ASTM D3084-20

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Designation: D3084 − 05 (Reapproved 2012) D3084 − 20
Standard Practice for
Alpha-Particle Spectrometry of Water
This standard is issued under the fixed designation D3084; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This practice covers the processes that are required to obtain well-resolved alpha-particle spectra from water samples and
discusses associated problems. This practice is generally combined with specific chemical separations, mounting techniques, and
counting instrumentation, as referenced.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
C859 Terminology Relating to Nuclear Materials
C1163 Practice for Mounting Actinides for Alpha Spectrometry Using Neodymium Fluoride
D1129 Terminology Relating to Water
D3648 Practices for the Measurement of Radioactivity
D3865 Test Method for Plutonium in Water
D3972 Test Method for Isotopic Uranium in Water by Radiochemistry
D7282 Practice for Set-up, Calibration, and Quality Control of Instruments Used for Radioactivity Measurements
D7902 Terminology for Radiochemical Analyses
3. Terminology
3.1 For definitions of terms used in this practice, refer to Terminologies D1129 and C859. For terms not found in these
terminologies, reference may be made to other published glossaries (1, 2). Definitions:
3.1.1 For definitions of terms used in this standard, refer to Terminologies D1129, D7902, and C859. For terms not found in
these terminologies, reference may be made to other published glossaries (1, 2).
4. Summary of Practice
4.1 Alpha-particle spectrometry of radionuclides in water (also called alpha-particle pulse-height analysis) has been carried out
by several methods involving magnetic spectrometers, gas counters, scintillation spectrometers, nuclear emulsion plates, cloud
chambers, absorption techniques, and solid-state counters. Gas counters, operating either as an ionization chamber or in the
proportional region, have been widely used to identify and measure the relative amounts of differentα -emitters.different
alpha-emitters. However, more recently, the solid-state counter has become the predominant system because of its excellent
resolution and compactness. Knoll (3) extensively discusses the characteristics of both detector types.
This practice is under the jurisdiction of ASTM Committee D19 on Water and is the direct responsibility of Subcommittee D19.04 on Methods of Radiochemical Analysis.
Current edition approved June 1, 2012July 1, 2020. Published August 2012July 2020. Originally approved in 1972. Last previous edition approved in 20052012 as
D3084 – 05.D3084 – 05 (2012). DOI: 10.1520/D3084-05R12.10.1520/D3084-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’sstandard’s Document Summary page on the ASTM website.
The boldface numbers in parentheses refer to thea list of references at the end of this document.standard.
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D3084 − 20
4.2 Of the two gas-counting techniques, the pulsed ionization chamber is more widely used as it gives much better resolution
than does the other. This is because there is no spread arising from multiplication or from imperfection of the wire such as occurs
with the proportional counter.
4.3 The semiconductor detectors used for alpha-particle spectrometry are similar in principle to ionization chambers. The
ionization of the gas by α-particles gives rise to electron-ion pairs, while in a semiconductor detector, electron-hole pairs are
produced. Subsequently, the liberated changescharges are collected by an electric field. In general, silicon detectors are used for
alpha-particle spectrometry. These detectors are n-type base material upon which gold is evaporated or ions such as boron are
implanted, making an electrical contact. A reversed bias is applied to the detector to reduce the leakage current and to create a
depletion layer of free-charge carriers. This layer is thin and the leakage current is very low. Therefore, the slight interactions of
photons with the detector produce no signal. The effect of any interactions of beta particles with the detector can be eliminated
by appropriate electronic discrimination (gating) of signals entering the multichannel analyzer. A semiconductor detector detects
all alpha particles emitted by radionuclides (approximately 2 to 10 MeV) with essentially equal efficiency, which simplifies its
calibration.
4.4 Semiconductor detectors have better resolution than gas detectors because the average energy required to produce an
electron-hole 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 an ion
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 FWHM increases with increasing detector area, but is typically between 15 and 60 keV. The
background is normally lower for a semiconductor detector than for an ionization chamber. Silicon detectors have four other
advantages compared to ionization chambers: they are lower in cost, have superior stability, have higher permissible counting rates,
and have better time resolution for coincidence measurements. However, the semiconductor detector requires sophisticated
electronics because of the low charge that is generated by the incident α-particle in the detector. Low-noise and high-stability,
charge-sensitive preamplifiers are used prior to the detection, analog-to-digital conversion, and storage of the voltage pulse by a
multichannel analyzer. The counting is nearly always performed in a vacuum chamber so that the α-particles will not lose energy
by collisions with air molecules between the source and the detector.
