ASTM D4962-2018 Standard Practice for NaI(Tl) Gamma-Ray Spectrometry of Water.pdf

1、Designation: D4962 17D4962 18Standard 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 revision. A numbe

2、r 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 tonuclides emitting gamm

3、a-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 should be avoided becau

4、se 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 initial concentrat

5、ion 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 identification of s

6、pecific gamma-ray emitters in a mixture ofradionuclides. radionuclides but that ability is limited when using low energy resolution Na(Tl) detectors as compared to HighPurity Germanium (HPGe) detectors. General information on radioactivity and the measurement of radiation has been published(11 and 1

7、2). Information on specific application of gamma-ray spectrometry is also available in the literature (13-16).1.3 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are included forinformation only and are not considered standard.1.4 This sta

8、ndard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability ofregulatory limitations prior to use.1.5 This inte

9、rnational standard was developed in accordance with internationally 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) Com

10、mittee.2. Referenced Documents2.1 ASTM Standards:3D1129 Terminology Relating to WaterD3648 Practices for the Measurement of RadioactivityD7902 Terminology for Radiochemical AnalysesE181 Test Methods for Detector Calibration and Analysis of Radionuclides3. Terminology3.1 Definitions:3.1.1 For definit

11、ions of 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, analog-to-digitalconverter, multi-channel analyzer device, and a computer (17 and 18).1 This pract

12、ice is under the jurisdiction ofASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.04 on Methods of RadiochemicalAnalysis.Current edition approved Nov. 1, 2017Oct. 1, 2018. Published November 2017November 2018. Originally approved in 1989. Last previous edition approved

13、 in 20092017as D4962 02 (2009).D4962 17. DOI: 10.1520/D4962-17.10.1520/D4962-18.2 The boldface 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 Annua

14、l Book of ASTM Standardsvolume information, refer to the standardsstandards Document Summary page 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

15、 not be technically possible to adequately depict all changes accurately, ASTM recommends that users 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 Harbo

16、r Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14.2 Thallium-activated sodium-iodide crystals, NaI(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

17、 of single nuclidesor simple mixtures of a few nuclides. A resolution of about 7 % (45 keV full width at one half the 137Cs peak height) at 662 keVcan be expected for a NaI(Tl) detector in a 76 mm by 76 mm-configuration. There are solid scintillators such as cerium dopedLaBr3 that may provide a perf

18、ormance advantage over NaI(Tl) in terms of energy resolution but whose suitability should beevaluated 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 aphotomultipli

19、er tube (PMT). The output from the PMT and its preamplifier is directly proportional to the energy deposited by theincident gamma-ray. These current pulses are fed into an amplifier of sufficient gain to produce voltage output pulses in theamplitude range from 0 to 10 V.4.4 A The combination of an a

20、nalog-to-digital converter and multichannel pulse-height analyzer is used to determine theamplitude of each pulse originating in the detector, and accumulates in a memory the number of pulses in each amplitude band(or channel) in a given counting time (17 and 18). For a 0 to 2 MeV spectrum two hundr

21、ed channels may be adequate but mostcurrent systems provide a thousand 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 distribu

22、tion and is the result 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 resul

23、ts from interactions wherein the gamma photons lose only part of their energy to the detector.4.6 Other peaks components, 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

24、.4.7 Escape peaks will be present when gamma-rays with energies greater than 1.02 MeV are emitted from the sample (19-24).The positron formed in pair production is usually annihilated in the detector and one or both of the 511 keV annihilation quantaFIG. 1 Compton ContinuumD4962 182may escape from t

25、he detector without interaction. This condition will cause single- or double-escape peaks at energies of 0.511or 1.022 MeV less than the photopeak energy.”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 a

26、nd energy of the incident photons, and is the basis for the quantitative and qualitative application of thespectrometer. The Compton continuum serves no useful quantitative purpose in photopeak analysis and must be subtracted fromthe photopeak to obtain the correct number of counts before peaks are

27、analyzed.4.9 If the analysis is being directed and monitored by an online computer program, the analysis period may be terminated byprerequisites incorporated 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 r

28、eached. Visual inspection of the computer monitor can also be used as a criterion for manuallyterminating the analysis.4.10 Upon completion of the analysis, the spectral data are interpreted and reduced to nuclide activity of becquerels(disintegrations per second) or related units suited to the part

