[P-PS08] Lunar Science and Exploration

[PPS08-12] Probing Low-Energy Cosmic Ray Flux and Its Temporal Variations through Nuclear Interactions on the Lunar Surface

*Tatsuki Fujiwara¹, Ellis R. Owen^1,2, Yoshiyuki Inoue¹, Manel Errando³, Kohei Fukuda¹, Kazuhiro Nakazawa⁴, Hirokazu Odaka¹, Keigo Okuma⁴, Kentaro Terada¹, Naomi Tsuji⁵, Hiroki Yoneda⁶, Ao Zhang³ (1.Department of Earth and Space Science, Graduate School of Science, Osaka University, 2.Astrophysical Big Bang Laboratory (ABBL), RIKEN Cluster for Pioneering Research, 3.Washington University in St. Louis, 4.Nagoya University, 5.Kanagawa University, 6.Julius-Maximilians-Universität Würzburg)

Keywords:Cosmic ray, Moon, Nuclear interaction, Gamma-ray astronomy

The Moon is the closest celestial gamma-ray emitting object due to nuclear interactions between Galactic cosmic rays (CRs) and its surface material. While the lunar GeV gamma-ray emission is dominated by a continuum, the MeV emission exhibits both continuum and distinctive spectral lines. In particular, the characteristic spectral lines in the MeV range offer valuable constraints on the poorly understood Galactic MeV CRs. In the late 2020s to 2030s, next-generation MeV gamma-ray instruments, such as COSI and GRAMS, are expected to achieve sensitivities more than an order of magnitude better than previous instruments. This significant improvement in sensitivity makes it timely to predict lunar MeV gamma-ray emission in preparation for these upcoming observations. In this study, we model the lunar MeV-GeV gamma-ray spectrum using the Geant4 Monte Carlo particle simulations. Our results are consistent with the lunar GeV gamma-ray observations by the Fermi Gamma-ray Space Telescope, and predict that future MeV instruments will detect both the lunar MeV continuum and key spectral line features--such as from electron-positron annihilation, ²⁸Si de-excitation, and ²⁶Al radioactive decay. Observations of the lunar MeV gamma-ray spectrum will open a new window into both the current state and long-term evolution of the Galactic CRs in the solar-terrestrial environment.