1、Designation: D 3649 06An American National StandardStandard Practice forHigh-Resolution Gamma-Ray Spectrometry of Water1This standard is issued under the fixed designation D 3649; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、 year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers the measurement of gamma-rayemitting radionuclides in water by means of gamma-rayspectr
3、ometry. It is applicable to nuclides emitting gamma-rayswith energies greater than 45 keV. For typical counting systemsand sample types, activity levels of about 40 Bq are easilymeasured and sensitivities as low as 0.4 Bq are found for manynuclides. Count rates in excess of 2000 counts per secondsho
4、uld be avoided because of electronic limitations. Highcount rate samples can be accommodated by dilution, byincreasing the sample to detector distance, or by using digitalsignal processors.1.2 This practice can be used for either quantitative orrelative determinations. In relative counting work, the
5、 resultsmay be expressed by comparison with an initial concentrationof a given nuclide which is taken as 100 %. For quantitativemeasurements, the results may be expressed in terms of knownnuclidic standards for the radionuclides known to be present.This practice can also be used just for the identif
6、ication ofgamma-ray emitting radionuclides in a sample without quan-tifying them. General information on radioactivity and themeasurement of radiation has been published (1,2).2Informa-tion on specific application of gamma spectrometry is alsoavailable in the literature (3-5). See also the reference
7、d ASTMStandards in 2.1 and the related material section at the end ofthis standard.1.3 This standard does not purport to address the safetyconcerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety andhealth practices and determine
8、 the applicability of regulatorylimitation prior to use.2. Referenced Documents2.1 ASTM Standards:3D 1066 Practice for Sampling SteamD 1129 Terminology Relating to WaterD 2777 Practice for Determination of Precision and Bias ofApplicable Test Methods of Committee D19 on WaterD 3370 Practices for Sam
9、pling Water from Closed ConduitsD 3648 Practices for the Measurement of RadioactivityD 4448 Guide for Sampling Ground-Water MonitoringWellsE 181 Test Methods for Detector Calibration and Analysisof Radionuclides3. Terminology3.1 DefinitionsFor definitions of terms used in this prac-tice, refer to Te
10、rminology D 1129. For terms not defined in thispractice or in Terminology D 1129, reference may be made toother published glossaries.4. Summary of Practice4.1 Gamma ray spectra are measured with modular equip-ment consisting of a detector, high-voltage power supply,preamplifier, amplifier and analog
11、-to-digital converter (or digi-tal signal processor), multichannel analyzer, as well as acomputer with display.4.2 High-purity germanium (HPGe) detectors, p-type orn-type, are used for the analysis of complex gamma-ray spectrabecause of their excellent energy resolution. These germaniumsystems, howe
12、ver, are characterized by high cost and requirecooling. Liquid nitrogen or electromechanical cooling, or both,can be used.4.3 In a germanium semiconductor detector, gamma-rayphotons produce electron-hole pairs. The charged pair is thencollected by an applied electric field. A very stable low noise1T
13、his practice is under the jurisdiction of ASTM Committee D19 on Water andis the direct responsibility of Subcommittee D19.04 on Methods of RadiochemicalAnalysis.Current edition approved Dec. 15, 2006. Published January 2007. Originallyapproved in 1978. Last previous edition approved in 1998 as D 364
14、9 98a.2The boldface numbers in parentheses refer to the list of references at the end ofthis test method.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standar
15、ds Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.preamplifier is needed to amplify the pulses of electric chargeresulting from gamma photon interactions. The output from thepreamplifier is d
16、irectly proportional to the energy deposited bythe incident gamma-ray. These current pulses are fed into anamplifier of sufficient gain to produce voltage output pulses inthe amplitude range from 0 to 10 V.4.4 A multichannel pulse-height analyzer is used to deter-mine the amplitude of each pulse ori
17、ginating in the detector,and accumulates in a memory the number of pulses in eachamplitude band (or channel) in a given counting time. Com-puterized systems with stored programs and interface hardwarecan accomplish the same functions as hardwired multichannelanalyzers. The primary advantages of the
18、computerized systeminclude the capability of programming the multi-channel ana-lyzer functions and the ability to immediately perform datareduction calculations using the spectral data stored in thecomputer memory or mass storage device. Fora0to2-MeVspectrum, 4000 or more channels are typically need
