Date of Award

12-2006

Document Type

Thesis

Degree Name

Master of Arts (MA)

Legacy Department

Environmental Engineering and Science

Advisor

DeVol, Timothy A

Abstract

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 high-level 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-244, Pu-239, and U-234/U-238 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-244, Am-241, Pu-239, and Th-230 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 Zn(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-238 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.

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