Lunar Albedo Protons and the Hunt for Subsurface Hydrogen

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) has been monitoring the presence of galactic cosmic rays (GCR) in the local interplanetary space environment since the launch of the Lunar Reconnaissance Orbiter (LRO) in 2009. CRaTER is a charged-particle linear energy transfer (LET) spectrometer capable of observing the primary GCR spectrum, as well as lunar surface albedo radiation. When the highly-energetic and heavy-charged particles comprising the GCR spectrum pass into the lunar bulk, a population of albedo protons is created via nuclear interactions within the regolith matter. These nuclear interactions include both elastic and inelastic processes, and the resulting albedo proton spectrum is a function of the incident GCR properties as well as the composition of the lunar regolith. Therefore, variations in lunar regolith composition are manifested as changes in the albedo proton spectrum observed by the CRaTER instrument [Schwadron et al. 2016, 2018]. In this work, we investigate how the presence and absence of subsurface hydrogen is expected to affect the albedo proton spectrum using the MCNP6 general purpose radiation transport code. Furthermore, we discuss how the observable population of albedo protons changes as a function of LRO's altitude, orientation, and detector coincidence.

Schwadron, N. A., Wilson, J. K., Looper, M. D., Jordan, A. P., Spence, H. E., Blake, J. B., Case, A. W., Iwata, Y., Kasper, J. C., Farrell, W. M., Lawrence, D. J., Livadiotis, G., Mazur, J., Petro, N., Pieters, C., Robinson, M. S., Smith, S., Townsend, L. W., and Zeitlin, C., Signatures of volatiles in the lunar proton albedo, Icarus, 273, 25, 2016

Schwadron, N. A., Wilson, J. K., Jordan, A. P., Looper, M. D., Zeitlin, C., Townsend, L. W., Spence, H. E., Legere, J., Bloser, P., Farrell, W. M., Hurley, D., Petro, N., Stubbs, T. J., and Pieters, C., Using proton radiation from the moon to search for diurnal variation of regolith hydrogenation, Planetary and Space Science, 162, 113, 2018