X-ray/Gamma-Ray Spectrometer

Instruments

MSI NIS NLR XGRS MAG RS

X-ray/Gamma-Ray Spectrometer = XGRS

XGRS Science Team
Educational Activities

XGRS fact sheet (61K PDF)
APL Tech Digest (1557K PDF)

The X-ray/Gamma-ray Spectrometer (XGRS) will develop global maps of the elemental composition of the surface of Eros. The instrument remotely senses characteristic x-ray and gamma-ray emissions to determine composition. 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. Gamma-ray emissions in the 0.3 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 surface depths on the order of 10 cm.

The X-ray/Gamma-Ray Spectrometer is really two instruments. The x-ray spectrometer measures x-ray florescence on the asteroid excited by solar flare x-rays using three gas-filled proportional counters: one bare, one with a Mg filter and one with an Al filter. Two solar monitors are mounted on the top deck of the spacecraft,facing away from the asteroid surface. One solar monitor is another gas-filled proportional counter. The second solar monitor is a new technology development: a high resolution, solid state Si detector with intrinsic thermo-electric cooler. The gas-filled proportional counters, collimated to a 5 degree FOV, have an energy resolution of about 0.84 keV Full Width at Half Maximum (FWHM) at 5.9 keV. The high resolution solid state solar monitor has a FWHM of about 0.60 keV, also at 5.9 keV. Risetime discrimination is used to filter out cosmic rays, energetic particles, and other sources of background noise. Throughput is at least 10 kHz. An Fe-55 calibration source can be used for inflight calibration of this instrument.

The gamma-ray spectrometer is a NaI scintillator with an active BGO shield. The scintillator and shield use separate photomultiplier tubes. The instrument measures gamma-rays from the asteroid surface in the energy range 0.3 to 10 MeV in approximately 10 keV steps and has a FOV of about 56 degrees. The NaI(Tl) and BGO detectors achieve energy resolutions of 8.7% and 14% FWHM, respectively at 0.662 MeV. Active anti-coincidence is used to reduce the high energy cosmic-ray background and other sources of interference. New coincidence techniques enable recovery of single and double escape events in the central detector. A unique feature of both the x-ray and gamma-ray spectrometers is the high accuracy high voltage control electronics that maintains about 0.5 V stability of the high voltage (1500 V) power supplies over the full temperature range.