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72. Variation of Cosmic-ray Intensity with Altitude in Asia: Results for Japan, China, and Korea
Masahide Furukawa, Shouzhi Zhang1, Shian Zhao1, Zhugang Jiang2, Lixia Nei2, Woonhyuk Chung3, Masaki Matsumoto and Shinji Tokonami
(1Laboratory of Industrial Hygiene, China; 2Epidemic Prevention Station of Tibet, China; 3Dept. Physics, Pusan National Univ., Korea)
Keywords: cosmic-ray intensity, ionizing component, variation with altitude, Asia
In the atmosphere, cosmic-ray intensity changes as a function of both altitude and latitude. To estimate the human dose from cosmic-rays, it is necessary to take into consideration these geographic factors. The purpose of this study is to investigate the actual distribution of cosmic-ray intensity by in situ measurement on a global scale.
A nationwide survey in Japan, including Mt. Fuji, Mt. Iwaki, Mt. Kaimon-dake etc., which aimed at the estimation of the variation of cosmic-ray intensity with altitude, was conducted from 1994 to 1998 by NIRS. The measurements were also carried out in the southern part of Korea in July 1996 and in Tibet, the highlands of China, in August 1998 by Korea-NIRS and China-NIRS co-operative research teams, respectively.
Cosmic rays were measured with a multichannel spectrometer with a 3"
3" cylindrical NaI(Tl) scintillation detector. Dose conversion analysis was made for the energy region from 3 MeV to 7 MeV, and the intensity was converted as the absorbed dose rate in air due to the ionizing component of cosmic rays.
Fig. 23 shows the relationship between cosmic-ray intensity and altitude which we obtained. The intensity increased from about 30 nGy/h to 140 nGy/h with increase of altitude. The minimum value of 28 nGy/h and the maximum value of 141 nGy/h were observed at sea level in Okinawa Prefecture, which is located in the southern part of Japan, and at the altitude of 4250 m in Tibet, respectively. Above 2500 m, the rate of change of the cosmic-ray intensity with altitude was estimated to be about 0.04 nGy/h/m. The rate from sea level to 2500 m was estimated to be about 0.01 nGy/h/m.
These results are considered to be useful information for the estimation of human exposure due to cosmic rays in Asian countries.
