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

• Published in: Journal of nuclear science and technology Vol. 42; no. 6; p. 495
• Main Authors: KOWATARI, Munehiko, NAGAOKA, Kazunori, SATOH, Shoji, OHTA, Yuji, ABUKAWA, Johji, TACHIMORI, Shoichi, NAKAMURA, Takashi
• Format: Journal Article
• Language: English
• Published: Tokyo Taylor & Francis Ltd 01.06.2005
• ISSN: 0022-3131; 1881-1248
• DOI: 10.3327/jnst.42.495

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 m) 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, ∼exp(αZ), where α=0.00071±0.00002 m-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.