4.5 A gridded pulse-ionization chamber was developed by Otto Frisch for high-resolution alpha spectrometry. The unit consists
of a standard ionization chamber fitted with a collimator between the source and the collector plate and a wire grid to shield the
collector from the effects of positive ions. The resolution of a griddedFrisch-grid pulse ionization chamber isranges from 35 to 100
keV for routine work. The detector parameters that affect resolution are primarily the following: statistical variations in the number
of ion pairs formed at a given alpha energy, the variation in rise time of pulses, and the effects of positive ions. An advantage of
gridded ionization chambers is their ability to count large-area sources with good efficiency.
4.6 There are two reasons for collimating a sample in a gridded ionization chamber. When thick-sample sources are
encountered, the alpha-particles emitted at a large solid angle would show an energy degradation upon ionization of the gas. The
effect leads to tailing of the alpha-particle spectrum. This problem is reduced significantly by use of the collimator. Secondly, when
the nucleus following anα -particlean α-particle emission does not decay to a ground state, the γ-rays that may be produced are
usually highly converted, and the conversion electrons ionize the gas. The special mesh-type collimators stop the conversion
electrons and collimate the source simultaneously.
4.7 A more recently developed measurement method is photon-electron-rejecting alpha liquid-scintillation spectrometry. The
sample is counted in a special liquid-scintillation spectrometer that discriminates electronically against non-alpha-particle pulses.
The resolution that can be achieved by this method is 250 to 300-keV 300 keV FWHM. This is superior to conventional
liquid-scintillation counting,counting but inferior to silicon detectors and gridded pulse-ionization chambers. An application of this
method is given in Ref (4.).
5. Significance and Use
5.1 Alpha-particle spectrometry can either be used either as a quantitative counting technique or as a qualitative method for
informing the analyst of the purity of a given sample.
5.2 The method may be used for evaporated alpha-particle sources, but the quality of the spectra obtained will be limited by
the absorbing material on the planchet and the surface finish of the planchet.
6. Interferences
6.1 There can be interferences due to tailing or spillover from one spectral region of interest into another, or to impurities in
the tracer, if any.
6.2 The resolution, or ability to separate alpha-particle peaks, will depend on the quality of the detector, the pressure inside the
counting chamber, the source-to-detector distance, the instrumentation, and the quality of the source. If peaks overlap, a better
spectrometer or additional chemical separations will be required.
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7. Apparatus
7.1 Alpha Particle Detector, either a silicon semiconductor or a Frisch-grid pulse-ionization chamber.
7.2 Counting Chamber, to house the detector, hold the source, and allow the detector system to be evacuated.
7.3 Counting Gas, for ionization chamber, typically a 90 % argon–10 % methane mixture, and associated gas-handling
equipment.
7.4 Pulse Amplification System, possibly including a preamplifier, amplifier, postamplifier, pulse stretcher, and a high-voltage
power supply, as directed by the quality and type of detector employed.
7.5 Multichannel Pulse-Height Analyzer, including data readout equipment. This is now often computer based.
7.6 Vacuum Pump, with low vapor-pressure oil and preferably with a trap to protect the detector from oil vapors.
8. Source Preparation
8.1 The technique employed for preparing the source should produce a low-mass, uniformly distributed deposit that is on a very
smooth surface. The three techniques that are generally employed are electrodeposition, microcoprecipitation, and evaporation.
The first two usually are preferred. Fig. 1 compares the alpha-particle spectrum of an electrodeposited source with that of an
evaporated source.
8.1.1 Electrodeposition of α-emittersalpha-emitters can provide a sample with optimum resolution, but quantitative deposition
is not necessarily achieved. Basically, the α-emitteralpha-emitter is deposited from solution on a polished stainless steel or platinum
disk, which is the cathode. The anode is normally made from platinum gauze or a spiralled platinum wire, which often is rotated
at a constant rate. Variants of this technique may be found in Refs (5) and (6.). See also Test Method D3865. Polonium can be
made to deposit spontaneously from solution onto a copper or nickel disk (7).
8.1.2 Micro-coprecipitation of actinide elements on a rare-earth fluoride, often neodymium fluoride, followed by filtration on
a specially prepared membrane-type filter (see Test Method Practice C1163) also produces a good-quality source for alpha-particle
spectrometry. The microgram quantity of precipitant only slightly degrades spectral resolution.
8.1.3 The evaporation technique involves depositing the solution onto a stainless steel or platinum disk. The liquid is applied
in small droplets over the entire surface area so that they dry separately, or a wetting agent is applied, which causes the solution
to evaporate uniformly over the entire surface. The total mass should not exceed 10μ g/cm10 μg/cm , otherwise self-absorption
losses will be significant. In addition, the alpha-particle spectrum will be poorly resolved, as evidenced by a long lower-energy
edge on the peak. This tailing effect can contribute counts to lower energy alpha peaks and create large uncertainties in peak areas.
