MEASUREMENTS AND MONTE CARLO SIMULATIONS OF THE SPECTRAL VARIATIONS OF THE COSMIC-RAY-INDUCED NEUTRONS AT THE PIC DU MIDI OVER A 2-Y PERIOD

摘要

In this paper, a Bonner Sphere Spectrometer extended to high energies (HERMEIS) was employed to measure continuously the cosmic-ray-induced neutron spectra over a long-term period (2 y) at mountain altitude and medium geomagnetic latitude (Pic du Midi de Bigorre in the French Pyrenees, + 2885 m, 5.6 G V). The results showed 1-y sinusoidal oscillations in the integrated fluence rates. The amplitude of these oscillations depends on the neutron energetic domain. The fluence rate of thermal neutrons was 53 % higher in August than that in February. Those of epithermal neutrons with energies between 0.4 eV and 0.1 MeV and evaporation neutrons (from 0.1 to 20 MeV) were ～25 % higher in the summer than those in the winter. Finally, the cascade neutron fluence rate (＞20 MeV) remained quite the same (＜10 % variation). To understand the effects of local and seasonal changes in the measurement environment, GEANT4 simulations were performed. The nature of rock and thickness of the snow cover during the winter period (given by meteorological data) were investigated. A reasonable agreement between experiments and calculations was found.