Instrument Overview
  ===================
    The Mars Odyssey Gamma-Ray Spectrometer is a suite of three
    different instruments working together to collect data that will
    permit the mapping of elemental concentrations of the surface of
    Mars.  The instruments are a gamma-ray spectrometer (GS), a
    neutron spectrometer (NS), and a high-energy neutron detector
    (HEND). The instruments are complementary in that the neutron
    instruments have much better counting statistics and can sample to
    greater depths than the GS, but the GS determines the abundances
    of many more elements. Working together the instruments are most
    powerful at mapping the distribution of hydrogen, both over the
    surface and as a function of depth in the upper few tens of
    centimeters.
 
    This information can be found in The Mars Odyssey Gamma-Ray
    Spectrometer Instrument Suite submitted to Space Science Reviews
    September 13, 2002 [BOYNTONETAL2002].
 
 
  Scientific Objectives
  =====================
    The chief scientific objectives are:
 
    - to quantitatively map the elemental abundances of the Martian
    surface,
 
    - to map the near surface hydrogen (and, by inference, water) and
    CO2 abundances and their stratigraphic distributions, and
    determine their seasonal variations, and
 
    - to determine the depth of the seasonal polar caps and their
    variation with time.
 
    The GRS will detect and count gamma rays and neutrons emitted from
    the Martian surface. By associating the energy of gamma rays with
    known nuclear transitions and determining the number of gamma rays
    emitted from a given portion of the Martian surface, it is
    possible to calculate the ratio of elemental abundances of the
    surface and discern their spatial distribution. By counting the
    number of neutrons and segregating those into thermal and
    epithermal energy bins it is possible to calculate the hydrogen
    abundance thus inferring the presence of water.
 
    These data permit a variety of Martian geoscience and life science
    problems to be addressed including the crust and mantle
    compositions, weathering processes, volcanism, and the volatile
    reservoirs and processes.
 
 
  Calibration
  ===========
    Calibration information for the GRS Gamma instrument is provided
    in the GRS Calibration Report (PDF file).
 
    Calibration information for the neutron spectrometer will be
    provided in the next PDS release.
 
    A calibration report for the HEND is not yet available (Should be
    provided in the next PDS release).
 
 
  Operational Considerations and Operational Modes
  ================================================
    The gamma-ray spectrometer data has been collected in different
    configurations.  This data is only useful for science when the
    gamma sensor high voltage (HV) is ramped to greater than 3000
    volts. This condition can only happen if the gamma sensor head
    anneal door is open and the detector is cold (below 143 degrees
    Kelvin). The gamma sensor is capable of generating data when the
    HV is not ramped, but it is only to verify that the electronics
    are functioning properly. Gamma sensor data has been taken in all
    mission phases thus far. During the mapping mission phase, data
    was taken with the GRS boom stowed and deployed. The GRS boom was
    deployed on June 4, 2002 which started the true mapping phase
    for the gamma sensor.
 
    The Neutron Spectrometer (NS) and High Energy Neutron Detector
    (HEND) data have also been taken in all mission phases: cruise,
    aerobraking, and mapping.  These data are also only valid if the
    high voltage is on for the instruments.
 
 
  Detectors
  =========
    The Gamma Sensor Head (GSH) consists of a high-purity germanium
    detector mounted in a passive cryogenic cooler, front stage
    electronics, and a pre-amplifier. The sensor-head must be isolated
    from all material that will interfere with the detection of gamma
    rays from the Martian surface. For this reason it must be isolated
    from the spacecraft by a boom, and the materials making up the
    sensor head are selected to avoid interference.
 
    The gamma-ray detector must be cooled to cryogenic temperatures to
    reduce the electronic noise induced by the thermal agitation of
    the germanium atoms. In addition, in the radiation flux of
    interplanetary space a warm detector will experience radiation
    damage. In order to remove the effects of radiation damage, the
    HP-GeD must be heated to temperatures of 105 degrees C and
    maintained at this temperature for a minimum of 48 hours and a
    maximum of 120 hours.
 
    The neutron spectrometer (NS) is comprised of the neutron
    sensor-head and its associated electronics. The neutron
    sensor-head is a four-section borated-plastic scintillator
    composed of BC454 coupled to photo-multiplier tubes (PMTs) and
    their associated high-voltage bleeder boards and
    pre-amplifiers. Each of the four segments is in the shape of a
    prism; mounted together the scintillators form a
    parallelepiped. One of the four segments must face in the
    direction of the spacecraft velocity vector; the opposite prism
    will then face in the negative velocity vector of the spacecraft,
    and the two remaining prisms will face towards Mars (nadir) and
    away from Mars (anti-nadir). This orientation together with
    coincident pulse logic allows separation of neutron radiation from
    other radiation and separation of neutron energies into broad
    energy bands. The neutron sensor-head must be separated from the
    spacecraft to reduce the effect of the flux of neutrons from the
    spacecraft. The required separation depends on the proximity and
    size of the neutron-moderating (hydrogen) materials in the
    spacecraft such as the fuel.
 
    The borated plastic scintillator has a heritage of space
    applications for detecting neutrons and uniquely identifying them
    as such in the presence of backgrounds of gamma rays and energetic
    charged particles that are generally encountered in space. Thermal
    and epithermal neutrons are identified by detection of a single
    interaction in one or two of the sensor segments that deposits an
    energy equal to the Q-value of the 10B(n,a)7Li* reaction. The
    signature of a fast neutron is the detection of a pair of
    interactions occurring within about 5 microseconds (the
    pulse-correlation time for fast neutrons interacting in BC454
    is t = 2.2 microseconds) where the second interaction of the
    pair has the signature of a thermal or epithermal neutron just
    mentioned. If this condition is satisfied, then the pulse
    height of the first interaction gives the energy of the fast
    neutron.
 
