1. introdudion - eckert & ziegler strahlen- und ... · reprint from vol. 1, no.2, 1990 15....

Robert J. GehrkeIdaho National Engineering Laboratory, EG&G Idaho, Inc.

lye tests are described to measure the per--spec-

trometer system. In combination with ac-cepted practices for setting up a germanium

gamma-ray spectrometer (see manufacturer's in-structions) and for measuring the spectrometer's en-ergy scale and efficiency, these tests play an integralrole in assuring that a germanium spectrometer andits associated software are operating within accept-able and known criteria. Conformance to these prac-tices will assure the practitioner that a germaniumspectrometer is yielding correct measurements ofgamma-ray-emission rates within the measured un-certainties and the limitations of the hardware andsoftware.

Along with these more apparent changes oc-curred some less visible but still very significantchanges. The sophistication of germanium spec-trometer hardware and software has increased dra-matically, but micro-computer automation has sim-plified the spectrometer operation. Circuitry hasbeen designed to automate some adjustments (e.g.,pole-zero, baseline restoration) that had earlier re-quired operator attention. Software that automat-ically finds the peaks in a spectrum, measures theirenergy and net area, and identifies the radionuclidesfrom which the gamma rays were emitted is com-mon. These developments are intended to improveperformance of a germanium spectrometer, simplifythe analysis process, and thereby give one manufac-turer an advantage over his competitors.

While the complexity of the hardware has in-creased significantly, the computer automation devel-opments have allowed germanium spectrometers tomove from the laboratory into the workplace. As aresult, it is not uncommon for the operator to haveless than five years of experience in germaniumgamma-ray spectrometry. Further, electronics sup-port is not always available on site, and many elec-tronic technicians are not knowledgeable in pulse cir-cuitry for nuclear radiation measurementinstrumentation. Therefore, it is essential that somebasic tests be developed that can be easily performedeven by the newcomer in order to measure the per-formance of a germanium spectrometer includingits spectral analysis program. The following testshave been designed to meet this need and to assure

1. IntrodudionThe practice of germanium gamma-ray spectrome-try has grown over the last twenty years. Today, al-most every college and university, government labo-ratory, and commercial nuclear facility are involvedin radioanalyses that require the use of germaniumspectrometers. Germanium spectrometer systems,that twenty years ago consisted of hard-wired com-ponents with hard copy, paper tape or, at that time,unreliable magnetic computer tape for output of thespectral data, have been replaced with componentsthat are totally under the control of a mini or microcomputer. Today, spectral data are usually automat-ically stored in the computer and hard copies of theanalysis results, with all pertinent corrections ap-plied, are provided to the user.

15Reprint from Vol. 1, No.2, 1990

Radioactivity

6- Radiochemistry

The Counting Room: Special Edition

the practitioner that the germanium spectrometer isperforming within recommended guidelines. Eventhough these tests do not cover all performance crite-ria, they can be useful in determining if a germa-nium spectrometer and its analysis software havebeen set up correctly and are ready for energy and ef-ficiency calibration. These tests were developed andare proposed for use in the forthcoming reissue ofthe American National Standards Institute standardANSI N42.14.1

for 65Zn); (4) the long half-life mixed radionuclidestandard for efficiency calibrations available fromthe National Institute of Standards and Technology(NIST) consisting of 125Sb, 154Eu, and 155Eu (SRM4275); and (5) 228Th (or its parent, 232U) in equilib-rium with the daughter activities.b Each of thesesources is available from one or more of the com-mercial radionuclide suppliers or from NIST eitheras a point-source standard or as a solution standard.Also, before proceeding with these tests the operatorshould know the total accuracy that will be appropri-ate [i.e., are 1 %,5%,10% or 30% results (always oneestimated standard deviation unless stated other-wise) acceptable?] for the analyses to be made.

