Monte Carlo simulation of the gamma ray spectrometer performance on Lunar Prospector

We present results of a simulation for the GRS instrument response expected in lunar orbit, using 90 spectral lines and a continuum gamma ray background taken from Apollo 15 and 16 data. The Monte Carlo program uses the exact dimensions and composition of the GRS in order to most accurately predict spectral performance, assuming an operating temperature on orbit of -30°C. We expect to obtain a resolution at 662 keV @ -30°C of about 9.6% FWHM. The data set is expected to be 2.5 to 8 times better in terms of peak intensity than the NaI data from Apollo orbiters. This factor depends on the gamma ray energy and is due to the high-Z BGO detector used in the GRS. Both coincidence and anticoincidence data will be returned, and a linear combination of the two spectra will be determined for best peak to background ratios