Cosmic‐ray‐induced neutron intensity in Taiwan and its variations during a typhoon

Abstract

Cosmic neutrons as a secondary cosmic radiation, strike the ground with intensity which varies at different latitude, altitude, and solar activity. In a 3‐year extensive survey period, the cosmic neutron intensity was measured over Taiwan islandwide; the average neutron dose equivalent rate is 0.65 nSv h−1 near sea level, up to 14.5 nSv h−1 at the highest mountaintop (altitude of 3952 m). In a rare event, the cosmic neutron intensity at a monitoring station was recorded during the entire period that Typhoon Herb swept across the island in 1996. The anomalous reading of cosmic neutron intensity in the typhoon eye, 2.17±0.40 nSv h−1 (or 4 times higher than the normal reading) is reported here for the first time. The onset of cosmic neutron intensity in the typhoon is closely associated with the steady, rapid air pressure drop as well as the 6×106 km3 spiral cloud; the dips in the onset are correlated with sustained gale winds and heavy precipitation. It is believed that the air pressure drop provides thinner air within the typhoon, and the spiral cloud containing dense moisture further slows down high energy neutrons from the ground, where we observed the neutrons showering down from the stratosphere where the cosmic neutrons have the maximum density. These combined effects result in more medium‐energy neutrons penetrating down to the ground through the typhoon; while the gale wind and heavy rain deflect, attenuate, and, to a lesser extent, absorb the cosmic neutrons from the atmosphere.