2.1 Verification of pulse pileup correctionLaboratories with heavy sample loads may be able tooptimize their sample throughput by reducing thesample-to-detector distance and thereby increasingthe sample counting rates. Although a long amplifiertime constant (-6 to 8 microseconds) will usually re-sult in a better energy resolution for the spectralpeaks, it also increases the width of the amplifierpulse and thereby the amount of pulse pileup.c Re-ducing the amplifier time constant to -2 microsec-onds reduces the pulse width and the amount ofpulse pileup in some cases without severely impact-ing the energy resolution or peak shape. In this way,a germanium spectrometer can operate at a highercounting rate before significant pileup occurs. Wheneither a correction or special pulse processing cir-cuitry for pulse pileup is employed, or when a stable,precision pulser of known repetition rate (real3,4 orvirtual5) is injected into the pulse train and the per-centage of pulser pulses stored (or would have beenstored) by the ADC are recorded, it is possible to op-

2. ExperimentalThe following tests should be performed after thegermanium spectrometer and the associated analysissoftware have been set up and adjusted according tothe manufacturer's instructions. If a counting-ratecorrection for pulse pileup needs to be measured orthe instrumentation adjusted to automatically makecounting-rate corrections, it should be done prior toperforming these tests (Appendix A of reference 1provides one such procedure; also see reference 2,p. 275).

These tests will assess: (1) the accuracy of thehigh-counting-rate correction; (2) the capabilities ofthe peak-fmding algorithm (if included in the analy-sis software); (3) the independence of the measuredpeak area to changes in the peak-height-to-baselineratio;a (4) the capabilities and limitations of the soft-ware to fit doublets; and (5) the magnitude (withinapproximately a factor of two) of true coincidencesumming that could be expected at a given countinggeometry due to summing of cascading gamma rays.

To perform these tests the following five radioac-tive sources are required: (1) 57 Co (a high-activity

source); (2) 137CS (a low- and a high-activitysource); (3) 65Zn f2Na or 60Co may be substituted

Baseline (same as continuum) is the part of the pulse height distribution lying underneath a peak including contributions associatedwith the source, detector, and measuring conditions that affect the spectral shape.

b Uranium-232, which has a 68 year half-life, will be made available in 1990 or 1991 as a Standard Reference Material (SRM) from

NIST to replace the shorter 1.9 year half-life 228Th SRM 4206-C which they had previously issued.

c Pulse pileup occurs when two or more pulses in the pulse train overlap in time so that a single distorted pulse is formed. This pulse is

processed by the analog-to-digital converter as one pulse and is stored in a part of the spectrum different from where either of theindividual piled-up pulses would have been stored. If either of the original pulses would have contributed to a spectral peak, thesecounts are now missing from the peak. The resulting net peak area is then less than it should be.

16 Reprint from Vol. 1, No.2, 1990

"Tests to Measure the Performance of a Germanium Gamma-Ray Spectrometer and Its Analysis Software"

Robert J. Gehrke

then the useful counting rate limit of the spectrome-ter has been exceeded. Either reconfigure (or adjust)the spectrometer for better rate performance or re-measure the pileup losses at successively lower ratesto establish an acceptable input rate limit.

Repeat the above test with a 228Th r32U may besubstituted for 228Th) source (primary source) emit-ting 2614-keV gamma rays and a high-activity 137Cssource emitting 661-keVgamma rays. Examples ofresults from these tests at several integral countingrates are given in Table 1.

2.2 Validation of the peak-finding algorithmMost spectral analysis programs contain subroutinesthat automatically detect the presence of peaks in anaccumulated spectrum. The positions and areas ofthese peaks are then measured by either integratingthe peak or by fitting the region of the spectrumcomprising these peaks with either a Normal (Gauss-ian) function or a function consisting of a Gaussianplus one or more exponentials. Because thoroughidentification and analysis of the spectral peaks isoften dependent on the capability of the peak-find-ing algorithm, it is important to know its limitations.The following test has been designed to determinehow well singlet peaks on a flat baseline that are ator above an "observable level" can be found with thepeak-finding algorithm.a This test is not intended todemonstrate nor should it be confused with the abil-ity to measure the lower limits of detection at a givenlevel of confidence. For this test the gain should bethe same as is used when counting samples for ra-

dioanalysis.This test uses a 65Zn r2Na or 60Co may be substi-

tuted; but if 60Co is used, a correction for the pres-ence of the single-escape peak of the 1173-keVgamma ray at 661 keY may need to be made) and a137 Cs source. The purity of the 65Zn source is exam-

ined by counting this source at a moderate countingrate (500 S-1 < rate < 2000 S-I) and at a reproduciblesource-to-detector distance until approximately10,000 counts are in each of the channels in the en-

erate a germanium spectrometer at counting ratesabove 20,000 S-1 and sometimes beyond 200,000 S-I.The following test assesses the counting rate atwhich additional corrections, beyond those madeautomatically by a measured correction factor, bythe pulse processing circuitry, or by a real or virtualpulser, must be applied.

