The Lunar Radiation Environment: LRO/CRaTER Observations and Geant4 Modeling

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) has been in orbit around the moon aboard the Lunar Reconnaissance Orbiter (LRO) for over a year. The purpose of CRaTER is to measure the radiation environment that will be experienced, in particular, by astronauts on and near the lunar surface; to that end, CRaTER consists of a "telescope" of six silicon solid state detectors arranged in three pairs, with two large blocks of Tissue-Equivalent Plastic between pairs to represent the shielding provided by the human body. The data we have collected to date are complex, and to understand our observations we have performed extensive modeling of the "albedo" particles produced by interactions of primary cosmic rays with the lunar surface and with the spacecraft itself, and of the response of the sensor to both primary and albedo particles. We will present measurements of the Linear Energy Transfer (LET) and dose from CRaTER, and will show more generic LET and dose spectra, using our models to remove the effects specific to the CRaTER sensor geometry and spacecraft environment (shielding, locally-produced albedo), for lunar orbit and at the lunar surface.