MESSENGER Neutron Spectrometer (NS) Instrument Description

Extracted from PDS3 Data Set Catalog File NS_INST..CAT, PDS3 Data Set ID
MESS-E/V/H-GRNS-2-NS-RAWDATA-V1.0, Volume MESSNS_1001, last updated 
2007-12-11.

Instrument Information
======================

  Instrument Name: Neutron Spectrometer
  Instrument Type: Neutron Spectrometer

  Instrument Description
  ======================

   The Mercury Surface, Space Environment, Geochemistry and Ranging
   (MESSENGER) mission orbited Mercury following one Earth flyby, two flybys
   of Venus and three of Mercury. It launched in August 2004 and used these
   flybys to achieve an orbit insertion around Mercury in March 2011.
   Initial data collection began during the three flybys of Mercury, and
   primarily consisted of global mapping and measurements of the surface,
   atmosphere and magnetosphere composition. MESSENGER made a series of
   observations using multiple instruments. These observations provided 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 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 (LiG) which respond to a combination of thermal and
   epithermal neutrons. The middle scintillator is a borated plastic (BP)
   scientillator that responds only to epithermal and fast neurtons due to
   its electronics setup. The interaction of galactic cosmic rays 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 near permanently 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
   [Goldstein et al., 2007).

Reference
=========

   Goldsten, J.O., E.A. Rhodes,, W.V. Boynton, W.C. Feldman, D.J. Lawrence, J.I.
   Trombka, D.M. Smith, L.G. Evans, J. White, N.W. Madden, P.C. Berg, G.A.
   Murphy, R.S. Gurnee, K. Strohbehn, B.D. Williams, E.D. Schaefer, C.A. Monaco,
   C.P. Cork, J.D. Eckels, W.O. Miller, M.T. Burks, L.B. Hagler, S.J. Deteresa,
   and M.C. Witte, The MESSENGER Gamma-Ray and Neutron Spectrometer, Space
   Science Reviews, 131, 339-391, 2007.