PDS_VERSION_ID               = PDS3
LABEL_REVISION_NOTE          = "
         2007-08-01 MESS:mick   V01 First Draft / Example;
         2007-08-10 MESS:chabot V02 Added Content;
         2007-12-11 GEO:slavney Separated GRS and NS;
         2012-07-02 MESS:harshman Revised for GRS RDR review;
         2012-12-07 GEO:ward Minor formatting."

RECORD_TYPE                       = STREAM

OBJECT                         = INSTRUMENT
  INSTRUMENT_HOST_ID             = "MESS"
  INSTRUMENT_ID                  = "GRS"

  OBJECT                         = INSTRUMENT_INFORMATION
   INSTRUMENT_NAME                = "GAMMA RAY SPECTROMETER"
   INSTRUMENT_TYPE                = "GAMMA RAY SPECTROMETER"
   INSTRUMENT_DESC                = "

   The Mercury Surface, Space Environment, Geochemistry and Ranging
   (MESSENGER) mission is designed to orbit Mercury following one
   Earth flyby, two flybys of Venus and three of Mercury.  It
   launched in August 2004 and will use these flybys to achieve an
   orbit insertion around Mercury in March 2011.  Initial data
   collection will begin during the three flybys of Mercury, and will
   primarily consist of global mapping and measurements of the
   surface, atmosphere and magnetosphere composition.  MESSENGER will
   remain in orbit for the rest of the nominal mission, which is
   scheduled to end in March 2012. Once in orbit around Mercury it
   will begin a series of observations using multiple instruments.
   These observations will provide data to answer questions about the
   nature and composition of Mercury's crust, tectonic history, the
   structure of the atmosphere and magnetosphere, and the nature of
   the polar deposits.

   The Gamma-Ray and Neutron Spectrometer (GRNS) instrument forms
   part of the geochemistry investigation of the MESSENGER mission
   and will yield information about the elemental composition of the
   planet's surface. The GRNS package is composed of two independent
   sensors: the Gamma-Ray Spectrometer (GRS) and the Neutron
   Spectrometer (NS). GRS detects gamma-ray emissions in the 0.1 - 10
   MeV range, allowing the identification of certain elements and
   their abundances to be determined. NS measures the flux of ejected
   neutrons in three energy ranges and is particularly sensitive to
   the H content of a body. Taken together, the gamma-ray and neutron
   measurements will be used to infer the composition of Mercury's
   surface over localized regions using established techniques, such
   as used recently on the Lunar Prospector and Mars Odyssey
   missions.

   The GRS detector is a coaxial germanium crystal 50 mm in diameter 
   and 50 mm in length, chosen for its superior energy resolution and 
   the ability to anneal the detector to remove accumulated radiation 
   damage. The detector is rigidly clamped in a hermetically sealed Al 
   capsule pressurized with clean, dry nitrogen. The capsule is cooled 
   to an operating temperature in the 80-95 K range by a mechanical 
   cryocooler. A plastic scintillator anti-coincidence shield 
   surrounds the germanium detector in its sides and back, for 
   rejection of cosmic-ray background. Galactic cosmic rays 
   continuously bombard the surface of Mercury, and through 
   interactions with the surface, gamma rays of discrete energies that 
   are characteristic of specific elements are created. A fraction of 
   these gamma rays, as well as those from the decay of radiogenic 
   elements escape from the surface, where they can be detected by the 
   orbiting GRS. Gamma-ray fluxes are measurable at altitudes up to 
   1000 km and for gamma rays up to about 10 MeV that emanate from 
   depths of up to tens of centimeter beneath the surface. Detected 
   fluxes are generally low and require numerous orbital passes over 
   a specific region to obtain a statistically well-defined energy 
   spectrum. The measurements of elements such as Fe, Si, Mg, Na, Al, 
   Ca, Ti, K and Th by GRS will provide insight into distinguishing 
   between different formation models for Mercury as well as other 
   planetary evolution issues.

   The NS detector is described in the catalog file NS_INST.CAT in
   the NS archive. Both instruments are described in full detail in
   [GOLDSTENETAL2007]."

  END_OBJECT                     = INSTRUMENT_INFORMATION

  OBJECT                         = INSTRUMENT_REFERENCE_INFO
    REFERENCE_KEY_ID             = "GOLDSTENETAL2007"
  END_OBJECT                     = INSTRUMENT_REFERENCE_INFO

END_OBJECT                     = INSTRUMENT
END