19、ed in orderto fully utilize a germanium detectors excellent energy reso-lution.4.5 The distribution of the amplitudes (pulse heights) of thepulses can be separated into two principal components. One ofthese components has a nearly Gaussian distribution and is theresult of total absorption of the gam
20、ma-ray energy in thedetector. This peak is normally referred to as the full-energypeak or photopeak. The other component is a continuous onelower in energy than that of the photopeak. This continuouscurve is referred to as the Compton continuum and is due tointeractions wherein the gamma photons dep
21、osit only part oftheir energy in the detector. These two portions of the curve areshown in Fig. 1. Other peaks, such as escape peaks, backscat-tered gamma rays or X rays from shields, are often superim-posed on the Compton continuum. Escape peaks will bepresent when gamma-rays with energies greater
22、than 1.02 MeVare emitted from the sample. The positron formed in pairproduction is usually annihilated in the detector and one orboth of the 511keV annihilation quanta may escape from thedetector without interaction. This condition will cause single ordouble escape peaks at energies of 0.511 or 1.02
23、2 MeV lessthan the photopeak energy. In the plot of pulse height versuscount rate, the size and location of the photopeak on the pulseheight axis is proportional to the number and energy of theincident photons, and is the basis for the quantitative andqualitative application of the spectrometer. The
24、 Comptoncontinuum serves no useful purpose in photopeak analysis andmust be subtracted when peaks are analyzed.4.6 If the analysis is being directed and monitored by anonline computer program, the analysis period may be termi-nated by prerequisites incorporated in the program. If theanalysis is bein
25、g performed with a modern multichannelanalyzer, analysis may be terminated when a preselected timeor total counts in a region of interest or in a specified channelis reached. Visual inspection of a display of accumulated datacan also be used as a criterion for manually terminating theanalysis on eit
26、her type of data acquisition systems.4.7 Upon completion of the analysis, the spectral data areinterpreted and reduced to include activity of Bq (disintegra-tion per second) or related units suited to the particularapplication. At this time the spectral data may be inspected toidentify the gamma-ray
27、 emitters present. This is accomplishedby reading the channel number from the x-axis and convertingto gamma-ray energy by multiplying by the appropriate keV/channel (system gain). In some systems the channel number orgamma-ray energy in keV can be displayed for any selectedchannel. Identification of
28、 nuclides may be aided by catalogs ofgamma-ray spectra and other nuclear data tabulations (3,6-8).4.7.1 Computer programs for data reduction have been usedextensively although calculations for some applications can beperformed effectively with the aid of a scientific calculator.Data reduction of spe
29、ctra taken with germanium spectrometrysystems is usually accomplished by integration of the photo-peaks above a definable background (or baseline) and subse-quent activity calculations using a library which includes dataFIG. 1 Cesium-137 SpectrumD 3649 062such as nuclide name, half-life, gamma-ray e
30、nergies, andabsolute gamma intensity.5. Significance and Use5.1 Gamma-ray spectrometry is of use in identifying radio-nuclides and in making quantitative measurements. Use of asemiconductor detector is necessary for high-resolution mea-surements.5.2 Variation of the physical geometry of the sample a
31、nd itsrelationship with the detector will produce both qualitative andquantitative variations in the gamma-ray spectrum. To ad-equately account for these geometry effects, calibrations aredesigned to duplicate all conditions including source-to-detector distance, sample shape and size, and sample ma
32、trixencountered when samples are measured.5.3 Since some spectrometry systems are calibrated at manydiscrete distances from the detector, a wide range of activitylevels can be measured on the same detector. For high-levelsamples, extremely low-efficiency geometries may be used.Quantitative measureme
33、nts can be made accurately and pre-cisely when high activity level samples are placed at distancesof 10 cm or more from the detector.5.4 Electronic problems, such as erroneous deadtime cor-rection, loss of resolution, and random summing, may beavoided by keeping the gross count rate below 2000 count
34、s persecond (s1) and also keeping the deadtime of the analyzerbelow 5 %. Total counting time is governed by the radioactiv-ity of the sample, the detector to source distance and theacceptable Poisson counting uncertainty.6. Interferences6.1 In complex mixtures of gamma-ray emitters, the degreeof int
