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: 40-3,700 in) 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 +/- 0.00029 cm(-2.)s(-1) and 6.4 +/- 0.25 nSv(.)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, similar to exp(alpha Z), where alpha=0.00071 +/- 0.00002 m(-1) for the fluence rate and alpha=0.00070 +/- 0.00001 m(-1) for the dose rate, respectively. These alpha-values were smaller than those obtained at high geomagnetic latitudes in other studies.