This test is designed to deduce the upper count-ing-rate limit for which no secondary corrections arenecessary. This test is conducted for two energies,661 keY (137CS) and 2614 keY (losTI) and should beconducted with the same instrument settings, includ-ing gain, as used to count routine samples.

A 137 Cs spectrum with a source that yields a

counting rate of approximately 500 s-1 is accumu-lated for a known counting time until at least 50,000counts are in the net area of the 661-keVfuIl-energypeak. The net-peak-area counting rate with its uncer-tainty is measured using the same method normallyused, including methods employed to correct forpileup and dead time. The resolution at one full-width-at-half-maximum (FWHM) is also measuredin channels for use in the test described in Section2.2. This count uses a counting rate for which pulsepileup is assumed negligible.

Without moving the 137 Cs source (primarysource) a high-activity 57 Co source is introduced, the

old spectrum is erased and a second count is accu-mulated for the same counting time as the firstcount. The 57 Co source should be positioned behindthe fixed 137 Cs source so that the gross counting rate

is about the maximum counting rate to be encoun-tered in the radionuclide analysis of samples.

Measure the net area counting rate of the 661-keY peak as in the first count using the same methodnormally used in the analysis of unknown samples,including methods employed to correct for pileupand dead time.

Compare the 661-keV peak area from the first(low-rate) and second (high-rate) spectra. If the dif-ferences in area (expressed as a percentage of thefirst) exceed 1;3 of the total acceptable uncertainty

a The peak-finding algorithm is expected to find a peak in a spectrum whose area, A = Lp SQRT[(2.55)(FWHM)yJ, where 2.55 is

based on :1:3 a for a Gaussian peak, (FWHM) is the width in channels at half maximum of the peak, yi are the average counts in eachbaseline channel, and Lp = 2.33 corresponds to the value of Lp initially suggested for this test. A multiplying factor 2.33 S Lp S 4.65may be substituted if the peak-location algorithm cannot find peaks for Lp = 2.33 or this sensitivity also finds too many false peaks.

Reprint from Vol. 1, No.2, 1990 17

Radioactivity

6- Radiochemistry


~

Table 1 Deviation of measured peak-area counting rate from

the measured ("true") peak-area counting rate of

500 S-1 for the 2614-keV gamma ray emitted in the

232U decay chain.

ing time that will result in an expected net peak areaof 50 counts in the 661-keVpeak (use measured net-peak-area counting rate to determine countingtime). Apply the peak-fmding algorithm to the spec-trum. The peak at 661 keY should be found everytime in five trials. Some peak-finding algorithms re-quire that the counts comprising the peak aresmoothly varying from channel to channel. In thiscase, peak areas with less than 50 counts in the netarea may not be consistently found. If this is a limita-tion of the peak-finding algorithm, it should be so re-corded as a limitation of the spectral analysis pro-

gram.Erase the spectrum and count the 137 Cs source

at its designated source-to-detector distance for acounting time that will result in an expected net areain the 661-keV peak of 100 counts. Remove the 137CSsource and insert the 65Zn source in the "65Zn" posi-tion. Continue the count, without erasing the spec-trum, for an additional counting time that will resultin each channel of the baseline on each side of the661- keY peak having an expected number of countsequal to (720)/(FWHM). FWHM is the energy reso-lution expressed in channels of the 661-keV peak atone full-width-at-half-maximum as measured inSection 2.1. Apply the peak-finding algorithm to thespectrum. The peak at 661 keY should be found.(Ideally, this test should be repeated many times.) Ifthe peak is not, found repeat the test. If it is notfound the second time, increase Lp (see footnote a,page 17) and rerun the test. Record the value of Lpfor which the peak is found.

