The Gamma-ray Albedo of the Moon

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.

arXiv comment6 pages, 4 figures, emulateapj.cls; to appear in the Astrophysical Journal

References in corpus (6)

• PAMELA - A Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics
• Dissipation of Magnetohydrodynamic Waves on Energetic Particles: Impact on Interstellar Turbulence and Cosmic Ray Transport
• Precise Measurement of Cosmic-Ray Proton and Helium Spectra with the BESS Spectrometer
• GLAST: Understanding the High Energy Gamma-Ray Sky
• Geant4 hadronic physics status and validation for large HEP detectors
• Extended solar emission - an analysis of the EGRET data

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• Fermi Large Area Telescope Measurements of the Diffuse Gamma-Ray Emission at Intermediate Galactic Latitudes
• Gamma-ray emission from the solar halo and disk: a study with EGRET data
• Fermi Gamma-ray Space Telescope: High-Energy Results from the First Year
• The Positron Puzzle
• Isotropic Gamma-Ray Background: Cosmic-Ray Induced Albedo from Debris in the Solar System?
• Measurement of the high-energy gamma-ray emission from the Moon with the Fermi Large Area Telescope
• A celestial gamma-ray foreground due to the albedo of small solar system bodies and a remote probe of the interstellar cosmic ray spectrum
• Yet Another Sunshine Mystery: Unexpected Asymmetry in GeV Emission from the Solar Disk
• Possible detection of gamma rays from Epsilon Eridani
• Detecting unresolved moving sources in a diffuse background
• Lunar neutrinos
• Angular Distribution of Gamma Rays Produced in Proton-Proton Collisions
• Lunar eclipse induces disturbance in the lunar exosphere