135Cs activity and 135Cs/137Cs atom ratio in environmental samples before and after the Fukushima Daiichi Nuclear Power Plant accident

Cs-135/Cs-137 is a potential tracer for radiocesium source identification. However, due to the challenge to measure Cs-135, there were no Cs-135 data available for Japanese environmental samples before the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. It was only 3 years after the accident that limited Cs-135 values could be measured in heavily contaminated environmental samples. In the present study, activities of Cs-134, Cs-135, and Cs-137, along with their ratios in 67 soil and plant samples heavily and lightly contaminated by the FDNPP accident were measured by combining gamma spectrometry with ICP-MS/MS. The arithmetic means of the Cs-134/Cs-137 activity ratio (1.033 +/- 0.006) and Cs-135/Cs-137 atom ratio (0.334 +/- 0.005) (decay corrected to March 11, 2011), from old leaves of plants collected immediately after the FDNPP accident, were confirmed to represent the FDNPP derived radiocesium signature. Subsequently, for the first time, trace Cs-135 amounts before the FDNPP accident were deduced according to the contribution of global and FDNPP accident-derived fallout. Apart from two soil samples with a tiny global fallout contribution, contributions of global fallout radiocesium in other soil samples were observed to be 0.338%-52.6%. The obtained Cs-135/Cs-137 database will be useful for its application as a geochemical tracer in the future.