The Gamma-ray Albedo of the Moon

TL;DR

This study models the Moon's gamma-ray albedo using GEANT4, confirming agreement with observations, and highlights its potential for energy calibration and cosmic ray monitoring via gamma-ray measurements.

Contribution

It provides a detailed Monte Carlo simulation of lunar gamma-ray emission, including spectral features, and proposes its use as a standard candle and cosmic ray monitor.

Findings

The gamma-ray spectrum of the Moon has a steep cutoff around 3-4 GeV.

A narrow pion-decay line at 67.5 MeV is identified.

The lunar albedo can serve as a calibration source for gamma-ray telescopes.

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 3-4 GeV (600 MeV for the inner part of the Moon disk) and exhibits a narrow pion-decay line at 67.5 MeV, perhaps unique in astrophysics. 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. The steep albedo spectrum also provides a unique opportunity for energy calibration of gamma-ray telescopes, such as the forthcoming Gamma Ray Large Area Space Telescope (GLAST). Since the albedo flux depends on the incident CR spectrum which changes over the solar cycle, 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 LAT to monitor the CR spectrum near the Earth beyond the lifetime of the PAMELA.