Thin-window gas flow proportional counting systems are used to measure the overall (gross) alpha and beta radioactivity of water samples and air particulate filters. Calibration for alpha counting was performed using ²⁴¹Am for both particulate filters and alpha in water determinations. Calibration of the proportional counting systems for beta counting was performed using ¹³⁷Cs sources. Both standards are traceable to the National Institute of Standards and Technology (NIST).

Analysis of Water Samples

A 250-mL volume of sample is first acidified to approximately a pH of 2 with nitric acid to maintain radionuclides in solution. Following acidification, samples are evaporated to a small volume in a beaker and then rinsed with nitric acid into a 5-cm diameter stainless steel planchet stamped with a concentric ring pattern to evenly distribute residue. Final evaporation to dryness takes place under heat lamps. Mass of the residue is determined by weighing the planchet before and after evaporating the samples.

Planchets are counted for gross alpha and gross beta activity on the Canberra 2404 proportional counter. Activities are calculated using empirically determined efficiency factors and self-absorption correction curves. These curves are developed using ²⁴¹Am as an alpha source and ¹³⁷Cs as a beta source. Pocatello tap water provides the water residues upon which the self-absorption correction curves are based. These corrections compensate for the change in the effective counting efficiency caused by the mass of residue material found in water. During analysis of environmental samples radioactive species are not identified, therefore, no decay corrections are made.

Analysis of Air Particulate Filters

The analysis of air particulate filters (Versapore 1200) used for air sampling is delayed for five or more days to allow the short-lived radioactive progeny of ²²²Rn and ²²⁰Rn to decay. Filters from low volume air sampling systems are analyzed for gross alpha and beta activity on a Canberra model 2404 proportional counter. Detection capability depends upon the air volume actually passed through the filter, the magnitude of background detected, and the efficiency of the counting instrument. Self-absorption is not considered in the case of air particulate filters because of the impracticality of determining the penetration depth of the deposit into the filter. No decay corrections are made because the radioactive species are not identified.

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Last modified: June 26, 2001