ASTM D3084-96
(Practice)Standard Practice for Alpha-Particle Spectrometry of Water
Standard Practice for Alpha-Particle Spectrometry of Water
SCOPE
1.1 This practice covers the process that is required to obtain well-resolved alpha-particle spectra from water samples and discusses associated problems. This practice is generally combined with specific chemical separations and mounting techniques, as referenced.
1.2 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 and health practices and determine the applicability of regulatory limitations prior to use.
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An American National Standard
Designation:D 3084–96
Standard Practice for
Alpha-Particle Spectrometry of Water
This standard is issued under the fixed designation D 3084; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope sion plates, cloud chambers, absorption techniques, and solid-
state counters. Gas counters, operating either as an ionization
1.1 This practice covers the process that is required to
chamber or in the proportional region, have been widely used
obtain well-resolved alpha-particle spectra from water samples
to identify and measure the relative amounts of differenta
and discusses associated problems. This practice is generally
-emitters. However, more recently, the solid-state counter has
combined with specific chemical separations and mounting
become the predominant system because of its excellent
techniques, as referenced.
resolution and compactness. Knoll (3) extensively discusses
1.2 This standard does not purport to address all of the
the characteristics of both detector types.
safety concerns, if any, associated with its use. It is the
4.2 Of the two gas-counting techniques, the pulsed ioniza-
responsibility of the user of this standard to establish appro-
tion chamber is more widely used as it gives much better
priate safety and health practices and determine the applica-
resolution than does the other. This is because there is no
bility of regulatory limitations prior to use.
spread arising from multiplication or from imperfection of the
2. Referenced Documents
wire such as occurs with the proportional counter.
4.3 The semiconductor detectors used for alpha-particle
2.1 ASTM Standards:
spectrometry are similar in principle to ionization chambers.
C 859 Terminology Relating to Nuclear Materials
The ionization of the gas by a-particles gives rise to electron-
C 1163 Test Method for Mounting Actinides for Alpha
ion pairs, while in a semiconductor detector, electron-hole
Spectrometry Using Neodymium Fluoride
pairs are produced. Subsequently, the liberated changes are
D 1129 Terminology Relating to Water
collected by an electric field. In general, silicon detectors are
D 3648 Practices for the Measurement of Radioactivity
usedforalpha-particlespectrometry.Thesedetectorsaren-type
D 3865 Test Method for Plutonium in Water
base material upon which gold is evaporated or ions such as
D 3972 Test Method for Isotopic Uranium in Water by
boron are implanted, making an electrical contact. A reversed
Radiochemistry
bias is applied to the detector to reduce the leakage current and
3. Terminology
to create a depletion layer of free-charge carriers. This layer is
thin and the leakage current is very low. Therefore, the slight
3.1 For definitions of terms used in this practice, refer to
interactions of photons with the detector produce no signal.
Terminologies D 1129 and C 859. For terms not found in these
Theeffectofanyinteractionsofbetaparticleswiththedetector
terminologies, reference may be made to other published
can be eliminated by appropriate electronic discrimination
glossaries (1, 2).
(gating) of signals entering the multichannel analyzer. A
4. Summary of Practice
semiconductor detector detects all alpha particles emitted by
radionuclides (approximately 2 to 10 MeV) with essentially
4.1 Alpha-particle spectrometry of radionuclides in water
equal efficiency, which simplifies its calibration.
(also called alpha-particle pulse-height analysis) has been
4.4 Semiconductor detectors have better resolution than gas
carried out by several methods involving magnetic spectrom-
detectors because the average energy required to produce an
eters, gas counters, scintillation spectrometers, nuclear emul-
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
This practice is under the jurisdiction ofASTM Committee D-19 on Water and
ion pair in a gas ionization chamber. Detector resolution,
is the direct responsibility of Subcommittee D19.04 on Methods of Radiochemical
defined as peak full-width at half-maximum height (FWHM),
Analysis.
is customarily expressed in kiloelectron-volts. The FWHM
Current edition approved May 10, 1996. Published July 1996. Originally
published as D 3084 – 72 T. Last previous edition D 3084 – 95.
increases with increasing detector area, but is typically be-
Annual Book of ASTM Standards, Vol 12.01.
tween 15 and 60 keV. The background is normally lower for a
Annual Book of ASTM Standards, Vol 11.01.
