Field measurements of the cosmic-ray neutron dose rate on the ground using the neutron dose-equivalent counter developed by Nakamura were conducted in order to allow an understanding of neutron dose rate distribution in Japan. The effects of altitude, geomagnetic latitude, snowfall and rainfall on the neutron dose rate were confirmed from the results of the field measurements. Altitude variation can be fitted to an exponential function (eαZ), where Z is altitude (km). The α value for the neutron dose rate increased with increasing geomagnetic latitude, 0.6-0.8(km-1) at latitudes 20°-40°. Roughly, α=0.01×L+0.4 (for Z under 2km), where L is the geomagnetic latitude(°). The effect of geomagnetic latitude on the neutron dose rate was approximately 2% per degree(°) at latitudes 20°-40° Reductions in neutron dose rate due to snowfall and rainfall are caused by a decrease in component backscattering from wet ground surfaces. The neutron dose rate (H*(10)) at sea level at a geomagnetic latitude near 25° in 2002 was approximately 4nSv/h, and the neutron dose rate obtained from the neutron energy spectrum up to 400MeV with the Bonner sphere counter developed by Uwamino was 5.7nSv/h. This difference may by due to the low sensitivity of the dose-equivalent counter to neutrons with energies above 15MeV.
