²³⁷Np and Pu isotopes activities in soils collected in Fukushima Prefecture

²³⁷Np and Pu isotopes activities such as ²³⁹Pu, ²⁴⁰Pu and, ²⁴¹Pu in soil collected at five stations (sta. A-E) in the Namie town in Fukushima Prefecture in September 2017 were reported. At sta. A-E, ²³⁷Np activities ranged from (0.058±0.05) to (5.7±0.1) mBq/kg-dry and these values were ∼2 orders of magnitude lower compared to those in environmental samples collected in Japan until 2000's. ²³⁹Pu, ²⁴⁰Pu and,^239+240Pu activities were (5.8±1) ∼ (1717±63) mBq/kg-dry, (3.9±1) ∼ (1051±32) mBq/kg-dry, and, (9.7±1) ∼ (2768±70) mBq/kg-dry, respectively, and these values were also ∼2 orders of magnitude lower compared to those before the Fukushima Dai-ich Nuclear Stations (FDNPS) accident. At sta. A, B, C and, E, there is a positive correlation (p value<0.05, R²=0.60∼0.93) between ²³⁷Np and ^239+240Pu activities and ²³⁷Np/^239+240Pu activity ratio was (0.15±0.2) x 10^-3 ∼ (4.1±0.4) x 10^-3. For total 6 layers of litter and surface soil (core depth 0-2.5 cm) at sta. D, ²⁴¹Pu was detected and the contribution rate of Pu derived from the FDNPS accident was very small (<1%) based on the two end-member mixing model for source apportionment. Interestingly, ²⁴¹Pu/²³⁹Pu atomic rations approached the GF value from litter to deeper layers of soil surface. Because there was no significant correlation for ²³⁷Np activity and ²³⁷Np/²³⁹Pu atomic ratio between the layers the ²⁴¹Pu was those not detected, ²³⁷Np in soils in this study was generally originated from GF. (author)