THE X-RAY GAMMA-RAY SPECTROMETER ON THE NEAR-EARTH ASTEROID RENDEZVOUS MISSION/

An X-ray/gamma-ray spectrometer has been developed as part of a rendez vous mission with the near-Earth asteroid, 433 Eros, in an effort to a nswer fundamental questions about the nature and origin of asteroids a nd comets. During about 10 months of orbital operations commencing in early 1999, the X-ray/Gamma-ray Spectrometer will develop global maps of the elemental composition of the surface of Eros. The instrument re motely senses characteristic X-ray and gamma-ray emissions to determin e composition. Solar excited X-ray fluorescence in the 1 to 10 keV ran ge will be used to measure the surface abundances of Mg, Al, Si, Ca, T i, and Fe with spatial resolutions down to 2 km. Gamma-ray emissions i n the 0.1 to 10 MeV range will be used to measure cosmic-ray excited e lements O, Si, Fe, H and naturally radioactive elements K, Th, U to su rface depths on the order of 10 cm. The X-ray spectrometer consists of three gas filled proportional counters with a collimated field of vie w of 5 degrees and an energy resolution of 850 eV @ 5.9 keV. Two sunwa rd looking X-ray detectors monitor the incident solar flux, one of whi ch is the first flight of a new, miniature solid-state detector which achieves 600 eV resolution @ 5.9 keV. The gamma-ray spectrometer consi sts of a NaI(Tl) scintillator situated within a Bismuth Germanate (BGO ) cup, which provides both active and passive shielding to confine the held of view and eliminate the need for a massive and costly boom. Ne w coincidence techniques enable recovery of single and double escape e vents in the central detector. The NaI(Tl) and BGO detectors achieve e nergy resolutions of 8.7% and 14%, respectively @ 0.662 MeV. A data pr ocessing unit based on an RTX2010 microprocessor provides the spacecra ft interface and produces 256-channel spectra for X-ray detectors and 1024-channel spectra for the raw, coincident, and anti-coincident gamm a-ray modes. This paper presents a detailed overview of the X-ray/Gamm a-ray Spectrometer and describes the science objectives, measurement o bjectives, instrument design, and shows some results from early in-fli ght data.