For the above and following procedures the ef-fort and time required may be reduced by stoppingthe count and analyzing it at a baseline whose ex-pected counts per channel correspond to Lp = 4.65(see footnote a, page 17). If the peak is found, thecount (only 65Zn source being counted) should becontinued (do not erase spectrum) for preset timescorresponding to sequentially smaller values of Lpuntil either the peak cannot be found or Lp = 2.33.

Repeat the above procedure with the followingdifferences. Accumulate a spectrum of 137 Cs at its

designated source-to-detector distance for a count-ing time that will result in an expected 1000 countsin the 661-keV 137CS peak. Remove the 137CS source,and insert the 65Zn source in the "65Zn" position and

ergy region of 661 keY. The counting rate should behigh enough to overwhelm the normal background.At the end of the counting period, the 661-keVen-ergy region is visually examined to be assured thatno peaks are within 5 keY of 661 keY and that thebaseline in the 661-keVregion is flat. The peak-find-ing algorithm should be applied to the spectrum toconfirm that the 661-keV energy region is void ofany peaks. Any visually observed or found peakswithin 5 keY of 661 keY should be avoided since itwill limit the sensitivity of the tests. If the baselineunderlying the 661-keVregion is not flat, it may notbe possible to confirm the independence of the peakarea as a function of the baseline height. If thesource of an extraneous peak in this energy regioncannot be eliminated and is due to a weak interfer-ence peak from the source or the background, thenet-peak-area counting rate of this interference peakshould be measured so that its contribution can besubtracted from the area of the 137 Cs 661- keY peak.

Also record the baseline counting rate in counts perchannel per second from the counting time and aver-age number of counts in each baseline channel.Erase the previous spectrum and count the 137 Cs

source at a moderate counting rate---(500 S-l < rate < 2000 s-l) and at a reproduciblesource-to-detector distance that will be used for thissource in the test for 1000 s counting time or untilthe 661-keV net area has 5000 counts. Record thenet-peak-area counting rate.

Erase the previous spectrum and count the 137 Cs

source at its source-to-detector distance for a count-



Robert J. Gehrke

Table 2 Measurement of the 661-keV peak area as a function

of the baseline height, yi.

continue the count for a counting time that will re-sult in an expected (7.2 x 104)/(FWHM) counts ineach of the baseline channels (FWHM in channels).This many counts may take an entire day to accumu-late even at a counting rate of 2000 S-l. The 661-keVpeak should be found when the peak-finding algo-rithm is applied. If the peak-finding algorithm can-not find the peak, repeat the test. If it is not foundthe second time, increase Lp (see footnote a, page 17)and rerun the test. Record the value of Lp for whichthe peak is found.

If the peak cannot be found for Lp = 4.65, thesensitivity parameter of the algorithm should be ad-justed so that the identification criteria(2.33 < Lp < 4.65) are met. Consult the Users Man-ual of the analytical program or the manufacturerfor additional help in adjusting the sensitivity of thepeak-finding-algorithm parameters. Informationpertaining to minimum detectable peak areas can befound in reference 6, 7 and 8.

the first (prior to adding additional baseline counts)within two estimated standard deviations.

If these criteria are not met, repeat the test. If itcannot be passed, there may be a systematic error inthe peak-area-measurement algorithm. Determinethe cause of the disagreement and rerun the test; oth-erwise the limitations encountered shall be docu-mented and the uncertainties in measured gamma-ray-emission rates increased to account for themagnitude in the discrepancy. Table 2 illustrates thedeviation of the net peak area from the "true" area asa function of the baseline height for a 661-keV 137CSpeak with a net area of 9844 counts.

2.3 Independence of the peak area from the

gross-peak-height-to-baseline height ratioThis test uses the same sources as are used in Section2.2 and requires the same high purity for the 65Znspectrum in the energy region covered by the 661-keY peak. With the 137 Cs source located at a repro-

ducible source-to-detector position yielding a mod-erate counting rate, acquire a spectrum untilapproximately 5,000 counts are recorded in thegross area of the 661-keVpeak. Measure the net peakarea and its uncertainty with the spectral analysisprogram. Remove the 137 Cs source and, without

erasing the spectrum, add counts to the baseline un-derlying the 661- keY peak by continuing the count-ing period with a 65Zn source positioned for approxi-mately the same counting rate as the 137 Cs source

until each channel of the baseline contains(4.9 x 104)/(FWHM) counts. This many counts maytake one or more days to accumulate. Measure thenet peak area and its uncertainty with the spectralanalysis program. If there is a 661- keY peak in the65Zn or background spectrum, its contribution mustbe subtracted from both of the measured net peak ar-eas before they are compared (see Section 2.2). Thesecond measured net peak area (the one taken afteradding additional baseline counts) shall agree with

