Present status and prospects of ultralow level radioactivity measurements (4). Ultralow level radioactivity measurements of fallout and deep sea samples

The radioactivity measurements have been carried out in Japan from 1958 to 2006. The long term change and behavior of concentration of 137Cs originated in air and sea from the atmospheric nuclear tests is reported. The high ground level concentration of 137Cs and 90Sr had been observed from 1960's, when many atmospheric nuclear tests were carried out, to 1980's, the 26th nuclear explosion in China. In the 1990s, the level of 137Cs, 90Sr and Pu in fallout decreased remarkably. The concentration of 137Cs in seawater showed the maximum value in the surface mixed layer at 30 to 50 degrees north latitude in 1960, and it 200 to 500 m in depth at about 20 degrees north latitude in 2002. The concentration of 137Cs in deep seawater and surface seawater after 1990 showed very low values, so that the low background measurement values of 137Cs in seawater samples were observed in Ogoya underground facility in Japan. The detection limit in Ogoya decreased 0.1 mBq or less. These results make possible to decrease amount of seawater sample. The long term changes of 137Cs in monthly fallout and in the surface seawater are illustrated. The distribution of 137Cs in the North Pacific Ocean in 2002 is shown. (S.Y.)