Experimental-theoretical response of a ZnS(Ag) scintillating disc for gross-alpha measurements of aqueous radioactivity

A ZnS(Ag) scintillating disc coupled to a photomultiplier window tube was used to quantify the experimental detection efficiency of thick alpha sources and compare it with the theoretical detection efficiency. The experimental detection efficiencies were determined to be ∼17%-18% for ²³³U and ²³⁷Np positioned 5.4 mm below the ZnS(Ag). The theoretical detection efficiency was calculated using an analytical model for an infinitely thick alpha source and was within 7% of the experimental values. In addition to these static measurements, online flow experiments were conducted. In these flow experiments, ²³³U wastewaters at three concentrations (14.19-53.65 Bq/mL) were quantified using the experimental detection efficiency and determined to be within 17%-18% of the expected concentrations. Pulse height analyses were performed for ²³³U spiked wastewater and for a 3700-Bq ⁹⁰Sr solid point source. For a very low discriminator setting, the ⁹⁰Sr/⁹⁰Y detection efficiency was about 3.5%, which was reduced to essentially zero by raising the discriminator level by a minimal amount that did not effect the alpha detection efficiency. Additional experiments, which included ²³³U (34.66 Bq/mL) on MnO₂-coated glass fiber filter resulted in a detection efficiency of 22%-23%.