1、Designation: D 4962 02Standard 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 revision. A number in p
2、arentheses 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 radionuclidesin water by means of gamma-ray spectrometry. It is applicableto nuclides emitting gamma-rays
3、 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 avoided because of elec
4、tronic 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 agive
5、n 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 specificgamma-ra
6、y 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 This standard does not purport to address all of t
7、hesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:D 3648 Practices for Measur
8、ement of Radioactivity3E 181 Test Methods for Detector Calibration and Analysisof Radionuclides43. 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-activate
9、d 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 their use to the analysis of singlenuclides or simple mixtures of a few nuclides. Resolution ofabout 7 % (45 keV full
10、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.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 phototub
11、eand its preamplifier is directly proportional to the energydeposited by the incident gamma-ray. These 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 a
12、mplitude of each pulse originating in the detector,and accumulates in a memory the number of pulses 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 energi
13、es, 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.The othe
14、r 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 components, such as escape peaks, backscatteredgamma-rays
15、, 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 positron formed in pair produc-tion is usually
16、annihilated in the detector and one or both of the511 keV annihilation quanta may escape from the detector1This 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 Feb. 1
17、0, 2002. Published May 2002. Originallypublished as D 4962 89. Last previous edition D 4962 95.2The boldface numbers in parentheses refer to the references at the end of thispractice.3Annual Book of ASTM Standards, Vol 11.02.4Annual Book of ASTM Standards, Vol 12.02.1Copyright ASTM International, 10
18、0 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.without interaction. This condition will cause single- ordouble-escape peaks at energies of 0.511 or 1.022 MeV lessthan the photopeak energy. In the plot of pulse height versuscount rate, the size and location of the p
19、hotopeak 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 quantitative purpose in photopeakanalysis and must be subtracted from the phot
20、opeak 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 program. Analysismay also be terminated when a preselected time or total
21、 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 data areinterpreted and reduced to nuclide activity of becquerels(disin
22、tegrations 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 converting to gamma-ray energy by meansof an eq
23、uation 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-rayenergy in keV can be displayed on the monitor for any selectedchannel. I
24、dentification 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 standardspectra of the individual nuclides acquired under conditionsidentical to
25、 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 forquantitative analysis of complex mixtures of nuclides.4.2 Variation o
26、f 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 aredesigned to duplicate all conditions including source-to-detector distanc
27、e, 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 detector distance combination that willbe used.4.3 Since some spectrometry systems are calibrated at manydiscrete distan
28、ces 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 measurements can be made accurately and pre-cisely when high activity level samples are placed at distancesof1mormore from t
29、he detector.4.4 Electronic problems, such as erroneous deadtime cor-rection, loss of resolution, and random summing, may beavoided by keeping the gross 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 t
30、heFIG. 1 Compton ContinuumFIG. 2 Single and Double Escape PeaksD4962022sample, the detector source distance, and the acceptablePoisson counting uncertainty.5. Interferences5.1 In complex mixtures of gamma-ray emitters, the degreeof interference of one nuclide in the determination of anotheris govern
31、ed 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 literature (31). If the nuclides are present
32、 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 the resolution of these interferences and,
33、 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 decaymay enter the detector to produce a sum peak at 2505 keV andcause the loss of co
34、unts 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 afunction of count rate. The total random summing rate isproportional to the squa
35、re 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 another factor that canaffect quantitative results. This source of error can be avoi
36、dedby 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 AssemblySodium iodide crystal, activatedwith about 0.1 % thallium iodide, cylindr
37、ical, 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 should be free of other radioactive materials.In order to establish freedom from
38、 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 photomultiplier requires a preamplifier or acathode follower compatible with the
39、 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, equivalentto 102 mm of lead in gamma-ray attenuation capability. It isdesirable that
40、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 tin lined with 0.4mm of copper, to attenuate lead x-rays at 88 keV, on the surface
41、near 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 operate a NaI(Tl) detector, photomulti-plier, and its preamplifier assembly. The po
42、wer 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 compatible with the pream-plifier or emitter follower and with the pulse-height analyzer.
43、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 separatecomputer, performs many functions required for gamma-rayspectrometry. An MCA or computer collects the data, providesa visual di
44、splay, 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 anddevices. TheADC digitizes the analog pulses from the detectoramplifier. These pulses represent energy. The digital resultselects
45、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 isaccomplished with microprocessors, which control factorssuch as the input/output, channel summing over set regions ofinterest, and system e
46、nergy calibration.6.1.6 Data StorageBecause of the use of microproces-sors, modern MCAs provide a wide range of input and outputFIG. 3 Gamma Spectrometry SystemD4962023(I/O) capabilities. Typically, these include the ability to transferany section of data to one or more of the following: printer,flo
47、ppy 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 geometry. Considerations include commercialavailability, ease of use and disposal, and the contai
48、nment 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 u
49、sedfor 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 the detector with standards of the samegeometry and density. A beta absorber consisting of about 6mm of aluminum, beryllium, or plastic may be used forsamples that have a significant beta activity and high betaenergies.8. Single or Simple Mixtures of Radionuclides8.1 Calibration and Standardization:8.1.1 Begin operation of the instrumentation and detectoraccording to the manufacturers ins
