1、Designation: D3084 05 (Reapproved 2012)Standard Practice forAlpha-Particle Spectrometry of Water1This standard is issued under the fixed designation D3084; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision.
2、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 processes that are required toobtain well-resolved alpha-particle spectra from water samplesand discusses as
3、sociated problems. This practice is generallycombined with specific chemical separations, mounting tech-niques, and counting instrumentation, as referenced.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of
4、 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:2C859 Terminology Relating to Nuclear MaterialsC1163 Practice for Mounting Actinides for Alpha Spec-trometry Using Ne
5、odymium FluorideD1129 Terminology Relating to WaterD3648 Practices for the Measurement of RadioactivityD3865 Test Method for Plutonium in WaterD3972 Test Method for Isotopic Uranium in Water byRadiochemistry3. Terminology3.1 For definitions of terms used in this practice, refer toTerminologies D1129
6、 and C859. For terms not found in theseterminologies, reference may be made to other publishedglossaries (1, 2).34. Summary of Practice4.1 Alpha-particle spectrometry of radionuclides in water(also called alpha-particle pulse-height analysis) has beencarried out by several methods involving magnetic
7、 spectrom-eters, gas counters, scintillation spectrometers, nuclear emul-sion plates, cloud chambers, absorption techniques, and solid-state counters. Gas counters, operating either as an ionizationchamber or in the proportional region, have been widely usedto identify and measure the relative amoun
8、ts of differenta-emitters. However, more recently, the solid-state counter hasbecome the predominant system because of its excellentresolution and compactness. Knoll (3) extensively discussesthe characteristics of both detector types.4.2 Of the two gas-counting techniques, the pulsed ioniza-tion cha
9、mber is more widely used as it gives much betterresolution than does the other. This is because there is nospread arising from multiplication or from imperfection of thewire such as occurs with the proportional counter.4.3 The semiconductor detectors used for alpha-particlespectrometry are similar i
10、n principle to ionization chambers.The ionization of the gas by a-particles gives rise to electron-ion pairs, while in a semiconductor detector, electron-holepairs are produced. Subsequently, the liberated changes arecollected by an electric field. In general, silicon detectors areused for alpha-par
11、ticle spectrometry. These detectors are n-typebase material upon which gold is evaporated or ions such asboron are implanted, making an electrical contact. A reversedbias is applied to the detector to reduce the leakage current andto create a depletion layer of free-charge carriers. This layer isthi
12、n and the leakage current is very low. Therefore, the slightinteractions of photons with the detector produce no signal.The effect of any interactions of beta particles with the detectorcan be eliminated by appropriate electronic discrimination(gating) of signals entering the multichannel analyzer.
13、Asemiconductor detector detects all alpha particles emitted byradionuclides (approximately 2 to 10 MeV) with essentiallyequal efficiency, which simplifies its calibration.4.4 Semiconductor detectors have better resolution than gasdetectors because the average energy required to produce anelectron-ho
14、le pair in silicon is 3.5 6 0.1 eV (0.56 6 0.02 aJ)compared with from 25 to 30 eV (4.0 to 4.8 aJ) to produce anion pair in a gas ionization chamber. Detector resolution,defined as peak full-width at half-maximum height (FWHM),is customarily expressed in kiloelectron-volts. The FWHMincreases with inc
15、reasing detector area, but is typically be-tween 15 and 60 keV. The background is normally lower for asemiconductor detector than for ionization chamber. Silicon1This practice is under the jurisdiction of ASTM Committee D19 on Water andis the direct responsibility of Subcommittee D19.04 on Methods o
16、f RadiochemicalAnalysis.Current edition approved June 1, 2012. Published August 2012. Originallyapproved in 1972. Last previous edition approved in 2005 as D3084 05. DOI:10.1520/D3084-05R12.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at servi
17、ceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The boldface numbers in parentheses refer to the list of references at the end ofthis document.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Con
18、shohocken, PA 19428-2959, United States.detectors have four other advantages compared to ionizationchambers: they are lower in cost, have superior stability, havehigher permissible counting rates, and have better time reso-lution for coincidence measurements. However, the semicon-ductor detector req
19、uires sophisticated electronics because ofthe low charge that is generated by the incident a-particle inthe detector. Low-noise and high-stability, charge-sensitivepreamplifiers are used prior to the detection, analog-to-digitalconversion, and storage of the voltage pulse by a multichannelanalyzer.
