ASTM D4962-17 Standard Practice for NaI(Tl) Gamma-Ray Spectrometry of Water.pdf

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1、Designation: 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 revision. A number in par

2、entheses 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 radionuclidesin water by means of gamma-ray spectrometry. It is applicableto nuclides emitting gamma-rays wi

3、th energies greater than 50keV. For typical counting systems and sample types, activitylevels of about 40 Bq (1080 pCi) are easily measured andsensitivities of about 0.4 Bq (11 pCi) are found for manynuclides (1-10).2Count rates in excess of 2000 counts persecond should be avoided because of electro

4、nic limitations.High count rate samples can be accommodated by dilution orby increasing the sample to detector distance.1.2 This practice can be used for either quantitative orrelative determinations. In tracer work, the results may beexpressed by comparison with an initial concentration of agiven n

5、uclide which is taken as 100 %. For radioassay, theresults may be expressed in terms of known nuclidic standardsfor the radionuclides known to be present. In addition to thequantitative measurement of gamma-ray activity, gamma-rayspectrometry can be used for the identification of specificgamma-ray e

6、mitters in a mixture of radionuclides. Generalinformation on radioactivity and the measurement of radiationhas been published (11 and 12). Information on specificapplication of gamma-ray spectrometry is also available in theliterature (13-16).1.3 The values stated in SI units are to be regarded asst

7、andard. The values given in parentheses after SI units areincluded for information only and are not considered standard.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-pr

8、iate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.5 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelo

9、pment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:3D1129 Terminology Relating to WaterD3648 Practices for the Measurement of RadioactivityD7902 Terminology for Radi

10、ochemical AnalysesE181 Test Methods for Detector Calibration and Analysis ofRadionuclides3. Terminology3.1 Definitions:3.1.1 For definitions of terms used in this standard, refer toTerminologies D1129 and D7902.4. Summary of Practice4.1 Gamma-ray spectra are commonly measured withmodular equipment c

11、onsisting of a detector, amplifier, multi-channel analyzer device, and a computer (17 and 18).4.2 Thallium-activated sodium-iodide crystals, NaI(Tl),which can be operated at ambient temperatures, are often usedas gamma-ray detectors in spectrometer systems. However,their energy resolution limits the

12、ir use to the analysis of singlenuclides or simple mixtures of a few nuclides. A resolution ofabout 7 % (45 keV full width at one half the137Cs peak height)at 662 keV can be expected for a NaI(Tl) detector in a 76 mmby 76 mm-configuration. There are solid scintillators such ascerium doped LaBr3that

13、may provide a performance advan-tage over NaI(Tl) in terms of energy resolution but whosesuitability should be evaluated and documented before beingconsidered 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

14、by aphotomultiplier tube (PMT). The output from the PMT and itspreamplifier is directly proportional to the energy deposited bythe incident gamma-ray. These current pulses are fed into an1This practice is under the jurisdiction of ASTM Committee D19 on Water andis the direct responsibility of Subcom

15、mittee D19.04 on Methods of RadiochemicalAnalysis.Current edition approved Nov. 1, 2017. Published November 2017. Originallyapproved in 1989. Last previous edition approved in 2009 as D4962 02 (2009).DOI: 10.1520/D4962-17.2The boldface numbers in parentheses refer to the references at the end of thi

16、spractice.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 standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive

17、, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issu

18、ed by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1amplifier 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 originating in the detector,an

19、d accumulates in a memory the number of pulses in eachamplitude band (or channel) in a given counting time (17 and18).Fora0to2MeVspectrum two hundred channels may beadequate but most current systems provide a thousand or morechannels.4.5 The distribution of the amplitudes (pulse heights) of thepulse

20、 energies, represented by the pulse height, can be sepa-rated into two principal components. One of these componentshas a nearly Gaussian distribution and is the result of totalabsorption of the gamma-ray energy in the detector; this peakis normally referred to as the full-energy peak or photopeak.T

21、he other component is a continuous one, lower in energy thanthe photopeak. This continuous curve is referred to as theCompton continuum and results from interactions wherein thegamma photons lose only part of their energy to the detector.4.6 Other peaks components, such as escape peaks, back-scatter

22、ed gamma-rays, or X-rays from shields, are oftensuperimposed on the Compton continuum. These portions ofthe curve are shown in Fig. 1 and Fig. 2.4.7 Escape peaks will be present when gamma-rays withenergies greater than 1.02 MeV are emitted from the sample(19-24). The positron formed in pair product

23、ion is usuallyannihilated in the detector and one or both of the 511 keVannihilation quanta may escape from the detector withoutinteraction. This condition will cause single- or double-escapepeaks at energies of 0.511 or 1.022 MeV less than thephotopeak energy.”4.8 In the plot of pulse height versus

24、 count rate, the size andlocation of the photopeak on the pulse height axis is propor-tional to the number and energy of the incident photons, and isthe basis for the quantitative and qualitative application of thespectrometer. The Compton continuum serves no useful quan-titative purpose in photopea

25、k analysis and must be subtractedfrom the photopeak to obtain the correct number of countsbefore peaks are analyzed.4.9 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. Analysismay also

26、be terminated when a preselected time or total countsin a region of interest or in a specified channel is reached.Visual inspection of the computer monitor can also be used asa criterion for manually terminating the analysis.4.10 Upon completion of the analysis, the spectral data areinterpreted and

27、reduced to nuclide activity of becquerels(disintegrations per second) or related units suited to theparticular application. At this time, the spectral data may beinspected on the monitor to identify the gamma-ray emitterspresent. This is accomplished by reading the channel numberfrom the x-axis and

