1、Designation: D 3084 05Standard Practice forAlpha-Particle Spectrometry of Water1This standard is issued under the fixed designation D 3084; 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 pare
2、ntheses 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 processes that are required toobtain well-resolved alpha-particle spectra from water samplesand discusses associated proble
3、ms. 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 this standard
4、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:2C 859 Terminology Relating to Nuclear MaterialsC 1163 Test Method for Mounting Actinides for AlphaSpectrometry Using Neodymium Fluo
5、rideD 1129 Terminology Relating to WaterD 3648 Practices for the Measurement of RadioactivityD 3865 Test Method for Plutonium in WaterD 3972 Test Method for Isotopic Uranium in Water byRadiochemistry3. Terminology3.1 For definitions of terms used in this practice, refer toTerminologies D 1129 and C
6、859. 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 spect
7、rom-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 amounts of
8、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 chamber i
9、s 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 in prin
10、ciple 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-particle
11、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 isthin and
12、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. Asemic
13、onductor 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-hole pai
14、r 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 increasin
15、g detector area, but is typically be-tween 15 and 60 keV. The background is normally lower for asemiconductor detector than for ionization chamber. Silicondetectors have four other advantages compared to ionization1This practice is under the jurisdiction of ASTM Committee D19 on Water andis the dire
16、ct responsibility of Subcommittee D19.04 on Methods of RadiochemicalAnalysis.Current edition approved Jan. 1, 2005. Published January 2005. Originallyapproved in 1972. Last previous edition approved in 1996 as D 3084 96.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact
17、ASTM Customer Service at serviceastm.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 Harbo
18、r Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.chambers: 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 requires sophisticated electroni
19、cs 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. The counting is nearly always
20、 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 standard ionization chamber fitted
21、 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 are primarily the following:
22、 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 are two reasons for collimati
23、ng 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 reduced significantly by use
24、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 electrons and collimate the sou
25、rce 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 pulses. The resolution that can
26、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-particle spectrometry can either
27、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 material on the planchet and the su
28、rface 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 source. If peaks overlap,a better
29、 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 be evacuated.7.3 Counting Ga
30、s, 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 quality and type ofdetector em
31、ployed.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 employed for preparing the source s
32、houldproduce 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 spectrum of an electrodeposite
33、d 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 platinum disk, which isthe cathode.
34、 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 D 3865. Polonium can be made todeposit spontaneously from solution onto a copper or nickeldisk (7).8.1.2
35、 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. 1 Resolution Obtained on Six
36、-Component MixtureD 3084 052on a specially prepared membrane-type filter (see Test MethodC 1163) also produces a good-quality source for alpha-particlespectrometry. The microgram quantity of precipitant onlyslightly degrades spectral resolution.8.1.3 The evaporation technique involves depositing the
37、solution 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 should not exceed 10 g/cm2, otherwis
38、e 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 peak areas. Alpha sources that ar
39、e 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 in Section 8. All standards shoul
40、d 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 alpha-particle activity by non-spec
41、trometric 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 andefficiency, and the gain of the multich
42、annel 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 interest. A chemical procedure is usually
43、required to isolate and purify the radionuclides. See TestMethods D 3865 and D 3972. 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 an alpha spectrometer,following
44、 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 recoiling atoms unless protective ste
45、ps 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.Recoiling atoms can also be reduced ele
46、ctrically (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 analyzer, alpha-particle emitters ma
47、y 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,corrections for overlapping pea
48、ks 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 polonium-210, plutonium-236 to trac
49、e 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 Radiotracer Net Counts:NT5 GT2 BC