    Separate determinations of the thermal, epithermal, and fast flux
    components of martian and spacecraft neutron flux spectra will be
    optimized if the sensor is oriented such that one of its prism
    segments faces in the direction of the spacecraft velocity vector
    (frontward), one faces in the opposite direction (backward), one
    faces downward toward the martian surface, and one faces upward
    from Mars. This configuration has the advantage that it allows
    discrimination between neutrons originating in Mars from those
    originating in the spacecraft, thereby reducing (but not
    eliminating) the need for separation from hydrogenous spacecraft
    materials.
 
    Given this orientation, all three neutron flux components from
    both the spacecraft and Mars can be measured and separately
    identified.  Specifically, the difference in counting rates of the
    front- and back-facing BC454 prism segments provides a measure of
    thermal neutrons from Mars alone. Contamination by thermals from
    the spacecraft, and all epithermal neutrons, is reduced by virtue
    of the motion of the sensor relative to Mars. The motion of the
    sensor relative to Mars ensures that the front-facing segment
    selectively scoops up the thermals from Mars while the back-facing
    segment outruns them. The fact that all four prism segments are
    stationary relative to the spacecraft ensures that the ratio of
    spacecraft contributions to each of them is fixed by geometry,
    which can be calibrated during cruise.
 
    A variant of the same technique can be used to separate martian
    and spacecraft epithermal neutrons. In this case the upward-facing
    segment is shielded from Mars by the other three segments. In
    consequence, the counting rate of the upward facing prism can be
    used to subtract the spacecraft contribution to the counting rate
    of the backward-facing prism to provide a measure of the martian
    epithermal flux alone. This result is possible because, as just
    mentioned, the backward facing prism will outrun the thermal
    neutrons coming from Mars, thus making it sensitive only to
    martian epithermal and fast neutrons. However, as discussed
    previously, the special properties of BC454 can be used to
    separate the epithermal and fast neutron events electronically. In
    addition, the normalized difference between single counts having
    the signature of a thermal or epithermal neutron registered in the
    downward- and upward-facing prisms will be used to provide a check
    on the combined thermal and epithermal neutron fluxes from Mars
    determined using the frontward-, backward- and upward-facing
    prisms, respectively.
 
    All four prism segments can be used to provide separate measures
    of martian and spacecraft fast neutrons. As for the epithermals,
    but to a lesser extent, this separation is facilitated by the
    absorption properties of the geometric arrangement of all four
    BC454 segments relative to the surface of Mars and the spacecraft,
    respectively. It is only feasible for this kind of an arrangement
    because the hydrogen moderator in the sensor is contained in an
    active element of the detector (the BC454 scintillator) as opposed
    to a passive element such as is provided by a polyethylene (or
    equivalent) moderator that is placed around a thermal neutron
    sensor.
 
    The High Energy Neutron Detector (HEND) is a self-contained
    experiment which relies on the GRS for power and command and data
    handling. HEND detects neutrons and soft gamma rays. Neutrons are
    detected in thermal (0.01 eV - 1 eV), epithermal (1 eV - 1 keV),
    fast (1 keV - 1 MeV), and high-energy (1 MeV - 10 MeV) energy
    ranges.  Provisions have been made to separate detected neutrons
    from gamma-rays and also provide anti-coincidence between protons
    and high-energy neutrons. HEND allows count rates from 0.01 per
    second for high-energy neutrons up to 104 for gamma rays. The
    energy resolution for neutrons and gamma-rays is 20% or better. A
    low background count rate due to the HEND is achieved through
    materials selection, active anti-coincidence shielding, and
    amplitude thresholding. Data is synchronized with the GRS so that
    all GRS data can be registered with respect to time. HEND also
    monitors the gamma-ray flux from solar flares and bursts with 0.25
    - 1 second time resolution. The high-energy neutron detector is
    comprised of three 3He sensors, a Stilben scintillator detector,
    and associated electronics. All HEND electronics are housed in the
    HEND sensor-head except for those circuits provided by the UA in
    the CEB to support the HEND RS-422 interface. HEND has a separate
    spacecraft power switch provided through the GRS CEB.
 
 
  Electronics
  ===========
    The remainder of the gamma-ray sensor electronics consists of the
    gamma analog processing system which provides peak-hold and 14-bit
    analog to digital conversion of the pulse as well as level
    discriminators and other instrument control functions plus the
    high-voltage bias supply to provide the bias voltage for the
    germanium detector. These electronics together with housekeeping
    electronics are housed in the MSO GRS Central Electronics Box
    (CEB).
 
    The remainder of the NS electronics consists of the neutron analog
    chain which shapes the pulses resulting from detection and
    provides 8-bit analog to digital conversion of the pulse as well
    as level discriminators and other instrument control
    functions. These electronics are housed in the CEB.
 
    The digital electronics process the output of the ADCs from the
    gamma-ray and neutron detectors. It also processes all
    housekeeping data. It receives and processes all instrument
    commands. It provides the interface with the spacecraft command
    and data handling system and provides the interface with all other
    components of the MSO GRS. It includes low-voltage power supplies
    for the MSO GRS including specially filtered low-voltage power
    supplies for the GRS analog electronics chain.
 
 
  Operational Modes
  =================
    Gamma-ray spectrometer
    1. Door open, closed
    2. Boom deployed, undeployed
    3. High voltage up, down
 
    Neutron Spectrometer (NS)
    1. High voltage up, down
 
    High Energy Neutron Detector (HEND)
    1. High voltage up, down