Evaluation of the Altitude Variation of the Cosmic-ray Induced Environmental Neutrons in the Mt. Fuji Area

To assess the natural background neutron dose rate in Japan, a series of measurements of the cosmic-ray induced neutron energy spectra and the neutron dose rates were carried out in the Mt. Fuji area (altitude: 40-3,700m) by means of a Bonner multi-sphere neutron spectrometer (Bonner Spectrometer: BS) and a commercially available neutron rem-counter. The following results were obtained: 1. The neutron energy spectra at each altitude were almost the same in their shapes.2. The total neutron fluence rate and the ambient dose equivalent H*(10) obtained by BS at sea level were estimated to be about 0.0075±0.00029cm−2.s−1 and 6.4+0.25nSv.h−1, respectively.3. It was confirmed that the neutron fluence rate and the dose rate varied with the altitude according to an exponential law, ~ exp(αZ), where α=0.00071±0.00002m−1 for the fluence rate and α=0.00070±0.00001 m−1 for the dose rate, respectively. These α-values were smaller than those obtained at high geomagnetic latitudes in other studies.