2.4 Validation of the doublet fitting softwareDue to the number and complexity of some fittingfunctions developed for spectral analysis, it is impor-tant to verify the analytical program's capability tomeasure the net areas of doublet peaks. The test de-scribed herein uses spectral data generated directlyby the germanium spectrometer system. This is ac-complished by acquiring a spectrum of a 65Znsource (a 22Na or 60Co source may be substituted)for a known counting time, changing the gain orzero slightly at the end of the first counting periodand then continuing to acquire the spectrum (donot erase the first spectrum) for a predeterminedcounting time. This technique will result in spectracontaining doublets whose components have knownpeak-area counting rates and a known separation.


Radioactivity & Radiochemistry


Table 3 Comparison of simulated equal area doublet compo-

nents with a generated peak area of 135,000 counts.a

Further, these doublet peaks truly represent the ger-manium spectrometer system being tested includingits nonideal "Gaussian" shaped peaks.

This procedure assumes that the gain will bechanged by adjusting the fine gain on the amplifier.Before beginning the procedure, the fine gain settingon the amplifier should be calibrated so that thepeak channel of interest can be raised or lowered inchannel increments corresponding to about one halfthe value of the full-width-at-half-maximum(FWHM). Alternatively, the zero level of the ADCcan be calibrated and used for positioning the dou-blet components.

For equal-sized doublet components positionthe source so as to have a counting rate <1000 S-1and acquire a spectrum of the source for a countingtime sufficient to accumulate -50,000 counts in thepeak area. At the end of the count find the peak withthe peak-finding algorithm, if available, and measurethe net peak area with the spectral analysis program.Adjust the gain so that the peak is raised the equiva-lent of one FWHM. Without erasing the acquiredspectrum, continue the count for an equal countingtime. At the end of the count find both peaks withthe peak-finding algorithm and measure the netpeak areas of each component. Each of the compo-nents of the doublet peak should be found and eachof the net areas should agree to within 1;3 of the ac-ceptable total accuracy. If the areas are not deter-mined to within 1/3 of the acceptable total accuracy,repeat the test. If the test cannot be passed on thesecond attempt, rerun the test with a peak separationof 11;2 FWHM or greater until the test is passed. Re-cord the separation at which the test can be passed.

Repeat the above procedure with the followingmodifications: (1) change the gain so that the peak israised two FWHM and reduce the second acquisi-tion time by a factor of ten; (2) change the gain sothat the peak is lowered two FWHM and reduce thesecond acquisition time by a factor of ten. Each ofthe components should be found and each of the netareas should agree (within 1;3 of the total acceptableaccuracy) with the net area measured from the singlepeak scaled by the relative acquisition times. If the ar-eas are not determined within 1;3 of the acceptabletotal accuracy, repeat the test. If the test cannot bepassed on the second attempt, rerun the test with the

Area of componentwith fewer counts

Ratio(relative to

primaryLow-energycomponent

deviation in %

+ 16.8+ 7.6+ 2.1+1.0

High-energycomponent

deviation in 0/0

-3.9

-0.8

-1.7

+ 0.2

0.10.20.51.0

a See Footnote from Table 3

Table 4 Comparison of simulated unequal area doublet compo-

nents separated by 2 FWHM with a generated peak

area of 135,000 counts,a

second acquisition time reduced by a factor of 5 orless until the test is passed. Recdrd the net-peak-arearatio for which the test can be passed.

Table 3 indicates the deviation in the net areasresulting from various separations of equal-sizeddoublets and Table 4 indicates the deviation in thenet areas resulting from the weaker of two unequal-sized doublets for a peak separation of two FWHMfor a simple Gaussian peak-fitting algorithm. As seenin Table 4, due to the nonideal Gaussian shape of thegamma-ray spectral peaks, the uncertainty is greaterin the measurement of the area of a lower energycomponent peak with fewer counts than its higher-energy companion peak than it is for a higher energycompanion peak with fewer counts than its lower en-ergy companion.