semiconductor detector than for ionization chamber. Silicon
Annual Book of ASTM Standards, Vol 11.02.
detectors have four other advantages compared to ionization
The boldface numbers in parentheses refer to the list of references at the end of
this document.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 3084–96
chambers: they are lower in cost, have superior stability, have instrumentation,andthequalityofthesource.Ifpeaksoverlap,
higher permissible counting rates, and have better time reso- abetterspectrometeroradditionalchemicalseparationswillbe
lution for coincidence measurements. However, the semicon- required.
ductor detector requires sophisticated electronics because of
7. Apparatus
the low charge that is generated by the incident a-particle in
the detector. Low-noise and high-stability, charge-sensitive
7.1 Alpha Particle Detector, either a silicon semiconductor
preamplifiers are used prior to the detection, analog-to-digital
or a Frisch-grid pulse-ionization chamber.
conversion, and storage of the voltage pulse by a multichannel
7.2 Counting Chamber, to house the detector, hold the
analyzer.Thecountingisnearlyalwaysperformedinavacuum
source, and allow the detector system to be evacuated.
chamber so that thea -particles will not lose energy by
7.3 Counting Gas, for ionization chamber, typically a 90 %
collisions with air molecules between the source and the
argon–10 % methane mixture, and associated gas-handling
detector.
equipment.
4.5 A gridded pulse-ionization chamber was developed by
7.4 Pulse Amplification System, possibly including a pream-
Frischforhigh-resolutionalphaspectrometry.Theunitconsists
plifier, amplifier, postamplifier, pulse stretcher, and a high-
of a standard ionization chamber fitted with a collimator
voltage power supply, as directed by the quality and type of
between the source and the collector plate and a wire grid to
detector employed.
shield the collector from the effects of positive ions. The
7.5 Multichannel Pulse-Height Analyzer, including data
resolution of a gridded pulse ionization chamber is from 35 to
readout equipment. This is now often computer based.
100 keV for routine work. The detector parameters that affect
7.6 Vacuum Pump, with low vapor-pressure oil and prefer-
resolution are primarily the following: statistical variations in
ably with a trap to protect the detector from oil vapors.
the number of ion pairs formed at a given alpha energy, the
variation in rise time of pulses, and the effects of positive ions. 8. Source Preparation
An advantage of gridded ionization chambers is their ability to
8.1 Thetechniqueemployedforpreparingthesourceshould
count large-area sources with good efficiency.
produce a low-mass, uniformly distributed deposit that is on a
4.6 There are two reasons for collimating a sample in a
very smooth surface. The three techniques that are generally
gridded ionization chamber. When thick-sample sources are
employed are electrodeposition, microcoprecipitation, and
encountered, the alpha-particles emitted at a large solid angle
evaporation. The first two usually are preferred. Fig. 1 com-
would show an energy degradation upon ionization of the gas.
pares the alpha-particle spectrum of an electrodeposited source
The effect leads to tailing of the alpha-particle spectrum. This
with that of an evaporated source.
problem is reduced significantly by use of the collimator.
8.1.1 Electrodeposition of a-emitters can provide a sample
Secondly, when the nucleus following ana -particle emission
with optimum resolution, but quantitative deposition is not
does not decay to a ground state, the g-rays that may be
necessarilyachieved.Basically,the a-emitterisdepositedfrom
produced are usually highly converted, and the conversion
solution on a polished stainless steel or platinum disk, which is
electrons ionize the gas. The special mesh-type collimators
the cathode. The anode is normally made from platinum gauze
stop the conversion electrons and collimate the source simul-
oraspiralledplatinumwire,whichoftenisrotatedataconstant
taneously.
rate. Variants of this technique may be found in Refs 5 and 6.
4.7 A more recently developed measurement method is
See also Test Method D 3865. Polonium can be made to
photon-electron-rejecting alpha liquid-scintillation spectrom-
deposit spontaneously from solution onto a copper or nickel
etry. The sample is counted in a special liquid-scintillation
disk (7).
spectrometer that discriminates electronically against non-
8.1.2 Micro-coprecipitation of actinide elements on a rare-
alpha-particle pulses. The resolution that can be achieved by
earthfluoride,oftenneodymiumfluoride,followedbyfiltration
this method is 250 to 300-keV FWHM. This is superior to
conventional liquid-scintillation counting, but inferior to sili-
con 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 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
bytheabsorbingmaterialontheplanchetandthesurfacefinish
of the planchet.
