RADIOISOTOPES

Radioactive materials, primarily radiocaesium (¹³⁴Cs+¹³⁷Cs), were released into the environment by the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011. The proportion of the soybean plants that had an activity concentration of radiocaesium over 100 Bq/kg was higher than that of other crops. To examine the reason why the activity concentration of radiocaesium in soybeans was higher, the concentration distributions of Cs in soybean seeds were analyzed using two methods, X-ray fluorescence microscopy and radioluminography with ¹³⁷Cs. Moreover, the Cs concentration of each organ and the ratio of absorbed Cs to seeds in matured soybeans were examined. The results of the two experiments show that Cs was uniformly distributed in the soybean seeds, as was potassium, both of which likely accumulated in the cotyledon. In addition, approximately 40% of absorbed Cs was accumulated in the soybean seeds. Conversely, it is reported that rice accumulates Cs only in part of its grain and that the accumulation in the grain is lower than that in soybeans. It has been suggested that soybean seeds have a large capacity for Cs accumulation, and this is one of the reasons why the activity concentrations of radiocaesium in soybeans are higher than that in other crops.