Gross Alpha by Liquid Scintillation

Carolyn T. Wong, Violeta M. Soliman, and S. Kusum Perera

California Department of Health Services

The standard procedure for the analysis of gross alpha in water is evaporation of the sample and counting of the resultant solids by proportional counting. This method generally lacks precision, and in the case of samples with high total dissolved solids also lacks sensitivity. Additionally, the analytical results are dependent on the choice of radionuclide standard used for the analysis as well as the sample matrix. Direct liquid scintillation counting of the sample has the advantage of higher counting efficiency and minimal sample preparation time. However, due to the small sample aliquots used for analysis, long count times are required to reach low detection limits. Preliminary tests indicate that a large sample volume can be evaporated to a small volume, mixed with scintillation cocktail and counted using alpha/beta liquid scintillation counting. Our tests show that a sample aliquot containing up to 500 mg of dissolved solids can be analyzed by alpha/beta liquid scintillation counting. Tests also indicate that the amount of total dissolved solids does not effect sample quench or counting efficiencies. Data will be presented demonstrating that this technique may be a viable alternative to proportional counting for gross alpha analyses.