Gamma-ray albedo of the moon

TL;DR

This paper models the Moon's gamma-ray albedo using GEANT4, confirming it as a standard candle for gamma-ray telescopes and proposing its use to monitor cosmic ray spectra over solar cycles.

Contribution

It provides a detailed GEANT4-based calculation of the Moon's gamma-ray albedo, aligning with observational data and highlighting its potential as a cosmic ray monitor.

Findings

The gamma-ray spectrum from the Moon has a steep cutoff around 4 GeV.

The Moon's albedo spectrum can serve as a calibration standard for gamma-ray telescopes.

Albedo flux varies with the solar cycle, reflecting changes in cosmic ray spectra.

Abstract

We use the GEANT4 Monte Carlo framework to calculate the gamma-ray albedo of the Moon due to interactions of cosmic ray (CR) nuclei with moon rock. Our calculation of the albedo spectrum agrees with the EGRET data. We show that the spectrum of gamma rays from the Moon is very steep with an effective cutoff around 4 GeV (600 MeV for the inner part of the Moon disc). Apart from other astrophysical sources, the albedo spectrum of the Moon is well understood, including its absolute normalisation; this makes it a useful "standard candle" for gamma-ray telescopes, such as the forthcoming Gamma Ray Large Area Space Telescope (GLAST). The albedo flux depends on the incident CR spectrum which changes over the solar cycle. Therefore, it is possible to monitor the CR spectrum using the albedo gamma-ray flux. Simultaneous measurements of CR proton and helium spectra by the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA), and observations of the albedo gamma rays by the GLAST Large Area Telescope (LAT), can be used to test the model predictions and will enable the GLAST LAT to monitor the CR spectrum near the Earth beyond the lifetime of PAMELA.