The X-Ray/Gamma-ray spectrometer on the near Earth Asteroid Rendezvous mission

Abstract

The X-ray/Gamma-ray Spectrometer on the Near Earth Asteroid Rendezvous spacecraft is a remote sensing instrument designed to develop elemental composition maps of the surface of the asteroid 433 Eros. This mapping operation will occur during about 10 months of orbital operations commencing in early 1999. Solar excited x-ray fluorescence in the 1 to 10 keV range will be used to measure the surface abundances of Mg, Al, Si, Ca, Ti, and Fe with spatial resolutions down to 2 km. Characteristic gamma-ray emissions in the 0.1 to 10 Mev range will be used to measure cosmic-ray excited elements O, Si, Fe, H and naturally radioactive elements K, Th, U to a depth of order 10 cm. The asteroid pointing portion of the x-ray spectrometer consists of three gas-filled proportional counters with an energy resolution of 850 eV full-width at half-maximum (FWHM) @ 5.9 keV. Two sunward looking x-ray detectors monitor the incident solar flux, one of which is the first flight of a new, miniature Si-PIN solid-state detector which achieves 600 eV FWHM @ 5.9 keV. The gamma-ray spectrometer consists of a NaI(Tl) scintillator situated in an active Bismuth Germanate (BGO) cup shield which confines the field of view, eliminates the need for a costly boom, and enables recovery of lost events in the central detector. The NaI(Tl) and BGO detectors achieve energy resolutions of 8.7% and 14% FWHM @ 0.662 MeV, respectively. A data processing unit using an RTX2010 provides the spacecraft interface and produces 256-channel spectra for x-ray detectors and 1024-channel spectra for integral, coincident, and anticoincident gamma-ray modes. We present an overview of the instrument and discuss design parameters and trade-offs.