peak height of 20,000 counts at 5.9 keVis recommended.Other PeaksThe resolution may be measured with lines of other energiesbesides those suggested here. X-ray peaks may beused, but many of these will be seen as unresolvabledoublets, and hence the measured resolution will not beexactly representative of the true system resolution atthat energy. The ""~mline at 60 keV is a recommendedsingle line low enough in energy to characterize thesystem noise.The methods described for the specific peaks mentionedare generally applicable. If there are not two knownpeaks with suitable separation for calibration, then thepulse generator method (as described for the 5.9-keVmeasurement) may be used.It is always important to achieve statistically valid countingdata. The IEEE recommended minimum of 6chsnnelsFWHM with 4000counts peak height should alwaysapply.Determining FWHM Expressed in ChannelsThese instructions assume the use of a spectroscopysystem similar to that shown in Fig. 7.1. FWHM can bedetermined with the same principles and even simplerinstrumentation (e.g., discriminators or single-channelanalyzers rather than an MCA). The determination willtake more time, but with good instrumentation the resultswill be accurate. Many computerized spectroscopysystems have routines which determine the FWHMalong principles similar to those listed. However, instead2. If necessary,' determine the background continuumunder the peak of interest such that the number of backgroundcounts in each channel is known. Obtain the netnumber of counts in each channel to be used by subtractingthe background counts from the total counts in thatchannel. All subsequent steps assume that the backgroundhas been subtracted.3. Locate the peak channel which is the channel containingthe maximum number of net counts of any channelin the peak of interest.4. Find the half maximum number of counts which is onehalfof the net number of counts in the peak channel.5. On the lower energy side of the peak, find the twoadjacent channels with net numbers of counts whichbracket the half maximum number.6. Perform a linear interpolation, to 0.01 channel, to findthe channel number which represents the half maximumnumber of counts. Record this number.7. Determine the channel number representing the halfmaximum number of counts on the higher energy side ofthe peak using the methods of steps 5 and 6. Record thisnumber.8. Subtract the channel number found in step 6from thatfound in step 7. The result is the FWHM expressed inchannels.8.2. PEAK WIDTH RATIOSIn addition to determining the FWHM of a spectral peak,its width may also be measured at other specific heights.The full width at tenth maximum (FWTM) and full widthat fiftieth maximum (FWFM) are frequently measured.From these values, the tenth-to-half ratio (FWTM/FWHM)and the fiftieth-to-half ratio (FWFM/FWHM) are calculated.These quantities are good indicators of the qualityof the peak shape. Good peak shape is especially importantwhen computer fitting is used.The ideal peak shape is a Gaussian curve for whichFWTM/FWHM = 1.82 and FWFM/FWHM = 2.38. However,for an actual spectral peak reasonably good values areFWTM/FWHM G 1.9 and FWFM/FWHM G 2.8. At higher'If a source is used which produces only a few lines, the backgroundlevel under a higher energy peak is often small enoughthat it may be neglected. However, if the background level ismore than 1% of ttie peak height, the background must be subtractedto determine the FWHM.('

energies, peak shape is less influenced by electronicnoise and these ratios are sensitive to peak asymmetryor tailing caused by charge collection problems (e.g.,charge trapping or the existence of weak field regions).Valuesfor these peak width ratios are representative onlyif the pole-zero is properly adjusted and the detector isoperated at the correct bias voltage.8.3. NOISEDetector system noise contributes significantly to theresolution specification of the system. Excess noise isoften symptomatic of an electronic problem in the detectorsystem or in other components. Therefore, it is usefulto measure and record for future reference the noise performanceof your detector system as part of the initialcheck-out.The spectroscopy system noise can be measured simplyby using a pulse generator. If all other elements of thespectroscopy system are functioning properly, thenessentially all of the measured noise is from the detectorand