35、erference of one nuclide in the determination of anotheris governed by several factors. If the gamma-ray emission ratesfrom different radionuclides are similar, interference will occurwhen the photopeaks are not completely resolved and overlap.If the nuclides are present in the mixture in unequal po
36、rtionsradiometrically, and if nuclides of higher gamma-ray energiesare predominant, there are serious interferences with theinterpretation of minor, less energetic gamma-ray photopeaks.The complexity of the analysis method is due to the resolutionof these interferences and, thus, one of the main rea
37、sons forcomputerized systems.6.2 Cascade summing may occur when nuclides that decayby a gamma-ray cascade are analyzed. Cobalt-60 is an ex-ample; 1172 and 1333-keV gamma rays from the same decaymay enter the detector to produce a sum peak at 2505 keV andcause the loss of counts from the other two pe
38、aks. Cascadesumming may be reduced by increasing the source to detectordistance. Summing is more significant if a well-type detector isused.6.3 Random summing is a function of counting rate andoccurs in all measurements. The random summing rate isproportional to the total count squared and the resol
39、ving timeof the detector. For most systems random summing losses canbe held to less than 1 % by limiting the total counting rate to2000 counts per second (s1). Refer to Test Methods E 181 formore information.6.4 The density of the sample is another factor that caneffect quantitative results. Errors
40、from this source can beavoided by preparing the standards for calibration in solutionsor other matrices with a density comparable to the samplebeing analyzed.7. Apparatus7.1 Gamma Ray Spectrometer, consisting of the followingcomponents:7.1.1 Detector Assembly:7.1.1.1 Germanium DetectorThe detector m
41、ay have avolume of about 50 to 150 cm3, with a full width at one-half thepeak maximum (FWHM) less than 2.2 keV at 1332 keV,certified by the manufacturer. A charge-sensitive preamplifierusing low noise field effect transistors should be an integralpart of the detector assembly. A convenient support s
42、hould beprovided for samples of the desired form.7.1.1.2 ShieldThe detector assembly may be surroundedby an external radiation shield made of a dense metal,equivalent to 102 mm of lead in gamma-ray attenuationcapability. It is desirable that the inner walls of the shield be atleast 127 mm distant fr
43、om the detector surfaces to reducebackscatter. If the shield is made of lead or a lead liner, theshield may have a graded inner shield of 1.6 mm of cadmiumor tin lined with 0.4 mm of copper, to attenuate the 88-keV PbX-rays. The shield should have a door or port for inserting andremoving samples.7.1
44、.1.3 High Voltage Power/Bias SupplyThe bias supplyrequired for germanium detectors usually provides a voltage upto 5000 V and up to 100 A. The power supply should beregulated to 0.1 % with a ripple of not more than 0.01 %. Linenoise caused by other equipment should be removed with rffilters and addi
45、tional regulators.7.1.1.4 AmplifierAn amplifier compatible with the pream-plifier and with the pulse-height analyzer shall be provided.7.1.2 Data Acquisition and Storage Equipment:7.1.2.1 Data AcquisitionsA multichannel pulse-heightanalyzer (MCA) or stand-alone analog-to-digital-converter(ADC) under
46、 software control of a separate computer, per-forms many functions required for gamma-ray spectrometry.An MCA or computer collects the data, provides a visualdisplay, and outputs final results or raw data for later analysis.The four major components of an MCA are the ADC, thememory, control, and inp
47、ut/output. More recently, digitalsignal processors (DSP) can directly amplify and digitizesignals from the preamplifier, replacing individual amplifierand ADC components. The ADC digitizes the analog pulsesfrom the amplifier. These pulses represent energy. The digitalresult is used by the MCAto sele
48、ct a memory location (channelnumber) which is used to store the number of events whichhave occurred with that energy. Simple data analysis andcontrol of the MCA is accomplished with microprocessors.These processors control the input/output, channel summingover set regions of interest, and system ene
49、rgy calibration toname a few examples.7.1.2.2 Data StorageBecause of the use of microproces-sors modern MCAs provide a wide range of input and output(I/O) capabilities.D 3649 0638. Sampling8.1 Collect the sample in accordance with Practice D 1066,Practices D 3370, Guide D 4448, or other documented proce-dures.8.2 Preserve the sample in a radioactively homogeneousstate.Asample can be made radioactively homogeneous by theaddition of a reagent in which the radionuclides or compoundsof the radionuclides present would be soluble in large concen-trations. Addition of acids, c
