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

This report documents measurements of cosmic ray induced background neutrons using a new Bonner sphere spectrometer (BSS) based on a 3He 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.