"Tests to Measure the Perfonnance of a Gennanium Garnrna-Ray Spectrometer and Its Analysis Software"

Robert J. Gehrke

2.5 Assessment of magnitude of coincidencesumming due to cascading gamma raysCascade summing is due to the simultaneous detec-tion of two or more photons originating from a sin-gle nuclear disintegration that results in only one ob-served (summed) pulse. The corrections for thiseffect are specific to each radionuclide being ana-lyzed because they are dependent upon a number ofparameters including the detector's total and full-en-ergy-peak efficiencies, the number of gamma rays incascade, their relative branching from each excitedlevel, and their emission probabilities. X rays result-ing from electron capture or internal electron con-version that are in coincidence with the gamma rayscan further complicate the calculation of the correc-tion factor for each gamma-ray peak. It is assumedhere that beta particles are prevented from reachingthe detector. A detailed discussion of the cascadesumming of gamma rays can be found in reference 2.

Even though coincidence summing is radionu-clide, geometry and detector specific, operators needa quick and simple method of assessing the relativemagnitude of this effect for various detection geome-tries and different size detectors in order to deter-mine if additional (and substantially more compli-cated) corrections are required. It is important tonote that the following test does not provide a cor-rection factor that can be applied to measurementsother than the test measurement but rather allowsan assessment of the relative magnitude of coinci-dence summing at a given geometry. This test is con-ducted with the NIST long half-life mixed radionu-clide standard which is composed of 125Sb, 154Eu,and 155Eu. If the source used in the test is a pointsource, the resultant curve of peak-area losses versusdistance should be a worst case result for summinglosses out of the 591-keV peak from the decay of 154Eu.

The test is designed to measure the loss ofcounts from the 591-keV gamma-ray peak of 154Eu,that is subject to cascade-summing losses (see partiallevel scheme in Figure I), by comparing it to the 600-

0.7033

Figure 1 Partial level scheme of 154Gd from the decay 01

154Eu 5howing the gamma rays in coincidence

with the 591-keV gamma ray.

keY gamma-ray peak of 125Sb which is essentiallyfree of summing if x rays and beta particles are notdetected. A gamma-ray spectrum of the NIST longhalf-life mixed radionuclide standard is illustrated inFigure 2.

To assess, within approximately a factor of two,the severity of cascade-summing losses for a particu-lar counting geometry, acquire a spectrum of theNIST long half-life mixed radionuclide at the dis-tance, i, and preferably the same sample geometry asused in routine sample analysis. a Acquire at least

20,000 and preferably 50,000 counts in the net areasof the 591- and 600-keV peaks. Determine the fol-

a If the detector does not observe the Te K x rays (27 to 32 keY) resulting from internal conversion in the decay of 125Sb, the 600-keVfull energy peak has no counts lost due to summing and only a very few added counts (due to 172-keV gamma rays summing with428-keV gamma rays to add counts to the 600-keV peak-see 125Sb decay scheme in reference 9, 7th edition, p. 612). A 1-mrn-thicktin or cadmium absorber can be placed between the detector and sample during this test to significantly reduce the sensitivity of the

detector's efficiency below 100 keY.


Radioactivity

& Radiochemistry


ClO 5I) 1M 1SO 2OD '5"

Figure 2 Spectrum of the NIST long half-life mixed radionuclide

standard containing 125Sb, 154Eu, and 155Eu.

Figure 3 Cascade summing of the 154Eu 591-keV peak as a func-

tion of the source-to-detector-distance, i, for point

source geometry and a large and small closed end co-

axial germanium spectrometer.

lowing ratio from the net peak areas, A(E)j, for ener-gies E = 591 and 600 keVat distance, i, at which thespectrum was accumulated:

tector housing (referred to as the zero distance posi-tion) and the 5-cm position. Hence a cascade-sum-ming correction may not be required at a given dis-tance, i, if extended samples are to be counted whosemean distance is 5 cm from the detector housing.fi = [A(591)i/A(600)i]

Acquire a second spectrum at a reference dis-tance (i.e., i = rat 10- or 15-cm source-to-detectordistance). The reference distance is where cascadesumming is assumed to be negligible. Calculate theabove ratio for r = i. Calculate Rj as follows: (at dis-tance i = r the ratio will be one by definition):

3. ConclusionMeeting the criteria of the tests described hereinshould provide the practitioner with an indication ofthe capabilities of the germanium spectrometer be-ing tested and an understanding of the capabilitiesand limitations of the associated analysis software.

