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NEAR X-ray/Gamma-ray Spectrometer (XRS-GRS)

NSSDCA ID: 1996-008A-02

Mission Name: NEAR Shoemaker
Principal Investigator:Dr. Jacob I. Trombka


The NEAR X-ray/Gamma-ray Spectrometer (XGRS) will be used to determine compositional properties of the asteroid. The experiment consists of two instruments, the X-Ray Fluorescence Spectrometer and the Gamma Ray Spectrometer. In addition, there are two X-ray solar monitors to determine the incident solar X-ray spectrum. The scientific objectives of this instrument are to provide a compositional map of the surface of asteroid 433 Eros to establish constraints on the origin and geologic history of the asteroid, and to determine what relationship, if any, Eros has to meteorite types collected on Earth. The maximum power dissipated by the instrument is 34 W.

The Gamma-Ray Spectrometer consists of a prime detector and shield detector mounted on the instrument deck. The prime detector is a 2.5 x 7.5 cm NaI (Tl) scintillator with a field of view of about 60 degrees and 8.5% FWHM minimum resolution at 662 keV. The shield detector is an 8.9 x 14 cm BGO scintillator cup with 15% FWHM minimum resolution at 662 keV. The Gamma-Ray Spectrometer can measure an energy range from 0.3 to 10 MeV in 10-keV channels. Gamma-rays are produced by naturally occurring radioactive elements such as K, Th, and U, and by the interaction of cosmic ray protons and energetic particles associate with solar flares with other elements near the surface such as Fe, Si, O, Mg, and H. They can be detected from material from the surface down to about 10 cm in depth.

The X-ray Flourescence Spectrometer detectors are three gas proportional counters collimated to 5 degrees field of view to determine X-ray line emissions from the asteroid. Balanced filters on two detectors (Al on one and Mg on the other) are used to separate Mg, Al, and Si lines and the Fe, S, Ti, and Ca lines can be resolved. The detectors have a 25 square cm active aperture area and a 25 micrometer beryllium window with a beryllium liner and window support. The detector energy range is 1 to 10 keV with rise-time discrimination to reduce background contamination. The energy resolution is better than 1 keV FWHM at 5.95 keV. Fe-55 is used as an in-flight calibration source. X-rays from the Sun produce flourescence X-rays from elements on the surface of Eros. The elements with low atomic number, such as Mg, Al, and Si, produce high count rates, while Fe, S, Ti, and Ca produce lower rates and the data take more time to accumulate. The X-rays are emitted from the top mm of surface material.

There two solar X-ray monitors are mounted on the antenna deck. One is a gas-filled proportional counterwith a pinhole active area and a graded shield, with a resolution of better than 1 keV FWHM at 5.95 keV. The other is a high-resolution solid state monitor with a resolution better than 600 eV FWHM at 5.95 keV.

Data from the XRGS experiment will allow production of surface abundance maps of Mg, Al, Si, Ca, Ti, Fe, S, K, U, and Th. For elements for which the X-ray and Gamma-ray results can be compared it will be possible to determine information on variation with depth.

Alternate Names

  • NEARShoemaker/XGRS
  • XGRS
  • urn:nasa:pds:context:instrument:grs.near
  • urn:nasa:pds:context:instrument:xrs.near

Facts in Brief

Mass: 26.9 kg
Power (avg): 24 W

Funding Agency

  • National Aeronautics and Space Administration (United States)


  • Planetary Science: Small Bodies

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Dr. Jacob I. TrombkaTeam LeaderNASA Goddard Space Flight Center
Dr. Steven W. SquyresTeam MemberCornell
Dr. William V. BoyntonTeam MemberUniversity of

Selected References

  • Goldsten, J. O., et al., The X-ray/Gamma-ray spectrometer on the Near Earth Asteroid Rendezvous mission, Space Sci. Rev., 82, No. 1-2, 169-216, doi:10.1016/S0094-5765(96)00146-4, 1997.
  • Trombka, J. I., et al., Compositional mapping with the NEAR X ray/gamma ray spectrometer, J. Geophys. Res., 102, No. E10, 23729-23750, doi:10.1029/97JE00270, Oct. 1997.
  • Goldsten, J. O., et al., The X-ray/Gamma-ray Spectrometer on the Near Earth Asteroid Rendezvous mission, Acta Astronaut., 39, No. 1-4, 281-291, doi:10.1016/S0094-5765(96)00146-4, 1996.
  • Starr, R., et al., Instrument calibrations and data analysis procedures for the NEAR X-ray spectrometer, Icarus, 147, No. 2, 498-519, Oct. 2000.
  • Evans, L. G., et al., Calibration of the NEAR Gamma-Ray spectrometer, Icarus, 148, No. 1, 95-117, Nov. 2000.
  • Trombka, J. I., et al., The NEAR-Shoemaker x-ray/gamma-ray spectrometer experiment: Overview and lessons learned, Meteor. Planet. Sci., 36, No. 12, 1605-1616, Dec. 2001.
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