The MESSENGER Gamma-Ray and Neutron Spectrometer |
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| Authors: | John O Goldsten Edgar A Rhodes William V Boynton William C Feldman David J Lawrence Jacob I Trombka David M Smith Larry G Evans Jack White Norman W Madden Peter C Berg Graham A Murphy Reid S Gurnee Kim Strohbehn Bruce D Williams Edward D Schaefer Christopher A Monaco Christopher P Cork J Del Eckels Wayne O Miller Morgan T Burks Lisle B Hagler Steve J DeTeresa Monika C Witte |
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| Institution: | (1) The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA;(2) Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA;(3) Los Alamos National Laboratory, Los Alamos, NM 87545, USA;(4) NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA;(5) Department of Physics, University of California, Santa Cruz, CA 95064, USA;(6) Computer Sciences Corporation, Science Programs, Lanham-Seabrook, MD 20706, USA;(7) Scientific and Engineering Solutions, Inc., Chagrin Falls, OH 44023, USA;(8) Lawrence Livermore National Laboratory, Livermore, CA 94550, USA;(9) Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA |
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| Abstract: | A Gamma-Ray and Neutron Spectrometer (GRNS) instrument has been developed as part of the science payload for NASA’s Discovery
Program mission to the planet Mercury. Mercury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) launched
successfully in 2004 and will journey more than six years before entering Mercury orbit to begin a one-year investigation.
The GRNS instrument forms part of the geochemistry investigation and will yield maps of the elemental composition of the planet
surface. Major elements include H, O, Na, Mg, Si, Ca, Ti, Fe, K, and Th. The Gamma-Ray Spectrometer (GRS) portion detects
gamma-ray emissions in the 0.1- to 10-MeV energy range and achieves an energy resolution of 3.5 keV full-width at half-maximum
for 60Co (1332 keV). It is the first interplanetary use of a mechanically cooled Ge detector. Special construction techniques provide
the necessary thermal isolation to maintain the sensor’s encapsulated detector at cryogenic temperatures (90 K) despite the
intense thermal environment. Given the mission constraints, the GRS sensor is necessarily body-mounted to the spacecraft,
but the outer housing is equipped with an anticoincidence shield to reduce the background from charged particles. The Neutron
Spectrometer (NS) sensor consists of a sandwich of three scintillation detectors working in concert to measure the flux of
ejected neutrons in three energy ranges from thermal to ∼7 MeV. The NS is particularly sensitive to H content and will help
resolve the composition of Mercury’s polar deposits. This paper provides an overview of the Gamma-Ray and Neutron Spectrometer
and describes its science and measurement objectives, the design and operation of the instrument, the ground calibration effort,
and a look at some early in-flight data. |
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| Keywords: | MESSENGER Mercury Gamma-ray spectrometry X-ray spectrometry Surface composition |
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