Ri = ri/rr = [A(591)i/A(600)i]/[A(591)r/A(600)r] AcknowledgementsThis work has been supported by the U .S. Depart~me~t of Energy under DOE contract No. DE-ACO7-76IDO1570. The author appreciates the encourage-ment of and technical discussions with D. D. Hop-pes, J. E. Cline and J. K. Hartwell in the preparationof this manuscript. The assistance ofR. L. Kynastonand R. K. Murray in conducting these tests is appre-ciated.

If the ratio Ri exceeds 1;3 of the total acceptableuncertainty in the measured gamma-ray-emissionrates, cascade-summing corrections will need to bemade. Debertin and Schoetzig,lO and Schima andHoppesll are two references that provide computerprograms for making coincidence summing correc-tions for gamma rays in cascade.

Figure 3 shows for point-source geometry thatthe ratio Ri rapidly approaches one as the source-to-detector distance approaches the reference distanceand that the change is most dramatic between the de-

References1. Calibration and Use of Germanium Spectrometers for the Meas-

urement of Gamma-ray Emission Rates of Radionuclides, R. J.



Robert J. Gehrke

8. J. E. cline, "A Comparison of Detection-Limit Compu~tions for

Four Commercial Gamma-Ray Analysis Programs," Nucl. Instr.

and Methods in Phys. Research, A286, 421, (1990).

9. Table of Isotopes, 7th edition, C. M. Lederer and V. S. Shirley, edi-

tors, John Wiley & Sons, Inc., (1978).

10. K. Debertin and V. Schoetzi& "Coincidence Summing Corrections

in Ge(Li) Spectrometry at Low Source-Detector Distances," Nucl.

Instr. and Methods, 158, 471, (1979).

11. F. J. Schima and D. D. Hoppes, "Tables for Cascade Summing Cor-

rections in Gamma-Ray Spectrometry," Int J. Appl. Radiat Iso-

topes, 34, 1039, (1983).

R&R

Editor's Comments:In 1991, the ANSI National Standards Institutepublished ANSI N42.14-1991, "Calibration andUse of Germanium Spectrometers for the Measure-ment of Gamma-Ray Emission Rates of Radionu-clides."

Gehrke, D. D. Hoppes, F. J. Schima, D. M. Montgomery, and J. E.

Cline (writing group), ANSI N42.14-revised, American National

Standards Institute, Inc., 1430 Broadway, New York, NY 10018,

to be published.

2. K. Debertin and R. G. Helmer, Gamma- and X-Ray Spectrometry

with Semiconductor Detectors, North Holland Publishing Com-

pany, Elsevier Science Publishers B. V., Amsterdam, The Nether-

lands, (1988).

3. H. H. Boltin, M. G. Strauss, and D. A. McClure, "Simple Technique

for Precise Determinations of Counting Losses in Nuclear Pulse

Processing Systems," Nucllnstr. and Methods, 83, 1, (1970).

4. L o. Johnson, E. W. Killian, R. G. Helmer, and R. Coates, "Utiliza-

tion of Concurrently Gathered Data for Complete Spectral Valida-

tion of Gamma-Ray Spectra from Germanium Detectors," IEEE

Transactions on Nuclear Science, NS-28, 638, (Feb. 1981).

5. G. P. Westphal, "Real-Time Corrections of Counting Losses in Nu-

clear Pulse Spectroscopy," Journal of Radioanalytical Chemistry,

70,387, (1982).

6. Lloyd A. Currie, limits for Quantitative Detection and Quantitative

Determination, Analytical Chemistry, 40, 586, (1968).

7. J. H. Head, "Minimum Detectable Photopeak Areas in Ge(Li)

Spectra," Nucllnstr. and Methods, 98, 419, (1972).

R&R


1. introdudion - eckert & ziegler strahlen- und ... · reprint from vol. 1, no.2, 1990 15....

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