Alpha sources that are prepared by evaporation may not adhere tenaciously and, therefore, can flake causing contamination of
equipment and sample losses.
9. Calibration
9.1 Calibrate the counter by measuring α-emittingalpha-emitting radionuclides that have been prepared by one of the techniques
described in Section 8. All standards should be traceable to the SI through a national metrology institute such as the National
Institute of Standards and Technology and in the case of nonquantitative mounting, standardized on a 2π or 4π alpha-particle
counter. Precautions should be taken to ensure that significant impurities are not present when standardizing the alpha-particle
activity by non-spectrometric means. The physical characteristics of the calibrating sources and their positioning relative to the
detector must be the same as the samples to be counted. A mixed radionuclide standard can be counted to measure simultaneously
the detector resolution and efficiency, and the gain of the multichannel analyzer. Check the instrumentation frequently for
consistent operation. Perform background measurements regularly and evaluate the results at the confidence level desired.
NOTE 1—Inner curve: nuclides separated on barium sulfate and then electrodeposited.
NOTE 2—Outer curve: carrier-free tracer solution evaporated directly.
FIG. 1 Resolution Obtained on Six-Component Mixture
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9.2 See Practice D7282 for additional information about setup, calibration, and quality control of nuclear counting instruments,
including alpha-particle spectrometers.
10. Procedure
10.1 The procedure of analysis is dependent upon the radionuclide(s) of interest. A chemical procedure is usually required to
isolate and purify the radionuclides. See Test Methods D3865 and D3972. Additional appropriate chemical procedures may be
found in Refs (7-10). A source is then prepared by a technique in accordance with Section 8. Measure the radioactivity of this
source in an alpha spectrometer, following the manufacturer’smanufacturer’s operating instructions. The counting period chosen
depends on the sensitivity required of the measurement and the degree of uncertainty in the result that is acceptable (see Section
12).
10.2 Silicon detectors will eventually become contaminated by recoiling atoms unless protective steps are taken. Controlling
the air pressure in the counting chamber so that 12 μg/cm of absorber is present between the source and the detector will cause
only a 1-keV 1 keV resolution loss; however, the recoil contamination will be reduced by a factor greater than 500. Recoiling atoms
can also be reduced electrically (11). Ruggedized detectors can be cleaned to a limited degree.
10.3 A silicon detector can also become contaminated by rapid venting of the vacuum chamber when a microprecipitated source
is present. Many systems now control the venting rate automatically; however, older systems might require careful operation of
the venting knob to avoid dispersing small amounts of the precipitate throughout the chamber.
10.4 The measurement of an alpha-emitting nuclide is based on the counts observed in a spectral peak resulting from the
nuclide’s alpha-particle emissions in a specified range of energies or channels, called a region of interest. Typically, the peak area
is estimated from the total counts observed in that region of interest, with a correction for the measured background counts in the
same region. Ideally, the region of interest includes virtually 100 % of all the emitted alpha-particles, or a well-estimated fraction
of them; however, tailing and spillover may reduce this fraction and may also produce interferences in other regions of interest.
10.5 Qualitative identifications sometimes can be made even on highly degraded spectra. By examining the highest energy
value, and using the energy calibration (keV/channel) (keV per channel) of the pulse-height analyzer, alpha-particle emitters
alpha-emitters may be identified. Fig. 2 shows a typical spectrum with very poor resolution.
11. Calculation
11.1 Analyze the data by first integrating the area under the alpha peak to obtain a gross count for the alpha emitter. When the
spectrum is complex and alpha peaks add to each other, corrections for overlapping peaks will be required. Some instrument
manufacturer’smanufacturer’s computer software can perform these and other data-analysis functions.
11.2 The preferred method for determination of chemical recovery is the use of another isotope of the same element (examples:
polonium-208 to trace polonium-210, plutonium-236 to trace plutonium-239, and americium-243 to trace americium-241). Add a
known activity of the appropriate isotope(s) to the sample at the beginning of the analysis, perform the appropriate chemical
separations, mount the sample, and measure it by alpha-particle spectrometry. The chemical yield is directly related to the reduction
in the activity of the added isotope.
11.2.1 When the recovery factorchemical yield is determined by the addition of a tracer, calculate the gross radioactivity activity
concentration, C,AC, of the analyte in becquerels per litre (Bq/L) as follows:
11.2.1.1 Radiotracer Net Counts: Tracer Net Count Rate and Associated Standard Uncertainty:
C C
ST BT
R 5 2 2 R (1)
NT IT
t t
S B
N 5 G 2 B 2 I (1)
T T C
FIG. 2 Poor Resolution Alpha-Spectrum Containing Minor Components a
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