20、The counting is nearly always performed in a vacuumchamber so that thea -particles will not lose energy bycollisions with air molecules between the source and thedetector.4.5 A gridded pulse-ionization chamber was developed byFrisch for high-resolution alpha spectrometry. The unit consistsof a stand
21、ard ionization chamber fitted with a collimatorbetween the source and the collector plate and a wire grid toshield the collector from the effects of positive ions. Theresolution of a gridded pulse ionization chamber is from 35 to100 keV for routine work. The detector parameters that affectresolution
22、 are primarily the following: statistical variations inthe number of ion pairs formed at a given alpha energy, thevariation in rise time of pulses, and the effects of positive ions.An advantage of gridded ionization chambers is their ability tocount large-area sources with good efficiency.4.6 There
23、are two reasons for collimating a sample in agridded ionization chamber. When thick-sample sources areencountered, the alpha-particles emitted at a large solid anglewould show an energy degradation upon ionization of the gas.The effect leads to tailing of the alpha-particle spectrum. Thisproblem is
24、reduced significantly by use of the collimator.Secondly, when the nucleus following ana -particle emissiondoes not decay to a ground state, the g-rays that may beproduced are usually highly converted, and the conversionelectrons ionize the gas. The special mesh-type collimatorsstop the conversion el
25、ectrons and collimate the source simul-taneously.4.7 A more recently developed measurement method isphoton-electron-rejecting alpha liquid-scintillation spectrom-etry. The sample is counted in a special liquid-scintillationspectrometer that discriminates electronically against non-alpha-particle pul
26、ses. The resolution that can be achieved bythis method is 250 to 300-keV FWHM. This is superior toconventional liquid-scintillation counting, but inferior to sili-con detectors and gridded pulse-ionization chambers. Anapplication of this method is given in Ref 4.5. Significance and Use5.1 Alpha-part
27、icle spectrometry can either be used as aquantitative counting technique or as a qualitative method forinforming the analyst of the purity of a given sample.5.2 The method may be used for evaporated alpha-particlesources, but the quality of the spectra obtained will be limitedby the absorbing materi
28、al on the planchet and the surface finishof the planchet.6. Interferences6.1 The resolution or ability to separate alpha-particle peakswill depend on the quality of the detector, the pressure insidethe counting chamber, the source-to-detector distance, theinstrumentation, and the quality of the sour
29、ce. If peaks overlap,a better spectrometer or additional chemical separations will berequired.7. Apparatus7.1 Alpha Particle Detector, either a silicon semiconductoror a Frisch-grid pulse-ionization chamber.7.2 Counting Chamber, to house the detector, hold thesource, and allow the detector system to
30、 be evacuated.7.3 Counting Gas, for ionization chamber, typically a 90 %argon10 % methane mixture, and associated gas-handlingequipment.7.4 Pulse Amplification System, possibly including a pream-plifier, amplifier, postamplifier, pulse stretcher, and a high-voltage power supply, as directed by the q
31、uality and type ofdetector employed.7.5 Multichannel Pulse-Height Analyzer, including datareadout equipment. This is now often computer based.7.6 Vacuum Pump, with low vapor-pressure oil and prefer-ably with a trap to protect the detector from oil vapors.8. Source Preparation8.1 The technique employ
32、ed for preparing the source shouldproduce a low-mass, uniformly distributed deposit that is on avery smooth surface. The three techniques that are generallyemployed are electrodeposition, microcoprecipitation, andevaporation. The first two usually are preferred. Fig. 1 com-pares the alpha-particle s
33、pectrum of an electrodeposited sourcewith that of an evaporated source.8.1.1 Electrodeposition of a-emitters can provide a samplewith optimum resolution, but quantitative deposition is notnecessarily achieved. Basically, the a-emitter is deposited fromsolution on a polished stainless steel or platin
34、um disk, which isthe cathode. The anode is normally made from platinum gauzeor a spiralled platinum wire, which often is rotated at a constantrate. Variants of this technique may be found in Refs 5 and 6.See also Test Method D3865. Polonium can be made to depositspontaneously from solution onto a co
35、pper or nickel disk (7).8.1.2 Micro-coprecipitation of actinide elements on a rare-earth fluoride, often neodymium fluoride, followed by filtrationNOTE 1Inner curve: nuclides separated on barium sulfate and thenelectrodeposited.NOTE 2Outer curve: carrier-free tracer solution evaporated directly.FIG.
