Spectral neutron flux oscillations of cosmic radiation on the Earth's surface

Two Bonner Sphere Spectrometers (BSS) were used - one at the Schneefernerhaus, Germany (altitude: 2650 m; geomagnetic cut-off: 4.1 GV), the other at the Koldewey station on Spitsbergen (sea level; geomagnetic: cut-off 0 GV) - to measure continuously the spectral flux distribution of secondary neutrons from cosmic radiation. At the Schneefernerhaus, the flux of thermal neutrons was about 75% higher in summer than in winter, that of epithermal neutrons about 80%, that of neutrons between 0.125 and 17.8 MeV about 32%, and that of neutrons above 17.8 MeV about 4%, respectively. The period of the observed oscillations was very close to one year. Similar oscillations were observed at the Koldewey station, with somewhat smaller amplitudes (40%, 45%, 22%, and 2%, respectively). At both stations, the flux of the neutrons above 17.8 MeV increased with time similar to the count rates measured by nearby neutron monitors. While this increase reflects changes in the Sun's activity, the observed oscillations are due to changes in ground albedo neutrons and their absorption due to snow. Consequently, the monthly averaged neutron ambient dose equivalent rates, H*(10), oscillated by about +/- 7% at the UFS and about +/- 4% at the Koldewey Station. The results demonstrate that BSS measurements could be used to monitor secondary neutrons from cosmic radiation above about 20 MeV. Below detailed neutron transport calculations are necessary to correct for changes in ground albedo neutrons and snow cover. The data presented here can be used as an experimental basis to perform such simulations.