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Gamma-Ray and Neutron Spectrometer (GRNS)

NSSDCA ID: 2004-030A-02

Mission Name: MESSENGER


The MESSENGER Gamma-Ray and Neutron Spectrometer (GRNS) is designed to return data on the elemental composition of Mercury's surface, particularly abundances of O, Si, S, Fe, H, K, Th, and U. This will allow production of global compositional maps. The prmary objectives of this experiment are to help characterize the chemical composition and geologic history of Mercury, and to determine the volatile inventory at the poles. A total of about 3.9 Gb of data return is expected over the one year nominal mission.

The experiment consists of two instruments, both mounted on the nadir facing instrument deck of MESSENGER. The Gamma-Ray Spectrometer subsystem measures discrete-line gamma ray emissions excited by galactic cosmic ray bombardment of elements (O, S, Si, Fe, and H) and given off by natural radioactive decay (K, Th, and U) to a depth of about 10 cm. The Gamma-Ray Spectrometer is a 31 cm high cylinder containing an active-shielded gamma-ray scintillator. The scintillator is mounted on a cup shaped 9.0 cm diameter x 9.5 cm, 1.25 cm thick active shield of bismuth germanate. The shield defines a 45 degree field of view and provides an anticoincidence veto for cosmic rays. The detector itself is a germanium semiconductor crystal with a cryo-cooler to maintain a temperature of -183 C. A photomultiplier tube sits below the shield. The energy range is 0.3 - 10 MeV, resolution is 8% full width half maximum at 0.662 MeV. Integration period is 300 s at periapsis, 1800 s at apoapsis.

The Neutron Spectrometer subsystem detects low energy neutrons produced by cosmic ray bombardment and moderated by collisions with hydrogen rich material within the upper 40 cm or so of the surface. The spectrometer consists of two paddle-shaped, 80 square cm x 6.35 cm, GS20 (6.6% lithium by weight) glass scintillators separated by two wedges of neutron absorbing BC454 (borated plastic) scintillator with a total area of 80 square cm. The scintillators are viewed by separate photomultiplier tubes. The GS20 scintillators count the thermal neutrons in the spacecraft ram and wake, the difference gives a measure of thermal neutron intensity near the planet. The BC454 scintillator measures epithermal and fast neutrons.

Alternate Names

  • GRNS

Facts in Brief

Mass: 13.1 kg
Power (avg): 4.5 W
Bit rate (avg): 0.122 kbps

Funding Agency

  • NASA-Office of Space Science Applications 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
Dr. Edward J. Rhodes, Jr.Experiment ScientistNASA Jet Propulsion
Dr. William V. BoyntonExperiment ScientistUniversity of
Dr. William V. BoyntonGeneral ContactUniversity of
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