1、Designation: D4962 02 (Reapproved 2009)D4962 17Standard Practice forNaI(Tl) Gamma-Ray Spectrometry of Water1This standard is issued under the fixed designation D4962; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last
2、 revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers the measurement of radionuclides in water by means of gamma-ray spectrometry. It is applicable tonucl
3、ides emitting gamma-rays with energies greater than 50 keV. For typical counting systems and sample types, activity levelsof about 40 Bq (1080 pCi) are easily measured and sensitivities of about 0.4 Bq (11 pCi) are found for many nuclides (1-10).2Count rates in excess of 2000 counts per second shoul
4、d be avoided because of electronic limitations. High count rate samples canbe accommodated by dilution or by increasing the sample to detector distance.1.2 This practice can be used for either quantitative or relative determinations. In tracer work, the results may be expressed bycomparison with an
5、initial concentration of a given nuclide which is taken as 100 %. For radioassay, the results may be expressedin terms of known nuclidic standards for the radionuclides known to be present. In addition to the quantitative measurement ofgamma-ray activity, gamma-ray spectrometry can be used for the i
6、dentification of specific gamma-ray emitters in a mixture ofradionuclides. General information on radioactivity and the measurement of radiation has been published (11 and 12). Informationon specific application of gamma-ray spectrometry is also available in the literature (13-16).1.3 The values sta
7、ted in SI units are to be regarded as standard. No other units of measurement are included in this The valuesgiven in parentheses after SI units are included for information only and are not considered standard.1.4 This standard does not purport to address all of the safety concerns, if any, associa
8、ted with its use. It is the responsibilityof the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine theapplicability of regulatory limitations prior to use.1.5 This international standard was developed in accordance with internationa
9、lly recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:3D1129 Terminol
10、ogy Relating to WaterD3648 Practices for the Measurement of RadioactivityD4962D7902 Practice for NaI(Tl) Gamma-Ray Spectrometry of WaterTerminology for Radiochemical AnalysesE181 Test Methods for Detector Calibration and Analysis of Radionuclides3. Terminology3.1 Definitions:3.1.1 For definitions of
11、 terms used in this standard, refer to Terminologies D1129 and D7902.4. Summary of Practice4.1 Gamma-ray spectra are commonly measured with modular equipment consisting of a detector, amplifier, multi-channelanalyzer device, and a computer (17 and 18).4.2 Thallium-activated sodium-iodide crystals, N
12、aI(Tl), which can be operated at ambient temperatures, are often used asgamma-ray detectors in spectrometer systems. However, their energy resolution limits their use to the analysis of single nuclides1 This practice is under the jurisdiction ofASTM Committee D19 on Water and is the direct responsib
13、ility of Subcommittee D19.04 on Methods of RadiochemicalAnalysis.Current edition approved Feb. 1, 2009Nov. 1, 2017. Published March 2009November 2017. Originally approved in 1989. Last previous edition approved in 20022009 asD4962D4962 02 (2009). 02. DOI: 10.1520/D4962-02R09.10.1520/D4962-17.2 The b
14、oldface numbers in parentheses refer to the references at the end of this practice.3 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary pag
15、e on the ASTM website.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that u
16、sers consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1or simple mixtures of a fe
17、w nuclides. Resolution A resolution of about 7 % (45 keV full width at one half the 137Cs peak height)at 662 keV can be expected for a NaI(Tl) detector in a 76 mm by 76 mm-configuration. There are solid scintillators such as ceriumdoped LaBr3 that may provide a performance advantage over NaI(Tl) in
18、terms of energy resolution but whose suitability shouldbe evaluated and documented before being considered as a substitute for NaI(Tl).4.3 Interaction of a gamma-ray with the atoms in a NaI(Tl) detector results in light photons that can be detected by a multiplierphototube. photomultiplier tube (PMT
19、). The output from the multiplier phototube PMT and its preamplifier is directly proportionalto the energy deposited by the incident gamma-ray. These current pulses are fed into an amplifier of sufficient gain to producevoltage output pulses in the amplitude range from 0 to 10 V.4.4 A multichannel p
