Thomas M. Semkow(a,b),
Pravin P. Parekh(a) and Amanda Bolden(c)
a) Wadsworth Center, New
York State Department of Health, Empire State Plaza,
Albany, NY 12201-0509,
Telephone 518-474-6071, Fax 518-474-8590, E-mail
b) School of Public
Health, University at Albany, State University of New York
c) St. Lawrence
University, Canton, New York (Ron McNair Scholar)
Detection of gross alpha
and beta radioactivities in water is an US Environmental Protection Agency
mandated program in the context of National Primary Drinking Water Regulations
(EPA, 1997). Gross alpha is more of a concern than gross beta for natural
radioactivity in water as it refers to the radioactivity of Th, U, Ra as well
as Rn and descendants. For anthropogenic radioactivity, gross alpha may pertain
to screening for transuranics in wastes, while gross beta to screening for
fission products in accidental reactor releases. Only when the results of
screening are positive, it is warranted to determine an isotopic content using
more sophisticated and time-consuming procedures.
It thus follows that
gross alpha and beta detection in water has many uses. The detection has to
involve minimum sample preparation and, if possible, be simultaneous for alpha
and beta particles. The EPA approved method 900.0 (EPA, 1980) satisfies many of
these requirements, and it is preformed in many laboratories across the US. It
involves evaporating of water and counting the residue deposited on a
planchette. This method is
characterized by low detection limits, especially for the alpha particles. The
method 900.0 has several possible interferences associated with sampling,
preservation, 222-Rn and daughters, 224-Ra and progeny as well as residue
nonuniformity in both calibration standards and samples. All of these
interferences may lead to erroneous results.
A gas proportional
detector is used to count the residue. 230-Th as well as 90-Sr/90-Y sources are
recommended for detector calibrations. The gas proportional detector can
operate in two modes to detect gross alpha and beta radioactivities:
alpha-then-beta or alpha-and-beta (ANSI, 1997). The latter mode is more useful,
as it allows for a simultaneous detection. In this case, alpha and beta
particles can be distinguished by either the pulse height or shape. The
interfering processes are crosstalk or spillover in the case of pulse height or
pulse shape, respectively.
While the sampling and
analytical procedures for gross alpha and beta detection in water, as well as
associated interferences mentioned above, are known and have been studied
extensively, the measurement on a gas proportional counter is known to a lesser
extent. This study has been therefore performed to investigate a detailed
response of a gas proportional counter to alpha and beta particles in the
health-related context of detecting them in water (Semkow and Parekh, 2000).
Two geometries were
studied: drop standards deposited by direct pipetting onto deep planchettes as
well as electroplated standards placed in shallow planchettes, closer to the
detector window. Three beta standards with increasing average beta energy were
used: 14-C, 137-Cs and 90-Sr/90-Y in the drop geometry. Four alpha standards
with increased alpha energy were used:
230-Th, 239-Pu, 210-Po
and 241-Am in both drop and electroplated geometry. A commercial XLB5 gas proportional detector (Canberra, Inc.) was
used in the simultaneous alpha-and-beta mode, where alpha and beta particles
were distinguished by pulse height. The detector consisted of a cylindrical
chamber and used a P-10 gas at approximately atmospheric pressure. It contained
a 0.08 mg/cm**2 Mylar window. The air gaps between the sample and window were 5
and 10 mm for the electroplated and drop geometries, respectively. In addition
to alpha and beta counts, spectral responses were also measured for all the
sources.
It has been observed
that the beta efficiency for intermediate to high energies is enhanced 33%
(relative to a geometrical efficiency defined via the solid angle) by
backscattering, reaching the values of 0.456 for 137-Cs and 0.446 for
90-Sr/90-Y. The beta efficiency includes all atomic processes for a particular
beta decay. The beta efficiency decreases considerably at low energies (14-C)
making this method less useful. The alpha efficiencies were found to be only
slightly dependent on the energy in the range studied (4665-5480 keV), whereas
considerably dependent on the geometry. The alpha efficiencies varied from
0.272 to 0.307 and from 0.368 to 0.396 for the drop and electroplated
geometries, respectively. On the average, they are smaller than the geometrical
efficiencies by 15% and 7.5%, respectively, due to energy loss in the air and
window.
While the method of
simultaneous alpha-and-beta counting can precisely determine the alpha
radioactivity in the presence of beta radioactivity, the converse is not
possible in general. The sources of this finding are atomic processes
associated with alpha decay. These processes include emissions of conversion as
well as Auger electrons in addition to x rays, which result in pulses
indistinguishable from the beta-particle pulses, causing a considerable
alpha-to-beta crosstalk and miscounting of beta radioactivity. The crosstalk is
dependent only on a particular alpha-decay scheme and not on the alpha energy.
The effect is the highest for 241-Am (28-35%) and 230-Th (22-24%), the
radionuclides recommended for detector calibration (EPA, 1997; EPA, 1980; ANSI,
1997).
A crosstalk correction
can be applied to the data. If the crosstalk correction is determined using
241-Am or 230-Th and applied to an unknown sample having small atomic effects,
it may lead to a serious underreporting of beta radioactivity in that sample.
The alpha-to-beta crosstalk is much smaller in 239-Pu (4.9-6.5%) and nearly
negligible in 210-Po (1.2-1.6%). In the
latter case, the crosstalk is caused predominately by alpha straggling and
self-absorption. Therefore, if the (small) crosstalk correction is determined
using radionuclide such as 210-Po (ANSI, 1997) and applied to an unknown sample
having large atomic effects, it may lead to a spurious detection or artificial
elevation of beta radioactivity.
Consequently, the crosstalk correction makes sense only if the
radionuclides assayed are exactly the same as used for standardization.
It was also investigated
how the EPA method for screening gross alpha and beta radioactivity can be
improved to overcome this limitation. One method found was to use absorbers in
a two step measurement. In the first step, the alpha radioactivity is measured
as before. In the second step, the sample is recounted for beta after covering
it with a 6.2 mg/cm**2 Al absorber.
This reduces the alpha-to-beta crosstalk to <3% for 241-Am and to
<1% in other radionuclides. This method has a drawback of reducing the beta
efficiency by 16-31%.
This work helps
understanding a quantitative detection of gross alpha and beta radioactivity
which is important for a quick surveying of both natural and anthropogenic
radioactivity in water. Since the EPA method has a substantial impact in the
US, a future research is warranted. It should involve determination of atomic
emissions of many other radionuclides of interest, besides those investigated,
since many of them have not been reported. The combined crosstalk due to atomic
effects and self-absorption should be studied. One concept, which might avoid
using a two-step measurement with the absorbers, could be alpha-beta separation
by both pulse height and shape. Since the atomic electrons have low energies,
they may be contained in a small volume inside the counter. This may lead to
pulses faster than those from more energetic beta particles penetrating the
counter, enabling the rejection of the atomic electrons online in a simultaneous
measurement. If verified, this concept would reduce the crosstalk to <3%,
however, with some possible loss to the beta efficiency.
References
American National
Standards Institute (1997) Calibration and usage of alpha/beta proportional
counters. IEEE N42.25, New York.
Environmental Protection
Agency (1980) Prescribed procedures for measurement of radioactivity in
drinking water. Environmental Monitoring and Support Laboratory. EPA
600/4-80-032.
Environmental Protection
Agency (1997) National primary drinking water regulations: Analytical methods
for radionuclides; Final rule and proposed rule. 40 CFR Part 141. Federal
Register 62(43):10168-10175.
Semkow T.M. and Parekh P.P. (2000) Principles of gross alpha and beta radioactivity detection in water. Submitted to Health Phys.