Plutonium concentration and isotopic ratio in soil samples from central-eastern Japan collected around the 1970s

摘要

Obtaining Pu background data in the environment is essential for contamination source identification and assessment of environmental impact of Pu released from the Fukushima Daiichi nuclear power plant (FDNPP) accident. However, no baseline information on Pu isotopes in Fukushima Prefecture has been reported. Here we analyzed 80 surface soil samples collected from the central-eastern Japan during 1969–1977 for ^239+240Pu activity concentration and ²⁴⁰Pu/²³⁹Pu atom ratio to establish the baseline before the FDNPP accident. We found that ^239+240Pu activity concentrations ranged from 0.004 –1.46 mBq g⁻¹, and ²⁴⁰Pu/²³⁹Pu atom ratios varied narrowly from 0.148 to 0.229 with a mean of 0.186 ± 0.015. We also reconstructed the surface deposition density of ²⁴¹Pu using the ²⁴¹Pu/²³⁹Pu atom ratio in the Japanese fallout reference material. The obtained results indicated that, for the FDNPP-accident released ²⁴¹Pu, a similar radiation impact can be estimated as was seen for the global fallout deposited ²⁴¹Pu in the last decades.