the preamplifier. System noise is independent ofenergy, unlike radiation source resolution. However, themeasured noise depends strongly upon the shaping timeand the method of pulse shaping used in the main amplifier.A spectral peak produced from a pulse generaior inputexhibits broadening only as a result of system noise.Therefore this technique excludes broadening effectscaused by fluctuations in the charge generation andcollection processes within the detector element.It is helpful to precede the precision pulse-generatormeasurement by a radiation source energy resolutionmeasurement (Section 8.1). Otherwise, a separate calibrationmeasurement must be made using a radiationsource. Do not readjust the gain or any other settings,and the same energy-slope calibration determined withthe radiation source will remain valid. Remove the radiationsource and turn on the pulse generator. Adjust thepulse generator output to produce a peak within the spectralregion of the pulse height analyzer. For a definitivemeasurement,~acquire a spectrum until the peak channelcontains at least 4000 counts. Measure the pulse generatorpeak FWHM in channels to the nearest 0.01 channel.Multiply this by the energy-slope calibration in eV or keVper channel to obtain the FWHM system noise in energyunits. A peak width of at least 6 channels at the FWHM isrequired for an accurate measurement. If the resultingpeak is too narrow, increase the system gain, recalibratethe energy slope with a radioactive source, and repeat themeasurement.It is important to understand the part played by systemnoise in determining total resolution. The contributionto spectral broadening due to electron-hole pair productionand collection effects is the detector contribution D.The detector contribution and system noise N are addedin quadrature to obtain the total resolution R:All quantities must be expressed in the same way, such asFWHM in units of energy.Preamplifier problems and other electronic problems cancause N to become excessive. Only changes in chargetrapping effects due to a change in bias voltage or radiationdamage will change the value of D, at a given energy,for a particular high-purity detector. (The warm-up of aGLI detector can also change D.) The theoretical valueof D is proportional to the square root of radiation energy:where the Fano factor F is -0.1, the energy t required tocreate an electron-hole pair is 2.97eVfor germanium, andE is energy in eV.8.4. PEAK-TO-COMPTONThe measurement of the peak-to-Compton ratio is basedon the same energy peak (1.33 MeV) used for the resolutionmeasurements (Section 8.1). The ratio is the numberof net counts in the 1.33-MeV peak channel divided by theaverage number of net counts in the channels representingthe range from 1.040 through 1.096 MeV, which is partof the Compton region associated with the 1.33-MeVpeak. 'The range of the multichannel analyzer must beadjusted to include the peak channel and the range ofinterest in the Compton plateau. Accumulate the spectrumuntil there are several thousand counts in the peakchannel and then calculate the ratio based on the informationin the spectrum.Normally, the calculated peak-to-Compton ratio can beexpected to match the quoted specification if the resolutionspecification has been met. A loss in the peak-to-Compton ratio that is not accompanied by a correspondingdegradation in the energy resolution measurementis probably due to the presence of some absorbingmaterial in the vicinity of either the detector or the sourceor to 4 0 background ~from concrete walls, or to the presenceof some other radiation source. Adjacent absorbingmaterial can increase the Compton background and canthus reduce the peak-to-Compton ratio. Of course, anycontribution of energy in the range of interest that is furnishedfrom extraneous radiation sources will add to thecount level in the Compton region. For this measurementbe sure that the detector is at least 3feet away from otherobjects and that no other radiation source is in thevicinityof the detector.8.5. RELATIVE EFFICIENCYThe detector system should be set up and adjusted in thesame way as for 60Co resolution measurements. The procedureused by EG&G ORTEC for efficiency measurementsis defined in IEEE Standard 325-1971 (Reaffirmed1979), "Standard Test Procedures for GermaniumGamma-Ray Detectors." This procedure must be followedexactly to achieve meaningful results.

