Design of a Low-Resolution Gamma-ray Spectrometer for Monitoring Radioactive Levels of Wastewater

Abstract

Wastewater containing radioactive materials is stored in decay tanks, which are typically installed in basements to restrict public access. Specific activity is measured by a gamma spectroscopy system or a gamma counter for several hours per sample before discharge. To simplify the sample collection and measurement procedures before draining, the specific activity conversion coefficient for converting a measured value from a 1.5″ NaI (Tl) detector installed on a tank into specific activity was calculated. The experimental calibration was performed in a test water tank with a volume of 14 L filled with diluted reference source 18F, which is most commonly used in hospitals. The specific activity of the test water was measured with a gamma spectroscopy system using a high-purity germanium detector calibrated with certified reference materials. The change in the conversion coefficient according to the change in the volume of the water tank was corrected based on a Monte Carlo calculation. Finally, it was determined whether the minimum detectable activity (MDA) of this system was lower than that of the draining standard. The counts for the specific activity conversion factor and MDA were evaluated as 5.5389 × 1010 and 2.09 × 105 for 18F and for the one-hour count, respectively. The MDA was lower than the standard value. Thus, this system can be used for discharge determination.