1、Designation: D 4962 02 (Reapproved 2009)Standard Practice forNaI(Tl) Gamma-Ray Spectrometry of Water1This standard is issued under the fixed designation D 4962; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revis
2、ion. 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 radionuclidesin water by means of gamma-ray spectrometry. It is applicableto nuclides em
3、itting gamma-rays with 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 avoide
4、d because of electronic 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 conce
5、ntration of agiven nuclide 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
6、 specificgamma-ray emitters 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
7、 to be regarded asstandard. No other units of measurement are included in thisstandard.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-priate safety and health practices
8、and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3D 3648 Practices for the Measurement of RadioactivityD 4962 Practice for NaI(Tl) Gamma-Ray Spectrometry ofWaterE 181 Test Methods for Detector Calibration and Analysisof Radionuclides3.
9、 Summary of Practice3.1 Gamma-ray spectra are commonly measured withmodular equipment consisting of a detector, amplifier, multi-channel analyzer device, and a computer (17 and 18).3.2 Thallium-activated sodium-iodide crystals, NaI(Tl),which can be operated at ambient temperatures, are often usedas
10、gamma-ray detectors in spectrometer systems. However,their energy resolution limits their use to the analysis of singlenuclides or simple mixtures of a few nuclides. 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 m
11、mby 76 mm-configuration.3.3 Interaction of a gamma-ray with the atoms in a NaI(Tl)detector results in light photons that can be detected by amultiplier phototube. The output from the multiplier phototubeand its preamplifier is directly proportional to the energydeposited by the incident gamma-ray. T
12、hese current pulses arefed into an amplifier of sufficient gain to produce voltageoutput pulses in the amplitude range from 0 to 10 V.3.4 A multichannel pulse-height analyzer is used to deter-mine the amplitude of each pulse originating in the detector,and accumulates in a memory the number of pulse
13、s in eachamplitude band (or channel) in a given counting time (17 and18).Fora0to2MeVspectrum two hundred data points areadequate.3.5 The distribution of the amplitudes (pulse heights) of thepulse energies, represented by the pulse height, can be sepa-rated into two principal components. One of these
14、 componentshas a nearly Gaussian distribution and is the result of totalabsorption of the gamma-ray energy in the detector; this peak1This practice is under the jurisdiction of ASTM Committee D19 on Water andis the direct responsibility of Subcommittee D19.04 on Methods of RadiochemicalAnalysis.Curr
15、ent edition approved Feb. 1, 2009. Published March 2009. Originallyapproved in 1989. Last previous edition approved in 2002 as D 4962 02.2The boldface numbers in parentheses refer to the references at the end of thispractice.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcon
16、tact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.is normally referred
17、to as the full-energy peak or photopeak.The 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.Other peaks component
18、s, such as escape peaks, backscatteredgamma-rays, or x-rays from shields, are often superimposed onthe Compton continuum. These portions of the curve areshown in Fig. 1 and Fig. 2. Escape peaks will be present whengamma-rays with energies greater than 1.02 MeV are emittedfrom the sample (19-24). The
19、 positron formed in pair produc-tion is usually annihilated in the detector and one or both of the511 keV annihilation quanta may escape from the detectorwithout interaction. This condition will cause single- ordouble-escape peaks at energies of 0.511 or 1.022 MeV lessthan the photopeak energy. In t
20、he 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 Comptoncontinuum serves no useful quanti
21、tative purpose in photopeakanalysis and must be subtracted from the photopeak to obtainthe correct number of counts before peaks are analyzed.3.6 If the analysis is being directed and monitored by anonline computer program, the analysis period may be termi-nated by prerequisites incorporated in the
22、program. Analysismay also 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.3.7 Upon completion of the analysis, the spectral
23、 data areinterpreted and 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 n
24、umberfrom the x-axis and converting to gamma-ray energy by meansof an equation relating channel number and gamma-ray energy.If the system is calibrated for 10 keV per channel with channelzero representing 0 keV, the energy can be immediatelycalculated. In some systems the channel number or gamma-ray
25、energy in keV can be displayed on the monitor for any selectedchannel. Identification of nuclides may be aided by libraries ofgamma-ray spectra and other nuclear data tabulations (25-30).3.7.1 Data reduction of spectra involving mixtures ofnuclides is usually accomplished using a library of standard
26、spectra of the individual nuclides acquired under conditionsidentical to that of the unknown sample (25-30).4. Significance and Use4.1 Gamma-ray spectrometry is used to identify radionu-clides and to make quantitative measurements. Use of acomputer and a library of standard spectra will be required
27、forquantitative analysis of complex mixtures of nuclides.4.2 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 these geometry effects, calibrations ared
