Optimizing calibration factors of plastic scintillation fibers for improved accuracy of <i>in situ</i> radiocesium concentration measurements in bottom sediments of agricultural ponds

Abstract

AbstractThe uncertainty of in situ radioactivity measurements can be influenced by the depth of sediment sampled by the spectrometer and the vertical distribution of the target radionuclide in the sediments. This study used Particle and Heavy Ion Transport Simulation code (PHITS) to evaluate the effective detection depth of a plastic scintillation fiber system (PSF) for monitoring radiocesium (RCs) in bottom sediments of agricultural ponds in Fukushima and thereafter, used 2015–2019 field measurement data from 47 ponds to evaluate the depth dependence of PSF calibration factors and its influence on measurement uncertainty. Though no significant differences were observed in PSF-derived RCs concentrations by calibration depth, PSF calibration factors and normalized mean square error (NMSE) suggest a potential dependence on the depth of sediment containing 90% of RCs inventory (L90) such that it may be the optimal choice for calibrating PSF. Accordingly, revision of PSF calibration depth from the currently adopted top 10 cm to the top 15–20 cm may be necessary for optimizing PSF measurement uncertainty not only because their calibration factors had the best coefficients of determination and yielded lowest NMSE, but also because they constituted the largest proportion of L90 among the 206 samples.