Place a calibrated 'OCo point source (l0/o accuracy inactivity) 25.0 cm from the center of the front face of theendcap on a line perpendicular to the endcap face. Thecurrent activity of this source must be calculated on thebasis of the "CO decay rate. The absolute efficiency of thegermanium detector for 1.33-MeV photons is measuredwith appropriate dead time corrections. The absoluteefficiency is given by the ratio of the total counts in the1.33-MeV peak to the total number of source disintegrationsduring the elapsed live time. (Live time =real time -dead time, including both amplifier and analyzer deadtime.) It is suggested that at least 20,000 counts be accumulatedin the 1.33-MeV peak. The ratio of theabsolutegermanium detector efficiency to theefficiency of a 341-1. X3-in. Nal(TI) scintillation detector at 25.0cm (known to be1.2 X lo-') is calculated. This ratio, expressed as a percentage,gives the relative efficiency of the detector:Relative Efficiency =(peak area)/[(activity) (live time)]1.2 X 10"X 100wherepeak area = number of counts in peak,activity = disintegrations/second,live time = real time minus total system dead time inseconds.When using EG&G ORTEC instrumentation, connect theamplifier Busy Out to the pulse height analyzer amplifier.Busy Pulse height analyzer live time will be corrected foramplifier pulse pileup, giving results to sufficient accuracyfor most efficiency determinations. If pulse pileup rejection isused, the inhibit output must also be connected to the pulseheight analyzer's anticoincidence gate.Window Attenuation Index - 22-keV Peakl88-keVPeak of loSCdThe ratio of the counting rates within the ""Cd peaks at22 keV and 88 keV is a measure of window attenuation.The 22-keV line is much more attenuated than the 88-keVline by any germanium dead layer which may be present.It will also be more attenuated by other absorbing materials,such as aluminum. Thus, the 22-to-88 ratio is usefulas a window attenuation index for coaxial detectors.To measure this index, a thin window ""'Cd source mustbe placed in front of the detector endcap at a distance ofat least 10 cm (to minimize angular effects). The systemgain should be adjusted so that the 22-keV and 88-keVpeaks appear in the same spectrum. The 22-keV peakmust be clearly resolvable from the neighboring 25-keVpeak. It is suggested that the counting time be such thatthe 88-keV peak has an area of at least 3000 counts.The most accurate determination of the 22-to-88 ratiowindow attenuation index is the net area of the 22-keVpeak divided by the net area of the 88-keV peak (backgroundsubtracted). In practice, the background (assumingonly IuYCd is present) is usually negligible. In fact,because the resolution at 22 keV and at 88 keV will besubstantially the same, a reasonable approximation of the2240-88 ratio may be achieved simply by dividing theheight of the 22-keV peak by the height of the 88-keVpeak.For a detector with a thick Ge dead layer (e.g., a lithiumdiffusion), the 22-keV line would not be visible. For aGAMMA-X detector equipped with a beryllium window, a2240-88 ratio window attenuation index of at least 17should be achievable. If the GAMMA-X detector had analuminum endcap of the usual thickness, the ratio-wouldbe 9 or more.8.6. MARINELLI BEAKER STANDARDSOURCE EFFICIENCYFor the effective measurement of low activity radioactivesamples, it is desirable to maximize the absolute efficiencyof the sample-detector geometry. For germaniumdetectors, there are two sample-detector geometrieswhich provide especially high absolute efficiencies. Forlarge volume, low activity samples, the Marinelli (reentrant)beaker geometry (Fig. 8.1) is preferred. This designplaces the sample material as close as possible to thedetector active volume. or small volume samples, thewell detector geometry (Fig. 8.2) gives the highest possiblecounting efficiency.To characterize detector performance for use with Marinellibeakers, a Marinelli Beaker Standard Source (MBSS)has been developed. The specifications of the MBSS anda description of its use as a standard are found in "IEEEStandard Techniques for Determination of GermaniumSemiconductor Detector Gamma-Ray Efficiency Using aStandard Marinelli (Reentrant) Beaker Geometry" (ANSI/Flg. 8.1.SampleGe DetectorDewarMarinelll Beaker Geometry.

IEEE Std. 680-1978).' An MBSS consists of a StandardMarinelli Beaker (of specified geometry and material)containing a given volume (450 ml + 2 ml) of a cariier (ofspecified density and mean atomic number) in whichradioactive material is dispersed. Marinelli Beaker StandardSources which meet the specifications of this standardare commercially availab1e.tAbsolute detector efficiency at several energies is measuredwith an MBSS placed over the endcap (Fig. 6.1). Aspectrum is accumulated for live time counting interval T.The area A, in number of counts, of each full energy peakis determined (after background subtraction). The numberof gamma rays N, of energy E originating in the MBSSduring time interval T is calculated from the known activity.The absolute efficiency E, of the detector at energyE is:Detector ElementSample\1As a minimum, measurements should be made at 122 keV("Co) and 1.33 MeV (6L'Co). In comparative evaluationsof different detectors for Marinelli beaker use, it is helpfulto examine their absolute efficiencies at these energies.Additionally, measurements are sometimes made at 88keV ('OYCd), 166 keV ('"Ce), 279 keV ('"'Hg). 392 keV(""n), 662 keV ("'Cs), 898 keV ("Y), 1.1 7 MeV (""Co), and1.84 MeV ("Y). Log-log plots of absolute efficiency as afunction of energy are also useful for thecharacterizationand comparison of detectors.The use of the MESS for detector evaluation does notrequire corrections for source geometry or self-attenuationeffects. The MBSS as it is, including source effects,is a standard with no additional corrections needed. Sincethe MBSS properties are fixed, the absolute efficiencyvalues obtained reflect detector characteristics such assensitive volume and window thickness.The same beaker used for the MBSS is available empty*and may be used to hold various samples being analyzed.In this case, the MBSS makes a convenient calibrationstandard. However, corrections must be made for sourcegeometry and self attenuation unless the sample understudy has the sam,e volume, mean atomic number, anddensity as the MBSS carrier.Flg. 8.2.Well Detector (EGaG ORTEC Style) Geometry.The Marinelli beaker may be used successfully with anycoaxial germanium detector. However, for lower energymeasurements, EG&G ORTEC's GAMMA-X (GMXSeries) detector gives exceptionally high efficiency whenused with a Marinelli beaker. This high efficiency is becauseof the very thin wrap-around entrance windowwhich is approximately 0.3 microns thick as comparedwith 500 to 1000 microns for other coaxial detectors.'Available from the IEEE'Service Center, 455 Hoes Lane, Piscataway.N.J. 08664.tNew England Nuclear is one supplier of the MBSS. Other potentialsuppliers are Amersharn Corporation and Isotope ProductsLaboratories.$Potential suppliers for this beaker are Bel-Art Products, ControlMolding Corporation and GA-MA Associates.

