1、Designation: D 3648 04Standard Practices for theMeasurement of Radioactivity1This standard is issued under the fixed designation D 3648; 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 parenth
2、eses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 These practices cover a review of the accepted countingpractices currently used in radiochemical analyses. The prac-tices are divided into four section
3、s:SectionGeneral Information 6 to 11Alpha Counting 12 to 22Beta Counting 23 to 33Gamma Counting 34 to 411.2 The general information sections contain informationapplicable to all types of radioactive measurements, while eachof the other sections is specific for a particular type ofradiation.1.3 This
4、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 and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2
5、.1 ASTM Standards:2D 1066 Practice for Sampling SteamD 1129 Terminology Relating to WaterD 1943 Test Method for Alpha Particle Radioactivity ofWaterD 2459 Test Method for Gamma Spectrometry of Water3D 3084 Practice for Alpha Spectrometry of WaterD 3085 Practice for Measurement of Low-Level Activity
6、inWater3D 3370 Practices for Sampling Water from Closed Con-duitsD 3649 Test Method for High-Resolution Gamma-RaySpectrometry of WaterE 380 Practice for Use of the International System of Units(SI) (the Modernized Metric System)2.2 ANSI/ISO Standards:4ANSI N42.14 Calibration and Use of Germanium Spe
7、c-trometers for the Measurement of Gamma-Ray EmissionRates of RadionuclidesISO Guide to the Expression of Uncertainty in Measure-ment, 19933. Terminology3.1 Definitions:3.1.1 For definitions of terms used in these practices, referto Terminology D 1129. For an explanation of the metricsystem, includi
8、ng units, symbols, and conversion factors, seePractice E 380.4. Summary of Practices4.1 The practices are a compilation of the various countingtechniques employed in the measurement of radioactivity. Theimportant variables that affect the accuracy or precision ofcounting data are presented. Because
9、a wide variety of instru-ments and techniques are available for radiochemical labora-tories, the types of instruments and techniques to be selectedwill be determined by the information desired. In a simpletracer application using a single radioactive isotope havingfavorable properties of high purity
10、, energy, and ample activity,a simple detector will probably be sufficient and techniquesmay offer no problems other than those related to reproduc-ibility. The other extreme would be a laboratory requiringquantitative identification of a variety of radionuclides, prepa-ration of standards, or studi
11、es of the characteristic radiationfrom radionuclides. For the latter, a variety of specializedinstruments are required. Most radiochemical laboratoriesrequire a level of information between these two extremes.4.2 A basic requirement for accurate measurements is theuse of accurate standards for instr
12、ument calibration. With thepresent availability of good standards, only the highly diverseradiochemistry laboratories require instrumentation suitable1These practices are under the jurisdiction of ASTM Committee D19 on Waterand are the direct responsibility of D19.04 on Methods of Radiochemical Anal
13、ysis.Current edition approved July 1, 2004. Published July 2004. Originally approvedin 1978. Last previous edition approved in 2003 as D 3648 03.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards
14、volume information, refer to the standards Document Summary page onthe ASTM website.3Withdrawn.4Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-295
15、9, United States.for producing their own radioactive standards. However, it isadvisable to compare each new standard received against theprevious standard.4.3 Thus, the typical laboratory may be equipped withproportional or Geiger-Mueller counters for beta counting,sodium iodide or germanium detecto
16、rs, or both, in conjunctionwith multichannel analyzers for gamma spectrometry, andscintillation counters suitable for alpha- or beta-emitting radio-nuclides.5. Significance and Use5.1 This practice was developed for the purpose of summa-rizing the various generic radiometric techniques, equipment,an
17、d practices that are used for the measurement of radioactiv-ity.GENERAL INFORMATION6. Experimental Design6.1 In order to properly design valid experimental proce-dures, careful consideration must be given to the following;6.1.1 radionuclide to be determined,6.1.2 relative activity levels of interfer
18、ences,6.1.3 type and energy of the radiation,6.1.4 original sample matrix, and6.1.5 required accuracy.6.2 Having considered 6.1.1-6.1.5, it is now possible tomake the following decisions:6.2.1 chemical or physical form that the sample must be infor radioassay,6.2.2 chemical purification steps,6.2.3
