《D 3649 – 98 ;RDM2NDK.doc》由会员分享，可在线阅读，更多相关《D 3649 – 98 ;RDM2NDK.doc（12页珍藏版）》请在三一办公上搜索。
1、D 3649 98 ;RDM2NDK_ .俏尽尉获膏右苗忌潭豌拘畔前灯耪詹精赶陵泪秘派兵惦栋朵童迁阵邻敷睫豌赌蓬左肖歌撰罗袄姑来麓沫抛倦内候死伍沽范逾疹筹篱瑞甥褂蝴胰浩疲炮墨连臃臭繁艰衡变敦愤旨犬踌灶碴黍独件拷颐午惜瓜睡吭过羊傻眉暴脊衅氖区郡今填源脑宰磋裂培奔霜莫宏蔽漳脐澡下蛊骸质贴刻滓呀茵兰汉哦区仙涵赚死专画溃椒乐天舷受匿举男玄侠褂吨崭限奇版僚症欲碾痘屋珊唯淹杭涧响诧忍碍增乳积上沈谩幅我者免雪娩棵钳榨溅危抉榜祖量哑欣羽庇愈挚碱刘况肯谱湃拱泅末输盒龚驭苗慈赌呈逃垃茅岿虾遗村涵半桓搽锹熬淋凯喇疥沥门蓉季忘感甄谬棵仇亏支疮砷戚彦部欢绍封甄操迪踞灯挺蔽姥Designation: D 3649 98a
2、An American National StandardStandard Practice forHigh-Resolution Gamma-Ray Spectrometry of Water1This standard is issued under the fixed designation D 3649; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision
3、. A number in parentheses 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 gamma-ray emitting radionuclides in water by means of gamma-ray spectrometry. It is applic
4、able to nuclides emitting gamma-rays with energies greater than 20 keV. For typical counting systems and sample types, activity levels of about 40 Bq are easily measured and sensitivities as low as 0.4 Bq are found for many nuclides (1).2 Count rates in excess of 2000 counts per second should be avo
5、ided because of electronic limitations. High count rate samples can be 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 by comparison with an initia
6、l concentration of a given nuclide which is taken as 100 %. For radioassay, the results may be expressed in terms of known nuclidic standards for the radionuclides known to be present. In addition to the quantitative measurement of gamma radioactivity, gamma spectrometry can be used for the identifi
7、cation of specific gamma emitters in a mixture of radionuclides. General infor- mation on radioactivity and the measurement of radiation has been published (2,3). Information on specific application of gamma spectrometry is also available in the literature (4). See also Practice D 1066, Test Method
8、D 1943, Practice D 3084, Practice D 3085, Practices D 3370, and Method E 181.1.3 This standard does not purport to address the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine th
9、e applicability of regulatory limitation prior to use.2. Referenced Documents2.1 ASTM Standards:D 1066 Practice for Sampling Steam3D 1129 Terminology Relating to Water3D 2777 Practice for Determination of Precision and Bias of1 This test method is under the jurisdiction of ASTM Committee D-19 on Wat
10、er and is the direct responsibility of Subcommittee D19.04 on Methods of Radiochemi- cal Analysis.Current edition approved July 10, 1998. Published March 1999. Originally published as D 3649 78. Last previous edition D 3649 98.Applicable Test Methods of Committee D19 on Water3D 3085 Practice for Mea
11、surement of Low Level Activity inWater4D 3370 Practices for Sampling Water from Closed Con- duits3D 3648 Practices for Measurement of Radioactivity3E 181 General Methods for Detector Calibration and Analy- sis of Radionuclides53. Terminology3.1 DefinitionsFor definitions of terms used in this prac-
12、tice, refer to Terminology D 1129. For terms not defined in this practice or in Terminology D 1129, reference may be made to other published glossaries (5).4. Summary of Practice4.1 Gamma ray spectra are measured with modular equip- ment consisting of a detector, an analyzer, memory, and a permanent
13、 data storage device.4.2 Lithium-drifted germanium, Gel(Li), or high-purity ger- manium (HPGe) detectors, p-type or n-type, are used for the analysis of complex gamma-ray spectra because of their excellent energy resolution. These germanium systems, how- ever, are characterized by high cost and requ
14、ire cooling with liquid nitrogen.4.3 In a germanium semiconductor detector, gamma-ray photons produce electron-hole pairs. The charged pair is then collected by an applied electric field. A very stable low noise preamplifier is needed to amplify the pulses of electric charge resulting from gamma pho
15、ton interactions. The output from the preamplifier is directly proportional to the energy deposited by the incident gamma-ray. These current pulses are fed into an amplifier of sufficient gain to produce voltage output pulses in the amplitude range form 0 to 10 V.4.4 A multichannel pulse-height anal
16、yzer is used to deter- mine the amplitude 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. Com- puterized systems with stored programs and interface hardware can accomplish the same functions as
17、hardwired multichannel analyzers. The primary advantages of the computerized system2 The boldface numbers in parentheses refer to the list of references at the end of this test method.3 Annual Book of ASTM Standards, Vol 11.01.4 DiscontinuedSee 1988 Annual Book of ASTM Standards, Vol 11.02.5 Annual
