Use of a commercial neutron counting module to validate neutron-particle transport code simulation of a new cosmic radiation neutron monitor design

Abstract

This paper reports on one of several experiments used to validate MCNP models and processes employed to define a cosmic radiation (CR) neutron source and benchmark to optimise a new ground level neutron monitor (NM) design. The new NM design aims to be more compact, non-toxic and capable of producing comparable counting efficiencies to the NM-64; a monitor standardised in 1964 and which makes up most of the established global network. A broad-energy neutron monitor, the N50L, originally designed for nuclear material accountancy and safeguards, is used to provide primary data in support of our new NM design. Ambient count rate data from the N50L were acquired from it bare and inside a lead (Pb) sarcophagus to boost the neutron signal. Measured data were compared with calculations made using a Monte Carlo N-Particle (MCNP) transport code model of the setup. The calculated compared to the measured count rate for the monitor inside the Pb sarcophagus are in good agreement with a ratio close to one. However, overpredictions (by almost a factor of two) of the calculated rate for the bare monitor suggest inadequacies of the model for high-energy physics. Pressure corrected count rate data were also compared with data from several existing NM-64s over the same period. Our data are in good agreement with variations observed by these existing monitors on the global network. The N50L uses helium-3 (3He) counters and roughly equates to about 1/17th of the proposed final NM design in terms of detector volume. Scaled count rates from the N50L are within 73% of the rates derived from an NM-64 for a similar latitude, geomagnetic cutoff rigidity and altitude, and differs by <8% of the MCNP simulated response of the proposed design.