Source evaluation of 137Cs in foodstuffs based on trace 134Cs radioactivity measurements following the Fukushima nuclear accident

摘要

We performed gamma-ray analysis to determine the amount of radioactive cesium-134 (¹³⁴Cs) and cesium-137 (¹³⁷Cs) in 259 foodstuffs five years after the Fukushima nuclear accident of 2011. Using measurements of trace ¹³⁴Cs radioactivity, we investigated the contribution ratio of ¹³⁷Cs derived from the Fukushima accident on 2011 and pre-Fukushima. The median detected concentration of radiocesium (¹³⁴Cs + ¹³⁷Cs) in foodstuffs was 0.33 Bq/kg-raw, a much lower radioactivity than the Japanese regulatory limit. However, a few samples had particularly high radioactivity, including some dried mushrooms sold in Iwate Prefecture that had a ¹³⁷Cs radioactivity concentration as high as 441 Bq/kg. Our analysis showed that 75.5% of the ¹³⁷Cs detected in these mushrooms originated from the Fukushima accident, and 24.5% was originated before the Fukushima event. Our study clarified the ¹³⁷Cs contamination in 75 of all 259 food samples before and after the Fukushima nuclear accident, showing that not only mushrooms but also fish had been contaminated before the Fukushima accident.