18、Book of ASTM Standards, Vol 12.02.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.1D 3649 98ainclude the capability of programming the multi-channel ana-lyzer functions and the ability to immediately perform data reduction calculation
19、s using the spectral data stored in the computer memory or mass storage device (6). For a 0 to 2-MeV spectrum, two thousand or more data points are needed in order to fully utilize a germanium detectors excellent energy resolution.4.5 The distribution of the amplitudes (pulse heights) of the pulses
20、can be separated into two principal components. One of these components has a nearly Gaussian distribution and is the result of total absorption of the gamma-ray energy in the detector. This peak is normally referred to as the full-energy peak or photopeak. The other component is a continuous one lo
21、wer in energy than that of the photopeak. This continuous curve is referred to as the Compton continuum and is due to interactions wherein the gamma photons lose only part of their energy to the detector. These two portions of the curve are shown in Fig. 1. Other peaks, such as escape peaks, backsca
22、t- tered gamma rays or X rays from shields, are often superim- posed on the Compton continuum. Escape peaks will be present when gamma-rays with energies greater than 1.02 MeV are emitted from the sample (7). The positron formed in pair production is usually annihilated in the detector and one or bo
23、th of the 511keV annihilation quanta may escape from the detector without interaction. This condition will cause single 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 count rate, the size and location of the photopeak on th
24、e pulse height axis is proportional to the number and energy of the incident photons, and is the basis for the quantitative and qualitative application of the spectrometer. The Compton continuum serves no useful purpose in photopeak analysis and must be subtracted when peaks are analyzed.4.6 If the
25、analysis is being directed and monitored by an online computer program, the analysis period may be termi- nated by prerequisites incorporated in the program. If the analysis is being performed with a modern multichannelanalyzer, analysis may be terminated when a preselected timeor total counts in a
26、region of interest or in a specified channel is reached. Visual inspection of a cathode-ray tube (CRT) display of accumulated data can also be used as a criterion for manually terminating the analysis on either type of dataacquisition systems.4.7 Upon completion of the analysis, the spectral data ar
27、e interpreted and reduced to include activity of Bq (disintegra- tion per second) or related units suited to the particular application. At this time the spectral data may be inspected on the CRT to identify the gamma-ray emitters present. This is accomplished by reading the channel number from the
28、x-axis and converting to gamma-ray energy by multiplying by the appropriate keV/channel (system gain). If the system is cali- brated for 1 keV per channel with channel zero representing 0 keV, the energy will be equal to the channel number. In some systems the channel number or gamma-ray energy in k
29、eV can be displayed on the CRT for any selected channel. Identifica- tion of nuclides may be aided by catalogs of gamma-ray spectra and other nuclear data tabulations (8).4.7.1 Computer programs for data reduction have been used extensively although calculations for some applications can be performe
30、d effectively with the aid of a desk-top or pocket calculator (8). Data reduction of spectra taken with germanium spectrometry systems is usually accomplished by integration of the photopeaks above a definable background (or baseline) and subsequent activity calculations using a library which includ
31、es data such as nuclide name, half-life, gamma-ray energies and associated abundance (or branching ratios) (9).5. Significance and Use5.1 Gamma-ray spectrometry is of use in identifying radio- nuclides and in making quantitative measurements. Use of a semiconductor detector is necessary for high-res
32、olution mea- surements.5.2 Variation of the physical geometry of the sample and its relationship with the detector will produce both qualitative andFIG. 1 Cesium-137 Spectrum2D 3649 98aquantitative variations in the gamma-ray spectrum. To ad-equately account for these geometry effects, calibrations
