Sequential measurements of cosmic-ray neutron energy spectrum and ambient dose equivalent on the ground

Abstract

The cosmic-ray neutron energy spectrum and dose rate were measured sequentially by using a 3He-loaded multi-moderator detector (Bonner ball) and rem counter, respectively. The Bonner ball is composed of a 5.08-cm diameter spherical 3He counter filled with 5 or 10 atm 3He gas, which is placed at the center of a set of polyethylene spheres of different diameters of 8.1, 11.0, 15.0, 23.0 cm. Neutron energy spectra were obtained by an unfolding technique using the SAND-II code. The rem counter consists of a 12.9-cm diameter 5 atm 3He gas filled spherical proportional counter. The measuring interval is every 12 hours for Bonner ball, and every 30 minutes for rem counter. This measurement was carried out at the Kawauchi-campus of Tohoku University in Japan.The neutron energy spectra and the neutron ambient dose equivalent rate were obtained on the ground level with an atmospheric pressure correction. The ambient neutron dose equivalent rate measured by the rem counter keeps an almost constant values of 4.1 [nSv/h], after atmospheric pressure correction, and it often decreased about 30% after a large Solar Flare, that is called as the Forbush decrease. The total neutron flux was also obtained by the Bonner ball measurements to be 7.5 × 10-3 [n cm-2 sec-1].