Click image for larger TIF file: 883K
Click image for larger TIF file: 883K
The X-ray/Gamma-Ray Spectrometer (XGRS) is really two instruments, whose purpose is to measure and map the abundances of key rock-forming elements on Eros:
The X-ray spectrometer (XRS) measures X-ray fluorescence of the asteroid stimulated by solar flares. Three gas-filled radiation counters are used, to resolve the nearly overlapping emissions from the key elements magnesium (Mg), silicon (Si) and aluminum (Al). One detector is bare, and the other two have different filters to block selective X-ray wavelengths higher than characteristic cutoff "edges." Combined, the outputs of all three detectors allow discrimination of these elements as well as measurement of calcium (Ca), iron (Fe), sulfur (S), and titanium (Ti). Two solar monitors, on the top deck of the spacecraft, face the sun and track variations in the solar radiation that causes the asteroid's X-ray fluorescence. One of the solar monitors is another gas-filled radiation counter. The second solar monitor is a new technology development, a solid state X-ray detector. The asteroid-facing gas-filled counters have a field of view of about 90 mrad (5°), or about 9 kilometers at a 100-kilometer range. The energy range measured by the X-ray spectrometer is 1000 to 10,000 electron volts (1 to 10 keV). A radioactive iron-55 source can be rotated into the field of view for inflight calibration. The gamma-ray spectrometer (GRS) detects gamma-ray fluoresence stimulated by cosmic rays, including emission "lines" from the elements oxygen (O), hydrogen (H), Si, and Fe. It also detects emissions from the naturally radioactive elements uranium (U), thorium (Th), and potassium (K). The sensor contains a sodium iodide (NaI) crystal sheathed in an open-ended tube of bismuth germanate (BGO). The open end of the sheath faces the asteroid and acts to restrict the field of view during a given measurement. Gamma rays penetrating both the NaI crystal and the BGO sheath cause emission of light which is detected by separate photomultiplier tubes. Light pulses seen during a very short period of time (0.5 microseconds) in both detectors are said to be in coincidence and are rejected as produced by cosmic rays traveling through both crystals. Light pulses seen only within the NaI crystal when nothing is seen in the BGO crystal correspond to lower-energy gamma-rays actually coming from the asrteroid and lying within the field of view of the detector. The gamma-ray spectrometer has a field of view of about 1.05 radians (60°), or 105 kilometers from a 100 kilometer range. It detects gamma rays in the energy range 0.3 million to 10 million electron volts (0.3 to 10 MeV).
MSI NIS NLR XGRS MAG RS
