Direct Method for Gross Alpha Beta Determination in Water Samples by LSC

Gross alpha/beta activity determination is utilized as a very popular Liquid Scintillation Counting (LSC) technique widely used in routine monitoring of drinking, surface and waste waters providing results that indicate whether water sample contains elevated levels of any radionuclide. Monitoring of drinking waters has been carried out in laboratory for low-level radioactivity measurements in Novi Sad for a few years according to ASTM D 7283-06 method on liquid scintillation counter Quantulus 1220TM. Development of rapid screening method for gross alpha/beta measurements has been presented in this paper. Modification of conventional ASTM method assumes direct mixing of water samples with liquid scintillation cocktail Ultima Gold AB, without any sample pretreatment. Optimization of the method involved optimal sample-to-scintillation cocktail ratio determination based on the achieved detection limit and quench level of sample. Optimal value of Pulse Shape Analysis (PSA) discriminator was investigated for different sample:cocktail ratios to ensure accurate and reliable alpha/beta spectra separation. Calibration experiments have been carried out with 241Am and 90Sr/90Y aqueous standards for different sample:cocktail ratios, which included efficiency, MDA (Minimal Detectable Activity) and optimal PSA parameters determination. Method's validity has been tested on spiked samples with 241Am/90Sr radionuclides. Direct alpha/beta method provides overestimated results up to 18 percent which should be corrected, but it offers advantages over conventional ASTM method, it is ultra fast, simple and inexpensive test for efficient screening of water samples. (author).