Session 25 - Solar System.
Oral session, Monday, January 13
Piers 2/3,

## [25.02] EGRET Detection of Gamma Rays from the Moon

D. J. Thompson, D. L. Bertsch (NASA/GSFC), D. J. Morris (UNH), R. Mukherjee (NASA/GSFC/USRA)

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory has detected gamma rays from the Moon as it passed through the instrument field of view several times between 1991 and 1994. The average flux, (4.7 \pm 0.7) \times 10^-7 ph(>100 MeV)/cm^2s, and the energy spectrum of the lunar gamma radiation are consistent with a model of gamma ray production by cosmic ray interactions with the lunar surface, and the flux varies as expected with the solar cycle. Although the same processes may occur on the Sun, EGRET does not detect the quiet Sun. The upper limit, 3.0 \times 10^-7 ph(>100 MeV)/cm^2s, does not contradict calculations of the expected solar gamma-ray flux. Thus, in high-energy gamma rays, the Moon is brighter than the quiet Sun.