About the Cover

Vol. 91 No. 4 (2015)

  The Great East Japan Earthquake with tsunami on March 11, 2011 caused successive meltdown of the Fukushima Nuclear Power Plant (NPP), which was followed by the discharge of large amounts of radioactive nuclides into environment a few days later. Principal efforts to figure out the distribution of the fallout have so far been focused on the main radioactive nuclides, 131I, 137Cs, and 134Cs, whereas a research group led by S. Mori trying to detect radioactive contamination in the local ecosystems discovered by chance the distinct biological concentration of radioactive silver 110mAg (half life 250 days) in two popular species of spider, Nephila clavata and Atypus karschi, as described in this issue on pp. 160-174.
  The first individual of N. clavata was collected on Sept. 20, 2011 at a site 44-km northwest from the NPP, which is indicated in an official map showing distribution of 110mAg in soil after the accident (right). The spider was found to emit unexpectedly strong γ-ray as shown in the autoradiograph (upper left), which was first assumed to be due to 137Cs, but the use of Ge-detector in γ-ray spectrometry revealed the presence of the signal of 110mAg distinct from that of 137Cs (lower left). The accumulation ratio of 110mAg in spiders to soil was three orders higher than that of 137Cs. Subsequent periodical surveillance has revealed the concentration of 110mAg in other faunal species especially several arthropods as well as molluscs (snails). This is the first detailed description of the biological concentration of 110mAg in the terrestrial faunal species. Although the released amount of 110mAg in this accident was far lower than those of 131I, 134Cs and 137Cs, this discovery suggests that close examination is required to reveal characteristic behaviors of the individual radionuclides in the ecosystems.