36、 1 Resolution Obtained on Six-Component MixtureD3084 05 (2012)2on a specially prepared membrane-type filter (see Test MethodC1163) also produces a good-quality source for alpha-particlespectrometry. The microgram quantity of precipitant onlyslightly degrades spectral resolution.8.1.3 The evaporation
37、 technique involves depositing thesolution onto a stainless steel or platinum disk. The liquid isapplied in small droplets over the entire surface area so thatthey dry separately, or a wetting agent is applied, which causesthe solution to evaporate uniformly over the entire surface. Thetotal mass sh
38、ould not exceed 10 g/cm2, otherwise self-absorption losses will be significant. In addition, the alpha-particle spectrum will be poorly resolved, as evidenced by along lower-energy edge on the peak. This tailing effect cancontribute counts to lower energy alpha peaks and create largeuncertainties in
39、 peak areas. Alpha sources that are prepared byevaporation may not adhere tenaciously and, therefore, canflake causing contamination of equipment and sample losses.9. Calibration9.1 Calibrate the counter by measuring a-emitting radionu-clides that have been prepared by one of the techniquesdescribed
40、 in Section 8. All standards should be traceable to theNational Institute of Standards and Technology and in the caseof nonquantitative mounting, standardized on a 2p or 4palpha-particle counter. Precautions should be taken to ensurethat significant impurities are not present when standardizingthe a
41、lpha-particle activity by non-spectrometric means. Thephysical characteristics of the calibrating sources and theirpositioning relative to the detector must be the same as thesamples to be counted. A mixed radionuclide standard can becounted to measure simultaneously the detector resolution andeffic
42、iency, and the gain of the multichannel analyzer. Check theinstrumentation frequently for consistent operation. Performbackground measurements regularly and evaluate the results atthe confidence level desired.10. Procedure10.1 The procedure of analysis is dependent upon theradionuclide(s) of interes
43、t. A chemical procedure is usuallyrequired to isolate and purify the radionuclides. See TestMethods D3865 and D3972. Additional appropriate chemicalprocedures may be found in Refs (78910. A source is thenprepared by a technique in accordance with Section 8. Measurethe radioactivity of this source in
44、 an alpha spectrometer,following the manufacturers operating instructions. Thecounting period chosen depends on the sensitivity required ofthe measurement and the degree of uncertainty in the result thatis acceptable (see Section 12).10.2 Silicon detectors will eventually become contaminatedby recoi
45、ling atoms unless protective steps are taken. Control-ling the air pressure in the counting chamber so that 12 g/cm2of absorber is present between the source and the detector willcause only a 1-keV resolution loss; however, the recoilcontamination will be reduced by a factor greater than 500.Recoili
46、ng atoms can also be reduced electrically (11). Rugge-dized detectors can be cleaned to a limited degree.10.3 Qualitative identifications sometimes can be madeeven on highly degraded spectra. By examining the highestenergy value, and using the energy calibration (keV/channel)of the pulse-height anal
47、yzer, alpha-particle emitters may beidentified. Fig. 2 shows a typical spectrum with very poorresolution.11. Calculation11.1 Analyze the data by first integrating the area under thealpha peak to obtain a gross count for the alpha emitter. Whenthe spectrum is complex and alpha peaks add to each other
48、,corrections for overlapping peaks will be required. Someinstrument manufacturers computer software can performthese and other data-analysis functions.11.2 The preferred method for determination of chemicalrecovery is the use of another isotope of the same element(examples: polonium-208 to trace pol
49、onium-210, plutonium-236 to trace plutonium-239, and americium-243 to traceamericium-241). Add a known activity of the appropriateisotope(s) to the sample at the beginning of the analysis,perform the appropriate chemical separations, mount thesample, and measure it by alpha-particle spectrometry. Thechemical yield is directly related to the reduction in the activityof the added isotope.11.2.1 When the recovery factor is determined by theaddition of a tracer, calculate the gross radioactivity concen-tration, C, of the analyte in becquerels per litre (Bq/L) asfollows:11.2.1.1
