Nationwide measurements of cosmic-ray dose rates throughout Japan.

Cosmic-ray dose rates on the ground were measured throughout Japan. Neutron dose rates were measured as ambient dose equivalent rates (H*(10)) at 240 points using high-sensitivity neutron REM counters. In addition, cosmic rays directly ionising plus photon components were measured with an ionisation chamber. Time variation due to solar modulation during this study was corrected based on the results of sequential measurements. The effects of altitude, geomagnetic latitude, rainfall and snowfall on the neutron dose rate were inferred from the measured results. The mean value of the neutron dose rates (H*(10)) measured at 47 points of prefectural capitals in Japan was 4.0 nSv h(-1). The value corrected for the energy response of the REM counter was 6.4 nSv h(-1), corresponding to 4.8 nSv h(-1) as an effective dose (ISO). The mean value of the cosmic ray directly ionising plus photon components as an effective dose was 31 nSv h(-1).