LUNAIRE - LUNAr Ionising Radiation Environment

Characterizing the radiation environment on the lunar surface is essential for a safe human and robotic exploration. Having a negligible atmosphere, the Moon is exposed to galactic cosmic rays (GCRs), a continuous high flux of very energetic particles, and solar energetic particles (SEPs), which are accelerated in the solar corona or in coronal mass ejections. These particles can damage biological, electronic systems and other materials and thus hinder or even terminate space missions.

To aid future mission planning and habitat design, we developed a Geant4 based model, LUNAIRE, that simulates GCR and SEP propagation through the lunar surface. The model accounts for secondary particle generation on the sub-surface and derives physical quantities such as absorbed dose and Linear Energy Transfer (LET) spectrum at the surface and underground. This model was adapted from the detailed Mars Energetic Radiation Environment (dMEREM) developed by LIP (Laboratory of Instrumentation and Experimental Particle Physics) for ESA (European Space Agency), and includes location dependent surface composition, as well as user custom particle spectra as inputs.

We validated the model by comparing the LET (Linear Energy Transfer) spectrum obtained with LUNAIRE to measurements of the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) aboard the Lunar Reconnaissance Orbiter (LRO). Additionally, we compared the spectrum of secondary particles production with those of the HZETRN (High Charge and Energy Transport) code available through OLTARIS (On-Line Tool for the Assessment of Radiation in Space).

 The results allow for a reconstructed GCR spectra that matches BadhwarO'Neill (BON) Galactic Cosmic Ray Model reference curves across species. We found the LET spectrum to be in good agreement with CRaTER data for both July 2009 (solar minimum available) and July 2015 (solar maximum available). Secondary particle fluxes also match HZETRN results for neutrons and protons but are not so according for electrons and gamma particles. This was attributed to differences in the physics processes of HZETRN comparing to Geant4.

 These results show that LUNAIRE accurately characterizes the lunar radiation environment that can lead to better forecasts of, and safer missions. Ongoing work includes the evaluation against SEP events, the incorporation of complex topography geometries and validation against other mission results.