29、icular application. At this time, the spectral data may be inspectedon the monitor to identify the gamma-ray emitters present. This is accomplished by reading the channel number from thex-axisX-axis and converting to gamma-ray energy by means of an equation relating channel number and gamma-ray ener

30、gy. If thesystem is calibrated for 2 keV per channel with channel zero representing 0 keV, the energy can be readily calculated. In somesystems 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

31、of gamma-ray spectra and other nuclear data tabulations (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 under conditions identical to that of the unknown sample (25-30).5. Significanc

32、e 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 mixtures of nuclides.5.2 Variation of the physical geometry of the sample and its rela

33、tionship with the detector will produce both qualitative andquantitative variations in the gamma-ray spectrum. To adequately account for these geometry effects, calibrations are designed toduplicate all conditions including source-to-detector distance, sample shape and size, and sample matrix encoun

34、tered 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 used.5.3 Since some spectrometry systems are calibrated at many discrete distances from the detector, a wide range of activity

35、 levelscan be measured on the same detector. For high-level samples, extremely low efficiency geometries may be used. Quantitativemeasurements can be made accurately and precisely when high activity level samples are placed at distances of 1 m or more fromthe detector.FIG. 2 Single and Double Escape

36、 PeaksD4962 1835.4 Electronic problems, such as erroneous deadtime correction, loss of resolution, and random summing, may be avoided bykeeping the gross count rate below 2 0002000 counts per second and also keeping the deadtime of the analyzer below 5 %. Totalcounting time is governed by the activi

37、ty 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 another isgoverned by several factors. If the gamma-ray emission rates from

38、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 nuclides are present in the mixture in unequal portions radiometrically, and n

39、uclides 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 interferences and, thus, one of the main reasonsfor computerized systems.6

40、.2 Cascade summing may occur when nuclides that decay by a gamma-ray cascade are analyzed. Cobalt-60 is an example;17731173 and 1333 keV gamma-rays from the same decay may enter the detector to produce a sum peak at 2506 keV and causethe loss of counts from the other two peaks. Cascade summing may b

41、e 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 rate is proportionalto the square of the total number of counts. For most system

42、s, random summing losses can be held to less than 1 % by limitingthe total counting rate to 2000 counts per second (see Test Methods E181).6.4 The density of the sample is another factor that can affect quantitative results. This source of error can be avoided bypreparing the standards for calibrati

43、on 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 incorporate the power supply, preamplifier, amplifier, analog-to-digital converter, and multi

44、channel analyzerinto a single unit.7.1.1 Detector AssemblySodium iodide crystal, activated with about 0.1 % thallium iodide, cylindrical, with or without aninner sample well, 51 to 102 mm in diameter, 44 to 102-mm high, and hermetically sealed in an opaque container with a transparentFIG. 3 Gamma Sp

45、ectrometry SystemD4962 184window. The crystal should contain less than 5 g/g of potassium, and should be free of other radioactive materials. In order toestablish freedom from other radioactive materials, the manufacturer should supply the gamma-ray spectrum of the backgroundof the crystal between 8

46、0 and 3000 keV. The crystal should be attached and optically coupled to a photomultiplier or other suitableoptical sensor such as an avalanche photodiode. A photomultiplier requires a preamplifier or a cathode follower compatible withthe amplifier. The resolution (FWHM) of the assembly for the photo

47、peak of 137Cs should be less than 9 %.7.1.2 ShieldThe detector assembly shall be surrounded by an external radiation shield made of dense metal, equivalent to 102mm of lead in gamma-ray attenuation capability. It is desirable that the inner walls of the shield be at least 127 mm distant fromthe dete

48、ctor surfaces to reduce backscatter. If the shield is made of lead or a lead liner, the shield may have a graded inner linerof of, for example, 1.6 mm of cadmium or tin lined with 0.4 mm of copper, to attenuate lead X-rays at 88 keV, on the surface nearthe detector. The shield must have a door or po

49、rt for inserting and removing samples.7.1.3 High Voltage Power/Bias SupplyHigh-voltage power supply of range (usually from 500 to 3000 V and up to 10 mA)sufficient to operate a NaI(Tl) detector, photomultiplier, and its preamplifier assembly. The power supply shall be regulated to0.1 % with a ripple of not more than 0.01 %. Line noise caused by other equipment shall be removed with radiofrequency filtersand additional regulators.7.1.4 Preamplifier/AmplifierAn amplifier compatible with the preamplifier or emitter follower and with the pulse-heightanalyzer.7.1.5 Scalar/T