[FPSPACE] The Gamma-ray Albedo of the Moon

[LARRY KLAES]

arXiv:0708.2742

Date: Tue, 21 Aug 2007 00:55:58 GMT   (136kb)

Title: The Gamma-ray Albedo of the Moon

Authors: Igor V. Moskalenko (Stanford), Troy A. Porter (UCSC)

Categories: astro-ph

Comments: 6 pages, 4 figures, emulateapj.cls; to appear in the Astrophysical
  Journal

  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.

\\ ( http://arxiv.org/abs/0708.2742 ,  136kb)