Monte-Carlo simulation of the response of bare neutron counters at the south pole to vertical secondary particles from cosmic rays

Abstract

Neutron monitors (NM64) are standard ground-based detectors that measure the flux of primary cosmic rays at GeV energies in space by counting secondary particles (mostly neutrons) from atmosphere cascades. The atmospheric neutrons are detected by induced nuclear fission in a gas proportional counter. In the standard design, there is a lead ring to generate evaporation neutrons that are moderated by polyethylene before being detected in the 10BF3 gas counter. By omitting the lead, so called “bare counters” respond to lower energy particles on average and can be used in conjunction with NM64 to estimate the energy spectrum of the primary cosmic rays. The specific objective of this research is to refine the understanding of the lead-free neutron monitor now installed at the South Pole using Monte-Carlo FLUKA simulations. This design uses paraffin and wood to moderate high-energy neutrons and detects them with 3He gas-filled proportional counters. Latitude surveys have shown that they have different detection efficiency from either the NM64 or polyethylene moderated bare counters, but they have never been adequately modelled. Understanding the differences quantitatively is the goal of this work. We will also report the detection efficiency of the paraffin bare for other atmospheric particles.