Comments on the investigation of environmental radioactivity. Importance of extremely low-level counting technique

Importance of adopting extremely low background Ge-detector is stressed for augmentation of the reliability of measurements of very low level natural radioactivity by showing actual examples. First, by comparing background spectra of various Ge-detectors at ground and underground laboratories (270 mwe), effectiveness of excluding contribution from cosmic rays to the background radiation has been demonstrated. Second, necessity of large sampling amounts is stressed in the case of measurement of atmospheric floating dusts or aerosols and their deposits. The present report describes some results obtained as demonstrative examples. (1) Analysis of radioactive cesium in air originating from the fire accident in an asphalt solidification facility of JNC held in 1997, (2) Measurements of atmospheric Be 7 and Pb 210 of cosmic ray source with a shot lifetime with high time resolution, (3) Investigation on environmental neutron induced activity producing Co 60 and Eu 152 and 154, which may be important for exposed dose estimation at Hiroshima and Nagasaki explosions. (S. Ohno)