Measurements of cosmic ray induced background neutrons near the ground using a Bonner sphere spectrometer

Abstract

This report documents measurements of cosmic ray induced background neutrons using a new Bonner sphere spectrometer (BSS) based on a 3 He thermal neutron counter (SP9 type). Three spheres with a lead or copper layer were added to a BSS to enhance its sensitivity in the high-energy range above 20 MeV and enable the spectrometer to cover the entire energy range from thermal to multi-GeV. The response function of the spectrometer was calculated using the Monte Carlo code MCNP6. Below 20 MeV, the responses of polyethylene spheres were experimentally verified with 6 mono-energetic neutron sources . A neutron spectrum was measured near the ground at one location in Beijing. Also deduced from the spectrum were the rates of neutron fluence , ambient dose equivalent, and effective dose. • A Bonner sphere spectrometer with three extended spheres was employed to measure cosmic ray induced background neutrons at Beijing. • The MCNP6 code was used for the response function calculation. • Each count rate of the BSS was obtained after neutron flux variation correction. • A neutron spectrum was unfolded, and the deduced flux and dose rate values were also compared with other works.