Measurement of cosmic-ray origin neutrons using a scintillation detector at the summit of Mt. Fuji

Abstract

Cosmic-ray origin neutrons were measured with a scintillation detector at the summit of Mt. Fuji at an altitude of 3776 m during the summer of 2008. The averaged neutron ambient dose equivalent rate converted from the count rates of the detector was 0.47 ± 0.001 μSv h−1, which is approximately four times greater than the calculated value of 0.11 ± 0.01 μSv h−1. It is postulated that the discrepancy between the measured and calculated neutron ambient dose equivalent rates is on account of the additional charged particle component of the mixed radiation field. To examine the characteristics of the detector at high altitude, a simple model equation which relates the count rates of the detector to the calculated fluence rates of cosmic radiation has been supposed. Based on the model equation it is estimated that the neutron ambient dose equivalent rate would be 0.14 ± 0.04 μSv h−1, and the proton contribution would be 0.28 ± 0.03 μSv h−1 converted as neutron ambient dose equivalent rate. The results agree to the values of the measured and calculated neutron dose equivalent rates, and the discrepancy between them within the errors.