9. MAINTENANCE AND TROUBLESHOO'rlNG9.1. LIQUID NITROGEN MAINTENANCE AND use oxygen or other gases which are potentially hazard-WARM-UP PROTECTIONous. Liquid nitrogen can condense liquid oxygen. If liquidAny GMX Series detector may be cycled between room Oxygen touches a combustible material such as oil ortemperature and liquid nitrogen temperature as needed. rubber* a fire Or explosion can result.However, when the system is frequently used, it is sug- One of the recommended filling procedures (Section 4)gested that a regular filling schedule be followed and un- should be followed. If your circumstances make thesescheduled warm-ups be avoided. This reduces the proba- methods impractical, contact EG&G ORTEC Customerbility that high voltage bias might be accidentally appliedto a detector which is not fully cold and cause seriousdamage. In addition, regular filling will make any unusualincrease in liquid nitrogen use readily apparent. An excessiveliquid nitrogen loss rate indicates a vacuumproblem in the cryostat or dewar.Protection against accidental detector warm up may beeffectively accomplished by either of two methods. A liquidnitrogen-levelmonitor (Model 729A) is available with dewarprobe, alarm, and high-voltage shutoff. The monitor activatesthe alarm and high-voltage shutoff if the liquid nitrogenlevel in the dewar falls below a safe level (-1 I4 full). Alsoavailable as an option with the streamline cryostat system isan automatic high-voltage shutoff triggered by a temperaturesensor within the cryostat. The hybrid monitoring circuit iscontained within the preamplifier electronics. This featureis activated simply by connecting the high-voltage shutoffoutput cable from the preamplifier to the remote shutdowninput on the detector bias supply. No alarm is provided, butthe bias supply voltage meter will register zero when theshutoff is activated. Either method of high-voltage shutoffmust be used with an appropriate bias supply such as theEG&G ORTEC Model 459. In any case, after the highvoltage has been shut off automatically, the bias supplyswitch must be turned off until the system has been filledwith liquid nitrogen and completely cooled for the recommendedperiod (see Quality Assurance Data Sheet in thefront of this manual or label on detector system). Accidentalapplication of high voltage to a detector which is not fullycold can cause serlous damage and void your warranty.Service to discuss possible alternatives.9.2. NEUTRON DAMAGE - IDENTIFICATIONAND TREATMENTThe GAMMA-X detector can withstand approximately 25times the neutron fluenceof aconventional p-typecoaxialdetectolr before damage is evident. However, a sufficientfluence of fast neutrons or high-energy charged particleswill result in neutron damage.Neutron damage results in low-side tailing on higherenergy peaks as a result of chargecarriertrapping by holetraps now created in the crystal (Fig. 9.1). The first indicationof neutron damage is noticeable degradation ofresolution above 1 MeV accompanied by little degradationof resolution below 200 keV. The system noise, asindicated by the FWHM resolution of a peak from a precisionpulse generator, will be unchanged by neutrondamage.It is necessary to observe the following precautions tofully use the superior radiation damage resistance of theGAMMA-X detector:1. NEVER WARM UP A GAMMA-X DETECTOR TOROOM TEMPERATLIRE IF IT SHOWS SIGNS OF NEU-TRON DAMAGE OR IF IT MAY HAVE BEEN EXPOSEDTO FAST NEUTRONS. Warming up a neutron damageddetector will result in significant resolution degradation.When the detector system is to be kept cold, a regular fillingschedule is best. Filling once a week is ideal for systemswith a 30-liter dewar even though the holding timeis about 3 weeks under typical laboratory conditions. Thisfrequency will ensure an adequate supply, and there isnever any doubt about whether or not it is the week forrefilling. A liquid nitrogen maintenance record form andprotective jacket ate provided with your detector. Beaware that frequent moving of the dewar will cause someincrease in liquid nitrogen loss rate. Dewa.rs in transit arelikely to have loss rates double those in laboratories.A regular delivery schedule should be established wi!h asupplier of liquid nitrogen. Keep the liquid nitrogensupply covered and free from contaminants at all times.When gas from an external source is used tosupply fillingpressure, only clean dry nitrogen gas should be used.Never use ordinary compressed air which containsoxygen and is likely to contain moisture and oil. Do not!~ig. 9.1. Tailing on the LOW-~nergy Side of 1.17- and 1.33-MeV ? . .Photopeaks Due to Radiation Damage. (Semilogarithmic Scale.)('

