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Lunar Prospector Gamma Ray Spectrometer (GRS)

NSSDCA ID: 1998-001A-01

Mission Name: Lunar Prospector
Principal Investigator:Mr. G. Scott Hubbard


The fundamental purpose of the Gamma Ray Spectrometer (GRS) experiment will be to provide global maps of elemental abundances on the lunar surface. The GRS is designed to record the spectrum of gamma rays emitted by: 1) the radioactive decay of elements contained in the Moon's crust; and 2) elements in the crust bombarded by cosmic rays and solar wind particles. The most important elements detectable by the GRS are uranium (U), thorium (Th), and potassium (K), radioactive elements which generate gamma rays spontaneously, and iron (Fe), titanium (Ti), oxygen (O), silicon (Si), aluminum (Al), magnesium (Mg), and calcium (Ca), elements which emit gamma rays when hit by cosmic rays or solar wind particles. The uranium, thorium, and potassium in particular will be used to map the location of KREEP (potassium, rare-earth element, and phosphorus containing material, which is believed to have developed late in the formation of the crust and upper mantle, and is therefore important to understanding lunar evolution.) The GRS is also capable of detecting fast (epithermal) neutrons, which will complement the neutron spectrometer in the search for water on the Moon.

The Gamma Ray Spectrometer is a small cylinder which is mounted on the end of one of the three 2.5 m radial booms extending from the Lunar Prospector. It consists of a cooled 7.1 cm diameter by 7.6 cm long bismuth germanate (BGO) crystal surrounded on all but one side by a 12 cm diameter, 20 cm long anticoincidence shield of borated plastic (BC454). Gamma rays striking the bismuth atoms produce a flash of light with an intensity proportional to the energy of the gamma ray. The energy of the gamma ray is associated with the element responsible for its emission. Photomultiplier tubes are mounted at each end of the cylinder, one against the bismuth germanate crystal and one against the anticoincidence shield. These are enclosed within a cylindrical (16.7 cm diameter, 55.4 cm length) graphite-epoxy laminate housing. The anticoincidence shield serves to detect penetrating charged particle events so they can be eliminated from the gamma-ray detections. It can also detect thermal and epithermal neutron signatures in concert with the bismuth germanate crystal. The GRS front end electronics is designed to digitize, record, and discriminate between the various events. Thermal control to a temperature of -28 degree C +/- 2 degrees was attempted using multilayered insulation blankets and a heater. Over the course of the experiment, however, the temperatures reached as high as -11 degrees C.

The GRS sensor can detect gamma-rays with energies between 0.3 and 9 MeV in channels 17.6 keV wide. Accumulation time is 32 seconds, corresponding to a ground track of 50 km. Due to a low signal to noise ratio, multiple passes were required to generate statistically significant results. At nine passes per month, it takes about three months to confidently estimate abundances of thorium, potassium, and uranium, and 12 months for the other elements. The precision varies according to element measured. For U, Th, and K, the precision is 7% to 15%, for Fe 45%, for Ti 20%, and for the overall distribution of KREEP 15% to 30%. The borated plastic shield is used in the detection of fast neutrons. The GRS will achieve global coverage from an altitude of approximately 100 km and with a surface resolution of 150 km.

Alternate Names

  • GRS
  • LunarProspector/GRS

Facts in Brief

Mass: 8.6 kg
Power (avg): 3 W
Bit rate (avg): 0.688 kbps

Funding Agency

  • National Aeronautics and Space Administration (United States)


  • Planetary Science: Geology and Geophysics

Additional Information

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



NameRoleOriginal AffiliationE-mail
Mr. G. Scott HubbardPrincipal InvestigatorNASA Ames Research

Selected References

  • Lawrence, D. J., et al., Global elemental maps of the moon: The Lunar Prospector Gamma-Ray Spectrometer, Science, 281, No. 5382, 1484-1489, Sept. 1998.
  • Maurice, S., et al., Reduction of neutron data from Lunar Prospector, J. Geophys. Res., 109, E07S04, doi:10.1029/2003JE002208, 2004.
  • Lawrence, D. J., et al., Gamma-ray measurements from Lunar Prospector: Time series data reduction for the Gamma-Ray Spectrometer, J. Geophys. Res., 109, E07S05, doi:10.1029/2003JE002206, 2004.
  • Feldman, W. C., et al., Gamma-Ray, Neutron, and Alpha-Particle Spectrometers for the Lunar Prospector mission, J. Geophys. Res., 109, E07S06, doi:10.1029/2003JE002207, 2004.
  • Prettyman, T. H., et al., Elemental composition of the lunar surface: Analysis of gamma ray spectroscopy data from Lunar Prospector, J. Geophys. Res., 111, E12007, doi:10.1029/2005JE002656, 2006.
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