Ogoya old copper mine as a monitoring station for various fields of earth sciences and radon anomaly observed at Tatsunokuchi

To utilize the tunnel of old Ogoya mine for measurement of low-level radioactivity, the intensity of cosmic ray have been determined at various points in the tunnel and basic data regarding radon level, wind velocity and temperature have been collected. Here, recent studies made in the underground laboratory in the tunnel were briefly outlined. The reciprocal of radon activity was found to be closely correlated with wind velocity. The mean activity was ca. 25 Bq/m3, slightly higher than that in an ordinal house at the wind velocity of 0.4 m/sec and it was highest in the conditions of no wind. Thus, the tunnel was found to be suitable for the measurement of low-level radioactivity. Then, the effects of radon on the background level for the Ge-detector set in the underground laboratory were investigated and an attempt for their elimination was made using nitrogen gas. The peak counting rates of 214Pb and 214Bi, both of which are daughter nuclides of radon were remarkably reduced by introducing the gas. Further, the radon level of the atmosphere upon the ground was determined using a radon detector with electrostatic trap aiming to catch the abnormality in radon level, which is regarded as an omen of earthquake. Thus, it was suggested that the radon anomaly observed in Tatsunokuchi during from Aug. 31 to Sep. 12 might be associated with the earthquake with a magnitude of 3.2 occurred 9 Km apart from the town on September 10. (M,N.)