2. Once the neutron damage process hasstarted, switchingthe bias voltage of the GAMMA-X off and then onagain shows a temporary dramatic deterioration of theenergy resolution. However, the energy resolution performancecan be recovered simply by keeping the detectoron bias for a few hours with a weak (-1 - 10 pCi)gamma-ray source such as '"Cs or "Co on the endcap.Following these precautions will allow you to postponerepair and get significant additional use from your detector.However, when unacceptable deterioration hasoccurred, EG&G ORTEC can repair your detectorpromptly (Section 6.1).9.3. NEUTRON DAMAGE RESISTANCE OF THEGAMMA-X DETECTORThe great difference in radiation (usually neutron) damageresistance between p- and n-type coaxial detectors isrelated to the charge collection process. Under the influenceof an electric field, electrons and holes movetowards opposite contacts. The signal pulse generatedduring the charge collection process is induced at thecontacts by the moving charge. Therefore those carrierswhich travel the longer path make the larger contributionto the resulting signal.In a coaxial detector many more electron-hole pairs aregenerated nearer the outer contact than the inner contact.This is because the volume of a cylinder is propor-) tional to the square of its radius, and thus most of theactive volume of the detector is nearer the outer contact.Also, because the intensity of impinging radiation is reducedby absorption within the detector, more radiationis absorbed in the outer layers. Therefore, the collectionof that carrier type which moves the greater averagedistance to the inner contact contributes the most to thesignal formation process.The direction of carrier motion is determined by electrodepolarity. The outer contact is positively biased in p-typeand negatively biased in n-type coaxial detectors. Thereforeholes in p-type and electrons in n-type coaxial detectorsmove towards the inner contact and contributethe most to signal pulse formation.Neutron damage produces hole traps which result inspectral degradation. For p-type coaxial detectors inwhich hole collection is more important, hole trappingbecause of neutron damage is especially detrimental. Incontrast, for n-type coaxial detectors, electron collectionis most important and hole trapping from neutrondamaae is less deleterious.-9.4. TROUBLESHOOTINGEach EG&G ORTEC GMX Coaxial Detector is guaranteedto meet all the performance specifications listed on theQuality Assurance Data Sheet. If there appears to be aproblem with the system, use the following troubleshootingprocedures to identify and eliminate the difficulty.The equipment that is required and .the connections areshown in Fig. 7.1. Review the connections of the componentsand the dial settings of the instruments to ensurea proper system setup. Instruction manuals of all elementsof the spectroscopy system should be availableto assist in determining which component of the systemis causing the problem.In detector system problem diagnosis it is important tocarefully examine the main amplifier output with an oscilloscope.This will be more effective if the user is familiarwith the normal appearance of the amplifier output baselineon an oscilloscope. Regular examination of the amplifieroutput will provide such familiarity and may helpdiscover problems in their early stages.Several common problems that may arise in agermaniumdetector system are listed as symptoms in the TroubleshootingGuide which follows. For each symptom themost probable causes are listed, and repair proceduresare suggested. Use the list to help perform an organizedanalysis of the problem, thereby avoiding lost use of thesystem while trying to decide whether or not the detectoris operating satisfactorily. This can :usually prevent anyunnecessary delays, costs, and hazards of shipping thedetector to and from the factory. When a problem isidentified that requires the detector to be returned toEG&G ORTEC, always summarize the symptoms observedand the tests that were performed and then callEG&G ORTEC to give us the details and to arrange forreturn and repair. In the United States call EG&G ORTECCustomer Services at (615) 482-441 1. Elsewhere, contactthe nearest EG&G Instruments Office.Check all power-supply voltages and the detector biassupply polarity. Set the main amplifier controls as follows:Gain 40l nputNegative (Positive