Neutron production in the lunar subsurface from alpha particles in galactic cosmic rays

Abstract

The neutron production from alpha particles in galactic cosmic rays (GCR) in the lunar subsurface has not been estimated with reliable precision despite its importance for lunar nuclear spectroscopy and space dosimetry. Here, we report our estimation of neutron production from GCR nuclei (protons and alpha particles) with the Particle and Heavy Ion Transport code System (PHITS), which includes several heavy ion interaction models. PHITS simulations of the equilibrium neutron density profiles in the lunar subsurface are compared with experimental data obtained in the Apollo 17 Lunar Neutron Probe Experiment. Our calculations successfully reproduced the data within an experimental error of 15%. Our estimation of neutron production from GCR nuclei, estimated by scaling that from protons by a factor of 1.27, is in good agreement within an error of 1% with the calculations using two different alpha particle interaction models in PHITS during a period of average activity of the solar cycle. However, we show that the factor depends on the incident GCR spectrum model used in the simulation. Therefore, we conclude that the use of heavy ion interaction models is important for estimating neutron production in the lunar subsurface.

Benchmarking Geant4 for Simulating Galactic Cosmic Ray Interactions Within Planetary Bodies

“…The epithermal neutron flux is also much larger than the thermal and fast neutron fluxes (note the two scales in Figure ). These trends are similar to what has been simulated in previous benchmarking to Apollo 17 LNPE data using MCNPX (McKinney et al, ) and PHITS (Ota et al, ).…”

“…Both of these choices can lead to 20% differences in the simulated results, and there is good agreement with the LNPE data for certain physics models when the LNPE drill core material is used. The radiation transport code PHITS has also been benchmarked against the LNPE data (Ota et al, ) and shows reasonable agreement. Geant4 has been used in a calculation of cosmogenic nuclide production in the Moon (Li et al, ); however, no extensive benchmarking of Geant4 physics models to validate its use for this type of application has been performed.…”

Measurement of the high-energy gamma-ray emission from the Moon with the Fermi Large Area Telescope

“…In both these models, the lunar surface composition is assumed to be independent of depth. Recently, another lunar surface model, based on the neutron and gamma-ray data from the Lunar Prospector mission, was proposed by Ota et al [ 37 ], in which the regolith composition and density are assumed to change with depth. In particular, in the Ota model, the lunar surface is described as a stack of four different layers, each with different thicknesses, compositions, and densities (the details of this model are given in Table I of Ref.…”

Fermi LAT observations of high energy gamma rays from the Moon

“…The choice of this composition is the result of ana analysis of the lunar gamma-ray emission data in the periods when direct measurements of the primary CR proton spectrum were taken by the PAMELA [3] (2008-09) and AMS [4] (May 2011-November 2013) experiments. We tested several composition models available in literature ( [16,18,17]) and we noticed that, to reproduce the measured lunar gamma-ray fluxes starting from the primary CR proton intensity spectra, a lighter composition of the lunar surface with respect to those models should be assumed. Hence we developed a custom composition model that differs from the models in refs.…”

“…Hence we developed a custom composition model that differs from the models in refs. [16,18,17] because the fractions of light oxides -mainly SiO 2 -are slightly higher (and, conversely, the fractions of heavy oxides -mainly FeO -are slightly lower) [19].…”