19、type of detector required,6.2.4 energy spectrometry, if required,6.2.5 length of time the sample must be counted in order toobtain statistically valid data,6.2.6 isotopic composition, if it must be determined, and6.2.7 size of sample required.6.3 For example, gamma-ray measurements can usually beper
20、formed with little or no sample preparation, whereas bothalpha and beta counting will almost always require chemicalprocessing. If low levels of radiation are to be determined, verylarge samples and complex counting equipment may be nec-essary.6.3.1 More detailed discussions of the problems and inte
21、r-ferences are included in the sections for each particular type ofradiation to be measured.7. Apparatus7.1 Location Requirements:7.1.1 The apparatus required for the measurement of radio-activity consists, in general, of the detector and associatedelectronic equipment. The latter usually includes a
22、 stablepower supply, preamplifiers, a device to store or display theelectrical pulses generated by the detector, or both, and one ormore devices to record information.7.1.2 Some detectors and high-gain amplifiers are tempera-ture sensitive; therefore, changes in pulse amplitude can occuras room temp
23、erature varies. For this reason, it is necessary toprovide temperature-controlled air conditioning in the countingroom.7.1.3 Instrumentation should never be located in a chemicallaboratory where corrosive vapors will cause rapid deteriora-tion and failure.7.2 Instrument Electrical Power SupplyDetect
24、or andelectronic responses are a function of the applied voltage;therefore, it is essential that only a very stable, low-noiseelectrical supply be used or that suitable stabilization beincluded in the system.7.3 Shielding:7.3.1 The purpose of shielding is to reduce the backgroundcount rate of a meas
25、urement system. Shielding reduces back-ground by absorbing some of the components of cosmicradiation and some of the radiations emitted from material inthe surroundings. Ideally, the material used for shieldingshould itself be free of any radioactive material that mightcontribute to the background.
26、In practice, this is difficult toachieve as most construction materials contain at least somenaturally radioactive isotopes (such as40K, members of theuranium and thorium series, and so forth). The thickness of theshielding material should be such that it will absorb most of thesoft components of co
27、smic radiation. This will reduce cosmic-ray background by approximately 25 %. Shielding of beta- orgamma-ray detectors with anticoincidence systems can furtherreduce the cosmic-ray or Compton scattering background forvery low-level counting.7.3.2 Detectors have a certain background counting ratefrom
28、 naturally occurring radionuclides and cosmic radiationfrom the surroundings; and from the radioactivity in thedetector itself. The background counting rate will depend onthe amounts of these types of radiation and on the sensitivity ofthe detector to the radiations.7.3.3 In alpha counting, low back
29、grounds are readilyachieved since the short range of alpha particles in mostmaterials makes effective shielding easy. Furthermore, alphadetectors are quite insensitive to the electromagnetic compo-nents of cosmic and other environmental radiation.7.4 Care of Instruments:7.4.1 The requirements for an
30、d advantages of operating allcounting equipment under conditions as constant and repro-ducible as possible have been pointed out earlier in this section.The same philosophy suggests the desirability of leaving allcounting equipment constantly powered. This implies leavingthe line voltage on the elec
31、trical components at all times. Theadvantage to be gained by this practice is the elimination of thestart-up surge voltage, which causes rapid aging, and theinstability that occurs during the time the instrument is comingup to normal temperature.7.4.2 A regularly scheduled and implemented program of
32、maintenance is helpful in obtaining satisfactory results. Themaintenance program should include not only checking thenecessary operating conditions and characteristics of the com-ponents, but also regular cleaning of the equipment.D36480427.5 Sample and Detector HoldersIn order to quantifycounting d
33、ata, it is necessary that all samples be presented tothe detector in the same “geometry.” This means that thesamples and standards should be prepared for counting in thesame way so that the distance between the source and thedetector remains as constant as possible. In practice, thisusually means th
34、at the detector and the sample are in a fixedposition. Another configuration often used is to have thedetector in a fixed position within the shield, and beneath it ashelf-like arrangement for the reproducible positioning of thesample at several distances from the detector.7.6 Special Instrumentatio