for Non-- ,streamline System withModel 120 Preamplifier.)OutputPositiveOutput Range 10 VShaping Time Consult Quality AssuranceData Sheet (front of Manual)BLROut or PZ AdjustConnect the preamplifier output to the input of the mainIn one experiment, an n-type and a p-type coax~al detecamplifierand the pulse generator output to theTest inputtor of the same size were made from a singiecrystal ingot.of the preamplifier. Connect an oscilloscope and a pulseThe two detectors were then subjected to radiation damaclefrom fast neutrons. The n-type detector withstoodapproximately 25 times more neuiron fluence than did the'R. H. Pehl. N. W. Madden, J. H. Elliot, T. W. Raudorf, R. C.p-type detector showing significantTrammel,, and L, S, Darken, Jr,, "Radiation Damage Resistance/ degradation.' However, resistance to radiation damage of Reverse Electrode Ge Coaxial Detectors," IEEE Trans. Nucl.will vary somewhat with individual detector geometry. Sci. NS-26. N1. 321 -323 (1979).

height analyzer to the unipolar output of the main amplifier.Then carefully examine the system's performance.If the system problem is poor y-ray resolution, a carefulstudy of the symptoms is needed before a diagnosis canbe made. Without the presence of a radioactive source,observe the output of the amplifier (gain of 40) with anoscilloscope (50 mV/cm vertical gain). Some nuclearpulses will still be seen. It is not abnormal to have occasionalnegative spikes or overload pulses followed bynegative excursions. Carefully observe the nature of anyother pulses or oscillations. Measure the noise of yourdetector system as the resolution obtained from a pulsegenerator spectrum (Section 8.3). A knowledge of thesystem resolution at both lower and higher energies isalso needed. Measurements at 1.33 MeV and 122 keV arerecommended. After these observations and measurementshave been made, consult the TroubleshootingGuide.Symptom1) Pulse height analyzerdoes not register anycounts.2) Pulse height analyzerregisters backgroundcounts, but no peaks arevisible.3) No amplifier output forgamma-ray source or pulsegenerator.4) No amplifier output, butsatisfactory preamplifieroutput.5) No preamplifier output atrecommended bias voltage.6) No preamplifier outputat any detector bias voltage.The baseline is virtually flat.7) Poor y-ray resolution.High noise (pulser resolution).Sinusoidal oscillationsseen on the baselinewith oscilloscope.TROUBLESHOOTING GUIDEProbable CausePulse height analyzernot receiving pulses.Pulse height analyzer controlsimproperly set.Possible improper amplifiersetting.Pulses corresponding tospectral peaks have voltageheights outside of the rangeof the pulse height analyzer.Amplifier not receivingpulses.Amplifier faulty or maladjusted.Excessive detector-leakagecurrent biasing off the preamplifier-inputstage.Power failure or preamplifierfailure, orblown input FET, or shortinside cryostat.Power line noise, RF pickup,or ground loop (especiallyif able to syncronize to acline with oscilloscopetrigger).ActlonUse an oscilloscope to examine the signal which is inputto the pulse height analyzer. If no pulses are present, see1 symptoms 3. 4, 5, and 6.If pulses are present at the input, see the pulse heightanalyzer manual and check the control settings. Be surethat the amplifier output is positive. You may be misled ifthe oscilloscope is accidentally set to "invert."Use an oscilloscope to examine the signal which is inputto the pulse height analyzer. Adjust amplifier gain toachieve puise heights within the input range of the pulseheight analyzer. (Make sure that you know this rangewhich may change with pulse height analyzer settings -e.g., digital offset or conversion gain.)Check preamplifier output with the oscilloscope set on50 mV/cm vertical gain. If no output, see symptoms 5and 6.Check amplifier settings and connections. Consult theamplifier manual. Replace the amplifier, if necessary.Decrease detector bias in steps of 100 V until outputpulses are obtained. If the detector does not performproperly at a lower bias, contact EG&G ORTEC CustomerService (or your local EG&G Instruments Office if outsidethe U.S.A.).Correct power supply or power cable problems. ContactEG&G ORTEC Customer Service (or your localEG&G Instruments Office if outside the U.S.A.).Eliminate ac power line noise by isolation or filtration.Ensure that there is one effective common ground. Eliminateany breaks in the ground path - especially badcables or connectors. If operation is in a strong RF field,consider extra shielding (Section 7.5).

