Selecting a spectrometric detector for drinking water monitoring in the case of nuclear and radiological emergencies

Rapid and reliable radionuclide-specific monitoring of drinking water may be required following a radiological or nuclear (RN) emergency. In this study, ten gamma-detector configurations were evaluated for continuous drinking-water monitoring using Monte Carlo simulations combined with measured detector backgrounds. Two source terms were investigated, representing fallout from a nuclear weapon detonation and a nuclear power plant accident, each at 48 h and 7 d after release. Two measurement geometries were simulated: a 4 L Marinelli beaker and a shielded detector positioned directly against a water pipe. A conservative scenario was assumed in which the complete source-term radionuclide composition was transferred to the drinking water. Minimum Detectable Activity (MDA) and Minimum Quantifiable Activity (MQA) were calculated for 131 I and 137 Cs, including contributions from other radionuclides within the respective regions of interest. Larger detectors generally provided lower MDA and MQA values, with the lowest values mainly obtained for HPGe. Despite substantially lower volume-normalised full-energy peak efficiency for the Pipe geometry, its concentration-based MDA and MQA values were relatively similar to those of the Marinelli geometry because of the much larger sampled water volume. Energy resolution strongly affected radionuclide-specific quantification. For 131 I, 7 d after a nuclear weapon detonation in a Marinelli geometry, non-target radionuclides contributed approximately 79% of the NaI(Tl) ROI counts, compared with approximately 29% for HPGe. The results show that both detection efficiency and energy resolution must be considered when selecting detector systems for radionuclide-specific drinking-water monitoring following an RN emergency.