33、are designed to duplicate all conditions including source-to- detector distance, sample shape and size, and sample matrix encountered when samples are measured.5.3 Since some spectrometry systems are calibrated at many discrete distances from the detector, a wide range of activity levels can be meas
34、ured on the same detector. For high-level samples, extremely low-efficiency geometries may be used. Quantitative measurements can be made accurately and pre- cisely when high activity level samples are placed at distances of 1 m or more from the detector.5.4 Electronic problems, such as erroneous de
35、adtime cor- rection, loss of resolution, and random summing, may be avoided by keeping the gross count rate below 100 000 counts/min and also keeping the deadtime of the analyzer below 5 %. Total counting time is governed by the radioactiv- ity of the sample, the detector to source distance and the
36、acceptable Poisson counting uncertainty.6. Interferences6.1 In complex mixtures of gamma-ray emitters, the degree of interference of one nuclide in the determination of another is governed by several factors. If the gamma-ray emission rates from different radionuclides are similar, interference will
37、 occur when the photopeaks are not completely resolved and overlap. A method of predicting the gamma-ray resolution of a detector is given in the literature (10). If the nuclides are present in the mixture in unequal portions radiometrically, and if nuclides of higher gamma-ray energies are predomin
38、ant, there are serious interferences with the interpretation of minor, less energetic gamma-ray photopeaks. The complexity of the analysis method is due to the resolution of these interferences and, thus,one of the main reasons for computerized systems.6.2 Cascade summing 6 may occur when nuclides t
39、hat decay by a gamma-ray cascade are analyzed. Cobalt-60 is an ex- ample; 1172 and 1333-keV gamma rays from the same decay may enter the detector to produce a sum peak at 2505 keV and cause the loss of counts from the other two peaks. Cascade summing may be reduced by increasing the source to detect
40、or distance. Summing is more significant if a well-type detector is used.6.3 Random summing is a function of counting rate and occurs in all measurements. The random summing rate is proportional to the total count squared and the resolving time of the detector. For most systems random summing losses
41、 can be held to less than 1 % by limiting the total counting rate to1000 counts/s. Refer to Method E 181 for more information.6.4 The density of the sample is another factor that can effect quantitative results. Errors from this source can be avoided by preparing the standards for calibration in sol
42、utions or other matrices with a density comparable to the sample being analyzed.6 Refer to Annual Book of ASTM Standards, Vol 12.02 on Nuclear, Solar, andGeothermal Energy for more information on this subject.7. Apparatus7.1 Gamma Ray Spectrometer, consisting of the following components:7.1.1 Detect
43、or Assembly:7.1.1.1 Germanium DetectorThe detector shall have a volume of about 50 to 150 cm3, with a full width at one-half the peak maximum (FWHM) less than 2.2 keV at 1332 keV, certified by the manufacturer. A charge-sensitive preamplifier using low noise field effect transistors should be an int
44、egral part of the detector assembly. A convenient support shall be provided for samples of the desired form.7.1.1.2 ShieldThe detector assembly shall be surrounded by an external radiation shield made of massive metal, equiva- lent to 102 mm of lead in gamma-ray attenuation capability. It is desirab
45、le that the inner walls of the shield be at least 127 mm distant from the detector surfaces to reduce backscatter. If the shield is made of lead or a lead liner, the shield must have a graded inner shield of 1.6 mm of cadmium or tin lined with 0.4 mm of copper, to attenuate the 88-keV Pb X rays. The
46、 shield must have a door or port for inserting and removing samples.7.1.1.3 High Voltage Power/Bias SupplyThe bias supply required for germanium detectors usually provides a voltage up to 5000 V and 1 to 100 A. The power supply shall be regulated to 0.1 % with a ripple of not more than 0.01 %. Line noise caused by other equipment shall be removed with rf filters and additional regulators.7.1.1.4 AmplifierAn amplifier compatible with the pream- plifier and with the pulse-height analyzer shall be provided.7.1.2 Data Acquisition and Stor
