In-situ gamma-ray measurement to estimate depth distribution of 137Cs 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 ¹³⁷Cs by employing the ratio of Compton-scattering and photo-peak components (r_C) 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_C and the net count of the 662-keV photo-peak, n_peak, were obtained from gamma-ray spectra measured using a portable CsI detector. Reasonable r_C was obtained by removing the contribution of naturally occurring radioactive materials through a simple and versatile procedure. The depth distribution of ¹³⁷Cs measured using the conventional sampling method was reproduced using the Fermi distribution function. The concentration of ¹³⁷Cs on the ground surface, N(0), and the depth at which the concentration becomes half of N(0), d_1/2, can be described by simple functions of n_peak and r_C, respectively. We also confirmed that the Monte Carlo simulation is useful to reproduce the present results, taking into account the contribution of ¹³⁴Cs and the detection system properly.