Monitoring of Environmental Radioactivity in Pine Needle and Soil at the Site of National Institute for Fusion Science

Abstract

At National Institute for Fusion Science (NIFS), the environmental radioactivity in soil and pine needles were measured from 2014 to 2019 as a part of the monitoring activities for deuterium plasma experiments using Large Helical Device (LHD). Among the monitoring nuclides of ⁵⁴Mn, ⁵⁹Fe, ⁵⁸Co, ⁶⁰Co, ¹³⁴Cs, ¹³⁷Cs, ⁷Be and ⁴⁰K, the nuclides of ¹³⁷Cs, ⁷Be and ⁴⁰K were detected by a HPGe detector. The concentration of ⁴⁰K in soil ranged from 530 Bq/kg to 1,068 Bq/kg, which was the same level at the Tono area reported by Hosoda et al. In addition, the concentration of ⁴⁰K in pine needles ranged from 125 Bq/kg to 300 Bq/kg and a peak was observed in summer. The maximum concentrations of ⁷Be on soil and pine needles were 69.4 Bq/kg and 31.9 Bq/kg, respectively. The concentration of ⁷Be on pine needles became low in summer. Although the concentration of ¹³⁷Cs on pine needles was not almost detectable, that on soil ranged from 0.56 Bq/kg to 3.69 Bq/kg. The concentrations of these nuclides remain the same level after starting the deuterium plasma experiment in LHD.