In-situ gamma-ray measurement to estimate depth distribution of Cs in farmland in Fukushima Prefecture.

Radioactive cesium released into the atmosphere caused by the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 has contaminated the surrounding area. We confirmed the applicability of in-situ methods to evaluate the depth distribution of Cs by employing the ratio of Compton-scattering and photo-peak components (r) obtained from measured gamma-ray spectra. In the present study, we applied the in-situ method to farmlands in Fukushima Prefecture whose sites were disturbed by decontamination and plowing operations. r and the net count of the 662-keV photo-peak, n, were obtained from gamma-ray spectra measured using a portable CsI detector. Reasonable r was obtained by removing the contribution of naturally occurring radioactive materials through a simple and versatile procedure. The depth distribution of Cs measured using the conventional sampling method was reproduced using the Fermi distribution function. The concentration of Cs on the ground surface, N(0), and the depth at which the concentration becomes half of N(0), d, can be described by simple functions of n and r, respectively. We also confirmed that the Monte Carlo simulation is useful to reproduce the present results, taking into account the contribution of Cs and the detection system properly.