20、ulse-height analyzer is used to determine the amplitude of each pulse originating in the detector, andaccumulates in a memory the number of pulses in each amplitude band (or channel) in a given counting time (17 and 18). Fora 0 to 2 MeV spectrum two hundred data points are adequate.channels may be a
21、dequate but most current systems provide athousand or more channels.4.5 The distribution of the amplitudes (pulse heights) of the pulse energies, represented by the pulse height, can be separatedinto two principal components. One of these components has a nearly Gaussian distribution and is the resu
22、lt of total absorptionof the gamma-ray energy in the detector; this peak is normally referred to as the full-energy peak or photopeak. The othercomponent is a continuous one, lower in energy than the photopeak.This continuous curve is referred to as the Compton continuumand results from interactions
23、 wherein the gamma photons lose only part of their energy to the detector. Other peaks components,such as escape peaks, backscattered gamma-rays, or x-rays from shields, are often superimposed on the Compton continuum.Theseportions of the curve are shown in Fig. 1 and Fig. 2. Escape peaks will be pr
24、esent when gamma-rays with energies greater than1.02 MeV are emitted from the sample (19-24). The positron formed in pair production is usually annihilated in the detector andone or both of the 511 keV annihilation quanta may escape from the detector without interaction. This condition will cause si
25、ngle-or double-escape peaks at energies of 0.511 or 1.022 MeV less than the photopeak energy. In the plot of pulse height versus countrate, the size and location of the photopeak on the pulse height axis is proportional to the number and energy of the incidentFIG. 1 Compton ContinuumD4962 172photons
26、, and is the basis for the quantitative and qualitative application of the spectrometer. The Compton continuum serves nouseful quantitative purpose in photopeak analysis and must be subtracted from the photopeak to obtain the correct number ofcounts before peaks are analyzed.4.6 Other peaks componen
27、ts, such as escape peaks, backscattered gamma-rays, or X-rays from shields, are often superimposedon the Compton continuum. These portions of the curve are shown in Fig. 1 and Fig. 2.4.7 Escape peaks will be present when gamma-rays with energies greater than 1.02 MeV are emitted from the sample (19-
28、24).The positron formed in pair production is usually annihilated in the detector and one or both of the 511 keV annihilation quantamay escape from the detector without interaction. This condition will cause single- or double-escape peaks at energies of 0.511or 1.022 MeV less than the photopeak ener
29、gy.”4.8 In the plot of pulse height versus count rate, the size and location of the photopeak on the pulse height axis is proportionalto the number and energy of the incident photons, and is the basis for the quantitative and qualitative application of thespectrometer. The Compton continuum serves n
30、o useful quantitative purpose in photopeak analysis and must be subtracted fromthe photopeak to obtain the correct number of counts before peaks are analyzed.4.9 If the analysis is being directed and monitored by an online computer program, the analysis period may be terminated byprerequisites incor
31、porated in the program. Analysis may also be terminated when a preselected time or total counts in a region ofinterest or in a specified channel is reached. Visual inspection of the computer monitor can also be used as a criterion for manuallyterminating the analysis.4.10 Upon completion of the anal
32、ysis, the spectral data are interpreted and reduced to nuclide activity of becquerels(disintegrations per second) or related units suited to the particular application. At this time, the spectral data may be inspectedon the monitor to identify the gamma-ray emitters present. This is accomplished by
33、reading the channel number from the x-axisand converting to gamma-ray energy by means of an equation relating channel number and gamma-ray energy. If the system iscalibrated for 102 keV per channel with channel zero representing 0 keV, the energy can be immediatelyreadily calculated. In somesystems
34、the channel number or gamma-ray energy in keV can be displayed on the monitor for any selected channel. Identificationof nuclides may be aided by libraries of gamma-ray spectra and other nuclear data tabulations (25-30).3.7.1 Data reduction of spectra involving mixtures of nuclides is usually accomp
35、lished using a library of standard spectra ofthe individual nuclides acquired under conditions identical to that of the unknown sample (25-30).4.11 Data reduction of spectra involving mixtures of nuclides is usually accomplished using a library of standard spectra of theindividual nuclides acquired
