In-situ gamma-ray measurement to estimate depth distribution of 137Cs in farmland in Fukushima Prefecture

Sadao Momota; Yoshihiro Okuda; Minoru Tanigaki; Yoshitaka Ohkubo; Takashi Saito

doi:10.1016/j.jenvrad.2024.107610

In-situ gamma-ray measurement to estimate depth distribution of ¹³⁷Cs in farmland in Fukushima Prefecture

J Environ Radioact. 2025 Jan 10:282:107610. doi: 10.1016/j.jenvrad.2024.107610. Online ahead of print.

Authors

Sadao Momota¹, Yoshihiro Okuda², Minoru Tanigaki³, Yoshitaka Ohkubo³, Takashi Saito⁴

Affiliations

¹ Kochi Univ. of Technology, Miyano-kuchi, Tosayamada, Kochi, 782-8502, Japan. Electronic address: momota.sadao@kochi-tech.ac.jp.
² Kochi Univ. of Technology, Miyano-kuchi, Tosayamada, Kochi, 782-8502, Japan.
³ Institute for Integrated Radiation and Nuclear Science, Kyoto Univ., Kumatori, Osaka, 590-0494, Japan.
⁴ Hama Agricultural Regeneration Research Centre, Fukushima Agricultural Technology Center, Minami-soma, Fukushima, 975-0036, Japan.

PMID: 39798231
DOI: 10.1016/j.jenvrad.2024.107610

Abstract

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.

Keywords: CsI(Tl); Depth distribution; In-situ gamma-ray measurement; Radioactive cesium; Soil.