A comparison of the ZnS(Ag) scintillation detector to the silicon semiconductor detector for quantification of alpha radioactivity in aqueous solutions

A ZnS(Ag) detector was successfully used in the direct detection of alpha particles from aqueous solutions and the results were compared to the passivated ion implanted planar silicon (PIPS) continuous air monitor (CAM) detector. The ZnS(Ag) detector is recommended for on-line detection of gross alpha radioactivity from highlevel liquid waste process streams; however, the detector suffers from limitations due to variations in detection efficiency with alpha energies. The beta and gamma interference did not significantly spillover into the alpha region of interest of the ZnS(Ag) detector which provided absolute detection efficiencies of 7.19 + 0.13%, 5.37 + 0.02%, and 4.21 + 0.03% for Cm, Pu, and U/U solutions, respectively. The detection efficiencies were found with a 3.5 mg/cm ZnS(Ag) scintillation layer, 4.15 cm detection surface, and a 5.2 mm air gap between the liquid surface and the detector. The absolute detection efficiencies of the PIPS CAM detector for Cm, Am, Pu, and Th solutions in the presence of beta and gamma activity were 4.61 + 0.01%, 3.87 + 0.01%, 2.84 + 0.01%, and 1.32 + 0.01%, respectively, for a geometrical set-up similar to the ZnS(Ag) experiments. Two ZnS(Ag) scintillator thicknesses were compared. The thicker scintillation layer suffered from light absorption and scattering in the ZnS(Ag) layer resulting in a lower energy signal. Despite the differences, the absolute detection efficiencies of the two thicknesses were equivalent. The large detection area available to the ZnS(Ag) detector proved to be valuable for the detection of radioactivity in liquids as the absolute detection efficiency more than doubled for a surface area of 78.5 cm and resulted in a minimum detection concentration of 0.32 Bq/mL for U solution for a 3600 second count. The detection capabilities and pulse height spectra of the ZnS(Ag) detector were compared to the PIPS CAM detector as well as computer simulations and theory. iii