31SymptomTROUBLESHOOTING GUIDE (Continued)Probable CauseAction8) Poor y-ray resolution.High noise (pulser resolution).Slowly damping sinusoidalbaseline oscillationsMicrophonics (Section 7.5).Place the system dewar on a foam pad if vibrationsthrough the floor are a problem. Arrange other suitableshock mounting. Try using a shorter time constant(Section 7.5). Contact EG&G ORTEC Customer Service(or your local EG&G lnstruments Office if outside theU.S.A.).9) Poor y-ray resolutionand high noise. Many negativespikes.10) Poor y-ray resolutionand high noise. A veryragged baseline with one ormore of the following typesof anomalous pulses: positivespikes, negative spikes,square shaped positive andnegative pulses.11) Poor y-ray resolution.No unusual pulses but awide baseline and highernoise.12) Poor y-ray resolution at1.33 MeV or higher energy.Normal noise as measuredby pulse generator resolution.Resolution at 122 keVor lower energy is much lessdegraded than higherenergy resolution.13) Low- or high-energyside tailing and poorresolution.14) Wandering peaks ormultiple peaks observed.Breakdown of filter capacitoror high voltage feedthrough.Breakdown across the surfaceof the detector or aninsulator.Excess detector leakagecurrent.Insufficient bias voltage.Neutron damage (or similarradiation damage).Incorrect pole-zerocancel lation.Unstable electronics(especially main amplifier).Contact EG&G ORTEC Customer Service (or your localEG8G Instruments Office if outside the U.S.A.).Be sure bias voltage is not more than specified. Reducedetector bias in steps of 100 V until baseline returns tonormal. If the system meets all performance requirements,it may be usable at the lower bias. If not, contactEG&G ORTEC Customer Service (or your local EG&GInstruments Office if outside the U.S.A.).-Same as for symptom 10.Be sure that the specified bias is used. Check highvoltagebias supply for proper connections and operation.Try another bias supply. Consult your bias supplymanual.If the detector is warmed-up, the resolution will get muchworse. Do not warm up the detector. Neutron damage canbe repaired. Contact EG&G ORTEC Customer Service(or your local EG&G Instruments Office if outside theU.S.A.).See Section 7.4 for instructions.Check each electronic component for proper operation.Repair or replace faulty units. Check all cables and connectors(including panel connectors) for intermittentsignal or ground connections:

32SymptomTROUBLESHOOTING GUIDE (Continued)Probable CauseActlon15) High loss rate of liquidnitrogen. Excessively coldcryostat with moisture condensation.'16) High loss rate of liquidnitrogen. Excessively colddewar with moisture condensation.Cryostat temperaturenormal.'Degradation of cryostatvacuum.Degradation of dewarvacuum.Measure liquid nitrogen loss rate by weighing (LN2weighs 0.807 kg/liter). For an accurate measurement, thedewar must not be moved from 2 h before the initialmeasurement until after the final measurement. Thenormal loss rate depends on cryostat/dewar configuration.Contact EG&G ORTEC Customer Service (or yourlocal EG&G ORTEC lnstruments Office if outside theU.S.A.) to find out if the loss rate is abnormal. For mostsystems, a loss rate of more than 1.8 liter/day isexcessive.For a dipstick system, the dewar loss rate (symptom 16)may be subtracted from the system loss rate to give thecryostat loss rate (which should not be above 1.1 liter/day).Check system loss rate as in symptom 15. If cryostat isa dipstick model, place it in another dewar and measurethe dewar loss rate alone. Place a stopper in the hole inthe white RTV silicone collar and measure by weight asin symptom 15. A 30-liter dewar with collar assemblyshould have a loss rate below 0.7 liter/day. To order areplacement dewar, contact EG&G ORTEC CustomerService (or your local EG&G lnstruments Office if outsidethe U.S.A.).'Some systems have a common cryostat-dewar vacuum. Symptoms 15 and 16 occur simultaneously in such systems.