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 polardeposits.

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 usedrecently on the Lunar Prospector and Mars Odyssey missions.

The NS sensor consists of three scintillators, each wrapped separately and coupled to separate photo-multiplier tubes. The three scintillators are sensitive to neutrons of different energies: thermal neutrons (0.025 - 1 eV), epithermal neutrons (1 eV - 500 keV), and fast neutrons (500 keV - 7 MeV). The first and third scintillators are lithium (6Li)-glass scintillators (LG1 and LG2) which respond to a combination of thermal and epithermal neutrons. The middle scintillator is a borated plastic (BP) scintillator that responds only to epithermal and fast neutrons due to its electronics setup. The interaction of galactic cosmic rays (GCR) with the surface of Mercury produces neutrons, some of which escape the surface and produce a neutron signal that can be measured by the orbiting NS sensor. The measured neutron energy spectrum reflects the transport properties of the surface composition and is sensitive to depths down to about 1 m. Thermal neutrons are sensitive to a variety of elements, including Fe, Ti, Gd, Sm, Cl, and C. Epithermal neutrons are mostly sensitive to H abundance. Fast neutrons can provide a good measure of average atomic mass. For the MESSENGER mission, the NS sensor will establish and map the abundance of H over most of the northern hemisphere of Mercury, providing significant new information regarding the potential presence of water ice within and nearpermanently shaded craters near the north pole.

The GRS detector is described in the catalog file GRS_INST.CAT in the GRS archive. Both instruments are described in full detail in[GOLDSTENETAL2007].