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.
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.