The Moon as a Cosmic-Ray Spectrometer: Prospects for MeV Gamma-Ray Observations

Abstract

The Moon is the closest celestial gamma-ray emitting object. Its gamma-ray emission arises from interactions between Galactic cosmic rays (CRs) and the lunar surface. While the lunar GeV gamma-ray spectrum is dominated by a continuum from hadronic decay processes, the MeV emission exhibits both continuum and distinctive spectral lines from nuclear de-excitation and radioactive decay processes. Using Geant4 Monte Carlo particle simulations, we model the lunar gamma-ray spectrum. Our results demonstrate its consistency with Fermi-LAT observations, and predict that next-generation MeV gamma-ray instruments will detect both the lunar MeV continuum and several key spectral line features, notably the $1.779\mathrm{MeV}$ line from ${}^{28}\mathrm{Si}$ de-excitation enhanced by the lunar surface composition, the $e^{+}{e}^{-}$ annihilation line, and radioactive decay lines from ${}^{22}\mathrm{Na}$ ($\tau \approx 3.75\mathrm{yr}$) and long-lived ${}^{26}\mathrm{Al}$ ($\tau \approx 1\mathrm{Myr}$). These gamma-ray lines are sensitive to CRs with energies $\lesssim 1\mathrm{GeV}\mathrm{nu}{\mathrm{c}}^{-1}$, offering unique temporal probes of CR activity over different timescales. Observations of the lunar MeV gamma-ray spectrum will therefore open a new window to study the current irradiation of the solar-terrestrial environment by low-energy CRs and its long-term temporal evolution.

15 pages, 8 figures, 1 table, accepted for publication in ApJ