28、esigned to duplicate all conditions including source-to-detector distance, sample shape and size, and sample matrixencountered when samples are measured. This means that aFIG. 1 Compton ContinuumFIG. 2 Single and Double Escape PeaksD 4962 02 (2009)2complete set of library standards may be required f
29、or eachgeometry and sample to detector distance combination that willbe used.4.3 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
30、 may be used.Quantitative measurements can be made accurately and pre-cisely when high activity level samples are placed at distancesof1mormore from the detector.4.4 Electronic problems, such as erroneous deadtime cor-rection, loss of resolution, and random summing, may beavoided by keeping the gros
31、s count rate below 2 000 counts persecond 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.5. Interferences5.1 In complex mixtures of gamma-ray emitters,
32、the degreeof interference 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.A method of predicting the gamma-ray res
33、olution of a detectoris given in the literature (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 comp
34、lexity of the analysismethod is due to the resolution of these interferences and, thus,one of the main reasons for computerized systems.5.2 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 deca
35、ymay enter the detector to produce a sum peak at 2505 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.5.3 Random summing occurs in all measurements but is
36、afunction of count rate. The total random 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 2 000 counts persecond (see Methods E 181).5.4 The density of the sample is ano
37、ther factor that canaffect quantitative results. This source of error can be avoidedby preparing the standards for calibration in matrices of thesame density of the sample under analysis.6. Apparatus6.1 Gamma Ray Spectrometer, consisting of the followingcomponents, as shown in Fig. 3:6.1.1 Detector
38、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 in an opaque container with atransparent window. The crystal should contain less than 5 g/gof potassium, and
39、 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. Thecrystal should be attached and optically coupled to a photo-multiplier. (The
40、 photomultiplier requires a preamplifier or acathode follower compatible with the amplifier). The resolu-tion (FWHM) of the assembly for the photopeak of cesium-137should be less than 9 %.6.1.2 ShieldThe detector assembly shall be surrounded byan external radiation shield made of massive metal, equi
41、valentto 102 mm of lead in gamma-ray attenuation capability. It isdesirable that the inner walls of the 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 must have agraded inner shield of 1.6 mm of cadmium or t
42、in lined with 0.4mm of copper, to attenuate lead x-rays at 88 keV, on the surfacenear the detector. The shield must have a door or port forinserting and removing samples.6.1.3 High Voltage Power/Bias SupplyHigh-voltagepower supply of range (usually from 500 to 3000 V and up to10 mA) sufficient to op
43、erate a NaI(Tl) detector, photomulti-plier, and its preamplifier assembly. The power supply shall beregulated to 0.1 % with a ripple of not more than 0.01 %. Linenoise caused by other equipment shall be removed withradiofrequency filters and additional regulators.6.1.4 AmplifierAn amplifier compatib
44、le with the pream-plifier or emitter follower and with the pulse-height analyzer.6.1.5 Data Acquisition and Storage EquipmentA multi-channel pulse-height analyzer (MCA) or stand-alone analog-to-digital-converter (ADC) under software control of a separateFIG. 3 Gamma Spectrometry SystemD 4962 02 (200
45、9)3computer, performs many functions required for gamma-rayspectrometry. An MCA or computer collects the data, providesa visual display, and outputs final results or raw data for lateranalysis. The four major components of an MCA are the ADC,the memory, the control, and the input/output circuitry an
46、ddevices. TheADC digitizes the analog pulses from the detectoramplifier. These pulses represent energy. The digital resultselects a memory location (channel number) which is used tostore the number of events which have occurred with thatenergy. Simple data analysis and control of the MCA isaccomplis
47、hed with microprocessors, which control factorssuch as the input/output, channel summing over set regions ofinterest, and system energy calibration.6.1.6 Data StorageBecause of the use of microproces-sors, modern MCAs provide a wide range of input and output(I/O) capabilities. Typically, these inclu
48、de the ability to transferany section of data to one or more of the following: printer,floppy or hard disk, x-y plotter, and computer interfaces byway of a serial, parallel, ethernet or USB port.7. Container for Test Specimen7.1 Sample mounts and containers must have a convenientreproducible geometr
49、y. Considerations include commercialavailability, ease of use and disposal, and the containment ofradioactivity for protection of the working environment andpersonnel from contamination. The evaporation of liquidsamples to dryness is not necessary and liquid samples up toseveral litres may be used. However, samples that have beenevaporated to dryness for gross beta counting can also be usedfor gamma-ray spectrometry. Massive samples may causesignificant self-absorption of low-energy gamma-rays anddegrade the higher-energy gamma-rays. Therefore, it is impor-tant to calibrate