6. Interferences
NOTE 1—Inner curve: nuclides separated on barium sulfate and then
6.1 The resolution or ability to separate alpha-particle peaks
electrodeposited.
will depend on the quality of the detector, the pressure inside
NOTE 2—Outer curve: carrier-free tracer solution evaporated directly.
the counting chamber, the source-to-detector distance, the FIG. 1 Resolution Obtained on Six-Component Mixture
D 3084–96
on a specially prepared membrane-type filter (see Test Method of the pulse-height analyzer, alpha-particle emitters may be
C 1163) also produces a good-quality source for alpha-particle identified. Fig. 2 shows a typical spectrum with very poor
spectrometry. The microgram quantity of precipitant only resolution.
slightly degrades spectral resolution.
11. Calculation
8.1.3 The evaporation technique involves depositing the
solution onto a stainless steel or platinum disk. The liquid is
11.1 Analyze the data by first integrating the area under the
applied in small droplets over the entire surface area so that
alpha peak to obtain a gross count for the alpha emitter. When
they dry separately, or a wetting agent is applied, which causes
the spectrum is complex and alpha peaks add to each other,
thesolutiontoevaporateuniformlyovertheentiresurface.The
corrections for overlapping peaks will be required. Some
total mass should not exceed 10µ g/cm , otherwise self-
instrument manufacturer’s computer software can perform
absorption losses will be significant. In addition, the alpha-
these and other data-analysis functions.
particle spectrum will be poorly resolved, as evidenced by a
11.2 The preferred method for determination of chemical
long lower-energy edge on the peak. This tailing effect can
recovery is the use of another isotope of the same element
contribute counts to lower energy alpha peaks and create large
(examples: polonium-208 to trace polonium-210, plutonium-
uncertainties in peak areas.Alpha sources that are prepared by
236 to trace plutonium-239, and americium-243 to trace
evaporation may not adhere tenaciously and, therefore, can
americium-241). Add a known activity of the appropriate
flake causing contamination of equipment and sample losses.
isotope(s) to the sample at the beginning of the analysis,
perform the appropriate chemical separations, mount the
9. Calibration
sample, and measure it by alpha-particle spectrometry. The
9.1 Calibrate the counter by measuring a-emitting radionu-
chemicalyieldisdirectlyrelatedtothereductionintheactivity
clides that have been prepared by one of the techniques
of the added isotope.
described in Section 8.All standards should be traceable to the
11.2.1 When the recovery factor is determined by the
National Institute of Standards and Technology and in the case
addition of a tracer, calculate the gross radioactivity concen-
of nonquantitative mounting, standardized on a 2p or 4p
tration, C, of the analyte in becquerels per litre (Bq/L) as
alpha-particle counter. Precautions should be taken to ensure
follows:
that significant impurities are not present when standardizing
11.2.1.1 Radiotracer Net Counts:
the alpha-particle activity by non-spectrometric means. The
N 5 G 2 B 2 I
T T C
physical characteristics of the calibrating sources and their
(1)
positioning relative to the detector must be the same as the
2 1/2
t
S
samples to be counted. A mixed radionuclide standard can be
s 5 G 1 I 1 B 3 (2)
F S D G
NT T
t
B
counted to measure simultaneously the detector resolution and
efficiency,andthegainofthemultichannelanalyzer.Checkthe
where:
instrumentation frequently for consistent operation. Perform
N = net counts in the tracer region of interest,
T
background measurements regularly and evaluate the results at
G = gross counts in the tracer region of interest,
T
the confidence level desired. B = background counts in the region of interest cor-
C
rected for sample count time,
10. Procedure
s = one-sigma uncertainty of the net tracer counts,
NT
10.1 The procedure of analysis is dependent upon the B = uncorrected background counts in the region of
interest,
radionuclide(s) of interest. A chemical procedure is usually
I = imprinity counts in the analyte’s region of interest,
required to isolate and purify the radionuclides. See Test
t = sample analysis time, s, and
Methods D 3865 and D 3972.Additional appropriate chemical S
t = background analysis time, s.
B
procedures may be found in Refs(789–10. A source is then
11.2.1.2 Analyte Net Counts:
preparedbyatechniqueinaccordancewithSection8.Measure
the radioactivity of this source in an alpha spectrometer,
following the manufacturer’s operating instructi
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