36、under conditions identical to that of the unknown sample (25-30).5. Significance and Use5.1 Gamma-ray spectrometry is used to identify radionuclides and to make quantitative measurements. Use of a computer anda library of standard spectra will be required for quantitative analysis of complex mixture
37、s of nuclides.FIG. 2 Single and Double Escape PeaksD4962 1735.2 Variation of the physical geometry of the sample and its relationship with the detector will produce both qualitative andquantitative variations in the gamma-ray spectrum. To adequately account for these geometry effects, calibrations a
38、re designed toduplicate all conditions including source-to-detector distance, sample shape and size, and sample matrix encountered whensamples are measured. This means that a complete set of library standards may be required for each geometry and sample todetector distance combination that will be u
39、sed.5.3 Since some spectrometry systems are calibrated at many discrete distances from the detector, a wide range of activity levelscan be measured on the same detector. For high-level samples, extremely low efficiency geometries may be used. Quantitativemeasurements can be made accurately and preci
40、sely when high activity level samples are placed at distances of 1 m or more fromthe detector.5.4 Electronic problems, such as erroneous deadtime correction, loss of resolution, and random summing, may be avoided bykeeping the gross count rate below 2 000 counts per second and also keeping the deadt
41、ime of the analyzer below 5 %. Totalcounting time is governed by the activity of the sample, the detector source distance, and the acceptable Poisson countinguncertainty.6. Interferences6.1 In complex mixtures of gamma-ray emitters, the degree of interference of one nuclide in the determination of a
42、nother isgoverned by several factors. If the gamma-ray emission rates from different radionuclides are similar, interference will occur whenthe photopeaks are not completely resolved and overlap. A method of predicting the gamma-ray resolution of a detector is givenin the literature (31). If the nuc
43、lides are present in the mixture in unequal portions radiometrically, and nuclides of highergamma-ray energies are predominant, there are serious interferences with the interpretation of minor, less energetic gamma-rayphotopeaks.The complexity of the analysis method is due to the resolution of these
44、 interferences and, thus, one of the main reasonsfor computerized systems.6.2 Cascade summing may occur when nuclides that decay by a gamma-ray cascade are analyzed. Cobalt-60 is an example;11721773 and 1333 keV gamma-rays from the same decay may enter the detector to produce a sum peak at 25052506
45、keV andcause the loss of counts from the other two peaks. Cascade summing may be reduced by increasing the source to detector distance.Summing is more significant if a well-type detector is used.6.3 Random summing occurs in all measurements but is a function of count rate. The total random summing r
46、ate is proportionalto the square of the total number of counts. For most systems, random summing losses can be held to less than 1 % by limitingthe total counting rate to 2 0002000 counts per second (see Test Methods E181).6.4 The density of the sample is another factor that can affect quantitative
47、results. This source of error can be avoided bypreparing the standards for calibration in matrices of the same density of the sample under analysis.7. Apparatus7.1 Gamma Ray Spectrometer, consisting of the following components, as shown in Fig. 3: . Some currently availablecommercial systems incorpo
48、rate the power supply, preamplifier, amplifier, and multichannel analyzer into a single unit.7.1.1 Detector AssemblySodium iodide crystal, activated with about 0.1 % 0.1 % thallium iodide, cylindrical, with or withoutan inner sample well, 51 to 102 mm in diameter, 44 to 102 mm 102-mm high, and herme
49、tically sealed in an opaque container witha transparent window. The crystal should contain less than 5 g/g of potassium, and should be free of other radioactive materials.In order to establish freedom from other radioactive materials, the manufacturer should supply the gamma-ray spectrum of thebackground of the crystal between 80 and 3000 keV. The crystal should be attached and optically coupled to a photomultiplier.(The photomultiplier photomultiplier or other suitable optical sensor such as an avalanche photodiode. A photomultiplier requiresa preamplifier or a cathode foll
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