28、converting to gamma-ray energy by meansof an equation relating channel number and gamma-ray energy.If the system is calibrated for 2 keV per channel with channelzero representing 0 keV, the energy can be readily calculated.In some systems the channel number or gamma-ray energy inkeV can be displayed

29、 on the monitor for any selected channel.FIG. 1 Compton ContinuumFIG. 2 Single and Double Escape PeaksD4962 172Identification of nuclides may be aided by libraries of gamma-ray spectra and other nuclear data tabulations (25-30).4.11 Data reduction of spectra involving mixtures of nu-clides is usuall

30、y accomplished using a library of standardspectra of the individual nuclides acquired under conditionsidentical to that of the unknown sample (25-30).5. Significance and Use5.1 Gamma-ray spectrometry is used to identify radionu-clides and to make quantitative measurements. Use of acomputer and a lib

31、rary of standard spectra will be required forquantitative analysis of complex mixtures of nuclides.5.2 Variation of the physical geometry of the sample and itsrelationship with the detector will produce both qualitative andquantitative variations in the gamma-ray spectrum. To ad-equately account for

32、 these geometry effects, calibrations aredesigned to duplicate all conditions including source-to-detector distance, sample shape and size, and sample matrixencountered when samples are measured. This means that acomplete set of library standards may be required for eachgeometry and sample to detect

33、or distance combination that willbe used.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 measuremen

34、ts can be made accurately and pre-cisely when high activity level samples are placed at distancesof1mormore from the detector.5.4 Electronic problems, such as erroneous deadtimecorrection, loss of resolution, and random summing, may beavoided by keeping the gross count rate below 2 000 counts persec

35、ond and also keeping the deadtime of the analyzer below5 %. Total counting time is governed by the activity of thesample, the detector source distance, and the acceptablePoisson counting uncertainty.6. Interferences6.1 In complex mixtures of gamma-ray emitters, the degreeof interference of one nucli

36、de 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.A method of predicting the gamma-ray resolution of a detectoris given in the l

37、iterature (31). If the nuclides are present in themixture in unequal portions radiometrically, and nuclides ofhigher gamma-ray energies are predominant, there are seriousinterferences with the interpretation of minor, less energeticgamma-ray photopeaks. The complexity of the analysismethod is due to

38、 the resolution of these interferences and, thus,one of the main reasons for computerized systems.6.2 Cascade summing may occur when nuclides that decayby a gamma-ray cascade are analyzed. Cobalt-60 is an ex-ample; 1773 and 1333 keV gamma-rays from the same decaymay enter the detector to produce a s

39、um peak at 2506 keV andcause the loss of counts from the other two peaks. Cascadesumming may be reduced by increasing the source to detectordistance. Summing is more significant if a well-type detector isused.6.3 Random summing occurs in all measurements but is afunction of count rate. The total ran

40、dom summing rate isproportional to the square of the total number of counts. Formost systems, random summing losses can be held to less than1 % by limiting the total counting rate to 2000 counts persecond (see Test Methods E181).6.4 The density of the sample is another factor that canaffect quantita

41、tive results. This source of error can be avoidedby preparing the standards for calibration in matrices of thesame density of the sample under analysis.7. Apparatus7.1 Gamma Ray Spectrometer, consisting of the followingcomponents, as shown in Fig. 3. Some currently availablecommercial systems incorp

42、orate the power supply,preamplifier, amplifier, and multichannel analyzer into a singleunit.7.1.1 Detector AssemblySodium iodide crystal, activatedwith about 0.1 % thallium iodide, cylindrical, with or withoutan inner sample well, 51 to 102 mm in diameter, 44 to 102-mmhigh, and hermetically sealed i

43、n an opaque container with atransparent window. The crystal should contain less than 5 g/gof 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 cryst

44、al between 80 and 3000 keV. TheFIG. 3 Gamma Spectrometry SystemD4962 173crystal should be attached and optically coupled to a photo-multiplier or other suitable optical sensor such as an avalanchephotodiode. A photomultiplier requires a preamplifier or acathode follower compatible with the amplifier

45、. The resolution(FWHM) of the assembly for the photopeak of137Cs should beless than 9 %.7.1.2 ShieldThe detector assembly shall be surrounded byan external radiation shield made of dense metal, equivalent to102 mm of lead in gamma-ray attenuation capability. It isdesirable that the inner walls of th

46、e shield be at least 127 mmdistant from the detector surfaces to reduce backscatter. If theshield is made of lead or a lead liner, the shield may have agraded inner liner of 1.6 mm of cadmium or tin lined with 0.4mm of copper, to attenuate lead X-rays at 88 keV, on thesurface near the detector. The

47、shield must have a door or portfor inserting and removing samples.7.1.3 High Voltage Power/Bias SupplyHigh-voltagepower supply of range (usually from 500 to 3000 V and up to10 mA) sufficient to operate a NaI(Tl) detector,photomultiplier, and its preamplifier assembly. The powersupply shall be regula

48、ted to 0.1 % with a ripple of not morethan 0.01 %. Line noise caused by other equipment shall beremoved with radiofrequency filters and additional regulators.7.1.4 Preamplifier/AmplifierAn amplifier compatible withthe preamplifier or emitter follower and with the pulse-heightanalyzer.7.1.5 Scalar/Ti

49、merA scalar/timer may be used to monitorthe count and regulate the spectral acquisition.7.1.6 ADC/MCAA multichannel pulse-height analyzer(MCA) or stand-alone analog-to-digital-converter (ADC) un-der software control of a separate computer, performs manyfunctions required for gamma-ray spectrometry. An MCA orcomputer collects the data, provides a visual display, andoutputs final results or raw data for later analysis. The fourmajor components of an MCA are the ADC, the memory, thecontrol, and the input/output circuitry and devices. The ADCdigitizes the analog