Neutron passive counts from DAN used to infer galactic cosmic ray environment at the surface of Mars

The neutron environment in planetary bodies is generated by interactions of highly energetic particles with the nuclei in the atmosphere, and/or the planetary surface. The Dynamic Albedo Neutron (DAN) instrument onboard Mars Science Laboratory (MSL) is capable of detecting thermal and epithermal neutrons in two modes, active and passive mode. The active mode uses a pulsed neutron generator (PNG) to study the geological characteristics while the passive mode (i.e., no pulsing) measures the background neutron environment. In addition to DAN, there are two other radiation-measuring instruments currently operating at Mars: MSL Radiation Assessment Detector (RAD) designed to characterize the dose exposures due to charged particles, gamma rays, and neutrons on the surface and Mars Odyssey High Energy Neutron Detector (HEND) which detects a wide energy range of neutron from orbit .

Previous efforts to correlate the DAN/HEND/RAD data were done to identify neutron count enhancements on orbit and at the surface of Mars simultaneously during solar particle events. No direct relationship was observed from this initial study because the strong dependence of DAN passive data to the regolith composition and multi-mission radioisotope thermoelectric generator (MMRTG) neutron flux and because of different spatial coverage between HEND, DAN and RAD.

Here we investigate the feasibility of using DAN data to understand the galactic cosmic ray (GCR)-induced neutron environment at the Mars surface. A series of Monte Carlo simulations were performed to de-couple the MMRTG and regolith composition dependences from the GCR-induced neutron environment for several MSL locations. In other words, at each location, a Monte Carlo simulation was performed using the composition estimated from the DAN active measurements at that location and from the MMRTG neutron source spectrum. The subtraction of the simulated MMRTG-only neutron counts from the "measured" DAN total count gives an estimate of GCR-only neutron counts. This can be compared directly with measurements of the GCR environment from RAD data [6]. Initial results indicate that the DAN passive data could be used to evaluate the long-term GCR-induced neutron environment. More thorough study is being performed and results will be presented in the final paper.