35、nThis section covers some ra-diation detection instruments and auxiliary equipment that maybe required for special application in the measurement ofradioactivity in water.7.6.1 4-p Counter:7.6.1.1 The 4-p counter is a detector designed for themeasurement of the absolute disintegration rate of a radi
36、oactivesource by counting the source under conditions that approacha geometry of 4-p steradians. Its most prevalent use is for theabsolute measurement of beta emitters (1, 2).5For thispurpose, a gas-flow proportional counter similar to that in Fig.1 is common. It consists of two hemispherical or cyl
37、indricalchambers whose walls form the cathode, and a looped wireanode in each chamber. The source is mounted on a thinsupporting film between the two halves, and the countsrecorded in each half are summed. A 10 % methane-90 %argon gas mixture can be used; however, pure methane givesflatter and longe
38、r plateaus and is preferred for the mostaccurate work. The disadvantage is that considerably highervoltages, about 3000 V, rather than the 2000 V suitable formethane-argon, are necessary. As with all gas-filled propor-tional counters, very pure gas is necessary for very highdetector efficiency. The
39、absence of electronegative gases thatattach electrons is particularly important since the negativepulse due to electrons is counted in this detector. Commercialchemically pure (cp) gases are ordinarily satisfactory, but theyshould be dried for best results. A high-voltage power supplyfor the detecto
40、r, an amplifier, discriminator, and a scalercomplete the system.7.6.1.2 To convert counting rate to disintegration rate, theprincipal corrections required are for self-absorption in thesource and for absorption in the support film. The support filmshould be as thin as practicable to minimize absorpt
41、ion of betaparticles emitted in the downward direction. Polyester filmwith a thickness of about 0.9 mg/cm2is readily available andeasily handled. However, it is too thick for accurate work withthe lower energy beta emitters. For this purpose, thin films (.5to 10 g/cm2) are prepared by spreading a so
42、lution of apolymer in an organic solvent on water. VYNS (1), Formvar(2), and Tygon (3) plastics have been used for this purpose.7.6.1.3 The films must be made electrically conducting(since they are a part of the chamber cathode) by covering themwith a thin layer (2 to 5 g/cm2) of gold or palladium b
43、yvacuum evaporation. The absorption loss of beta particles inthe film must be known. Published values can be used, ifnecessary, but for accurate work an absorption curve usingvery thin absorbers should be taken (1). The “sandwich”method, in which the film absorption is calculated from thedecrease in
44、 counting rate that occurs when the source surfaceis covered with a film of the same thickness as the backingfilm, is suitable for the higher beta energies.7.6.1.4 The source itself must be very thin and depositeduniformly on the support to obtain negligible self-absorption.Various techniques have b
45、een used for spreading the source;for example, the evaporation of63Ni-dimethylglyoxime ontothe support film (1), the addition of a TFE-fluorocarbonsuspension (3), collodial silica, or insulin to the film asspreading agents, and hydrolysis (2). Self-absorption in thesource or mount can be measured by
46、 4-p beta-gamma coinci-dence counting (4, 5). The 4-p beta counter is placed next to aNaI(Tl) detector, or a portion of the chamber wall is replacedby a NaI(Tl) detector, and the absolute disintegration rate isevaluated by coincidence counting (6, 7). By adding a suitablebeta-gamma tracer, the metho
47、d has been used for pure beta aswell as beta-gamma emitters (8). Accurate standardization ofpure low-energy beta emitters (for example,63Ni) is difficult,and the original literature should be consulted by thoseinexperienced with this technique.7.6.1.5 Photon (gamma and strong X-ray) scintillationcou
48、nters with geometries approaching 4-p steradians can beconstructed from NaI(Tl) crystals in either of two ways.Awellcrystal (that is, a cylindrical crystal with a small axial holecovered with a second crystal) will provide nearly 4-p geom-etry for small sources, as will two solid crystals placed ver
49、yclose together with a small source between them. The countsfrom both crystals are summed as in the gas-flow counter. Thedeviation for 4-p geometry can be calculated from the physicaldimensions. For absolute gamma-ray counting, the efficiencyof the crystal for the gamma energy being measured and theabsorption in the crystal cover must be taken into account.Additional information on scintillation counting is given in7.6.4. The liquid scintillation counter is also essentially a 4-pcounter for beta particles, since nearly all the radiations areemitted into and interact with the dete
