Anisotropy of the Albedo Radiation Environment on the Lunar Surface from GEANT4 Simulations

We use the Geant4 Monte Carlo code to study the angular dependences of different radiation particles on the lunar surface in the 1977 solar minimum galactic-cosmic-ray environment when there is no habitat. In particular, we study the anisotropy of albedo particles on the lunar surface. We find that albedo particles are in general not isotropic in the upper hemisphere, and for neutrons or photons the deviations from isotropy at lower energies have opposite signs as those at higher energies. In terms of fluence rates, deviations from the corresponding isotropic fluence rate, i.e., the rate if the particles were isotropic in a hemisphere, range from -8% for albedo neutrons up to +58% for albedo protons. Results on other albedo particles such as electrons, positrons, photons, and charged pions are also presented.