Comparison of high-purity germanium detectors and gamma-ray water monitors in radioactive wastewater management for targeted radionuclide therapy.

OBJECTIVE: Gamma-ray water monitors (GWMs) are widely used for radioactive wastewater monitoring but can be inaccurate in mixed radionuclide conditions. This study compared radioactivity concentrations measured using a GWM and a high-purity germanium (HPGe) detector and evaluated the validity of derived conversion factors. METHODS: Wastewater containing 131I and 177Lu with 177mLu impurity was sampled from storage tanks in a targeted radionuclide therapy facility. We then measured radioactivity concentrations using GWM and HPGe detectors. GWM reproducibility was evaluated, and the effects of sampling location and stirring were assessed. Conversion factors were derived from paired measurements and applied to independent samples. RESULTS: Radioactivity concentrations did not differ significantly between sampling locations or before and after stirring. GWM measurements showed high reproducibility (CV = 1.7%) but consistently produced higher radioactivity concentrations than the HPGe detector. Summed concentration limit ratios were 30.9 and 3.0 for the GWM and the HPGe detectors, respectively, whereas a conversion factor-based estimate was 13.9. CONCLUSION: Conversion factors reduced the gap between GWM and HPGe detection and provided estimates closer to HPGe-based values, facilitating more efficient discharge management in targeted radionuclide therapy facilities.