Instrument Information
Acronyms and Abbreviations
  ADC             Analog to Digital Converter
  ATLO            Assembly, Test, and Launch Operations
  BGO             Bismuth Germanate
  BLP             Boron-loaded plastic
  CAT             Category
  CPT             Comprehensive Performance Test
  CZT             Cadmium Zinc Telluride
  FPGA            Field Programmable Gate Array
  FWHM            Full-Width-at-Half-Maximum
  GRaND           Gamma Ray and Neutron Detector
  GRaND_Data_Proc GRaND data processing documents accompanying the
                  EDR and RDR archive
  HGA             High Gain Antenna
  Li-glass        Lithium-loaded glass
  MCA             Mars Closest Approach
  S/C             Spacecraft
  SCLK            Spacecraft clock ticks (s)
  Instrument Overview
  The Dawn Mission's Gamma Ray and Neutron Detector (GRaND) is a nuclear
  spectrometer that will collect data needed to map the elemental
  composition of the surfaces of 4-Vesta and 1-Ceres [PRETTYMANETAL2003B,
  PRETTYMANETAL2011, PRETTYMANETAL2012].  GRaND measures the spectrum of
  planetary gamma rays and neutrons, which originate from galactic cosmic
  ray interactions and the decay of radioelements within the regolith,
  while the S/C is in orbit around each body. The instrument, which is
  mounted on the +Z deck of the spacecraft, consists of 21 sensors
  designed to separately measure radiation originating from the surface of
  each asteroid and background sources, including the space energetic
  particle environment and cosmic ray interactions with spacecraft
  materials.  The nuclear spectroscopy data provided by GRaND will be
  analyzed to determine the abundance of major, rock forming elements,
  such as O, Mg, Si, Al, Ca, Ti, and Fe; incompatible elements, including
  K and Th, detected by gamma ray emissions from the decay of long-lived
  radioisotopes; and H, C, N, and Cl, which are constituents of ices and
  products of aqueous alteration of silicate minerals.
  A complete description of the GRaND and the performance of the
  instrument during cruise and Mars Flyby is provided by
  PRETTYMANETAL2011.  Additional details on the instrument and data
  reduction can be found in the GRaND PDS Data Processing Documents
  (GRaND_Data_Proc), which accompanies the archived data and in
  PRETTYMANETAL2003B.  Measurements of Vesta's elemental composition are
  presented by PRETTYMANETAL2012.
  Scientific Objectives
  Scientific objectives include:
  - provide geochemical data needed to constrain the thermal evolution of
    Vesta and Ceres, and to determine the role of water in their
  - if detectable, determine the contribution of long-lived
    radioisotopes to the global heat balance;
  - constrain the composition of the primordial solar nebula as a function
    of heliocentric distance (for example, measure the K/Th ratio to
    determine the proportion of volatile to refractory elements in the
    source material from which Vesta and Ceres accreted);
  - constrain the interior composition of Vesta and Ceres by measuring
    stratigraphic variations within large impact basins that probe the
    crust and mantle (for example, the large, south-polar basin on Vesta);
  - determine sources of near-surface hydrogen and detect and characterize
    compositional layering (for example, determine the depth of the ice
    table at high latitudes on Ceres);
  - at Vesta, determine the relationship between compositional terranes
    and howardite, eucrite and diogenite (HED) meteorites (Is the
    chemistry of Vesta more diverse than suggested by the HEDs?);
  - at Ceres, search for chemical evidence of the primitive crust and
    aqueous alteration products to constrain internal structure (e.g.
    subsurface ocean?) and crustal evolution
    Nuclear spectroscopy is used to determine the elemental composition of
    planetary surfaces and atmospheres.  Radiation, including gamma rays
    and neutrons, is produced steadily by cosmic ray bombardment of the
    surfaces and atmospheres of planetary bodies and by the decay of
    radionuclides in the regolith.  The leakage flux of gamma rays and
    neutrons contains information about the abundance of major elements,
    selected trace elements, and ice constituents (e.g., H, C, and N) as
    well as elements associated with aqueous alteration products such as
    Cl.  Gamma rays and neutrons can be measured at altitudes less than a
    planetary radius, enabling global mapping of elemental composition by
    an orbiting spacecraft.  Radiation that escapes into space originates
    from shallow depths (< 1 m within the solid surface).  Consequently,
    nuclear spectroscopy is complementary to other surface mapping
    techniques, such as reflectance spectroscopy, which is used to
    determine the mineralogy of planetary surfaces.  The main benefit of
    gamma ray and neutron spectroscopy is the ability to reliably identify
    elements important to planetary geochemistry and to accurately
    determine their abundance.  This information can be combined with
    other remote sensing data, including surface thermal inertia and
    mineralogy, to investigate many aspects of planetary science.
    Nuclear reactions and radioactive decay result in the emission of
    gamma rays with discrete energies, which provide a fingerprint that
    can uniquely identify specific elements in the surface.  Depending on
    the composition of  the surface, the abundance of major rock-forming
    elements such as O, Mg,  Al, Si, Cl, Ca, Ti, Fe, as well as Cl, a
    tracer of aqueous alteration, H, and elements with radioisotopes (40K,
    U series, Th series) can be determined from measurements of the gamma
    ray spectrum when they are present in detectable quantities.  High
    energy neutrons produced by cosmic ray interactions loose energy in
    successive collisions with nuclei in the regolith, and are ultimately
    absorbed or escape into space. Their sensitivity to elemental
    composition depends on three main types of reactions that are
    important in three broad energy ranges measured by GRaND:  inelastic
    scattering (important for fast neutrons greater than about 0.7 MeV);
    elastic scattering (epithermal neutrons between 0.1 eV to 0.7 MeV);
    and absorption (thermal neutrons less than 0.1 MeV). Fast neutrons are
    sensitive to the average atomic mass of the regolith when H is present
    in small quantities (H weight fractions less than a few hundred ppm).
    Epithermal neutrons are very sensitive to the abundance of H and are
    relatively insensitive to variations in the abundance of major
    elements.  Thermal neutrons are sensitive to strong absorbers such as
    Fe, Ti, N, Cl, Gd, and Sm.
    Close proximity to the planetary body is needed to measure neutrons
    and gamma rays because their production rate is relatively low in
    comparison, for example, to reflected sunlight. In addition, sensors
    used for gamma ray and neutron spectroscopy are generally insensitive
    to incident direction.  Consequently, spatial resolution depends on
    orbital altitude, and higher resolution can be achieved by moving
    closer to the planet.  Regional scale measurements are generally made
    using nuclear spectroscopy, in contrast to the meter to kilometer
    scale generally achieved by reflectance and thermal-emission
    spectroscopy. As a rough guide, a nuclear spectrometer can resolve
    distinct, sources of radiation on planetary surfaces that are
    separated by an arc length of about 1.5 times the orbital altitude of
    the spacecraft. For the 210-km mean altitude achieved by Dawn at
    Vesta, the spatial resolution was about 300-km, which is smaller in
    scale than the 500-km diameter Rheasilvia basin.
  Calibration data for GRaND was acquired during assembly, test, and
  launch operations (ATLO), before and after delivery of the instrument
  for integration with the spacecraft.  Prior to delivery, the instrument
  was characterized at a calibration facility at Los Alamos National
  Laboratory and on the bench using neutron and gamma ray sources.  The
  main goals of the calibration exercise were to:
  - verify the functionality of each of the sensors;
  - determine the energy calibration for each sensor and event category;
  - determine the absolute calibration (relationship between flux and
    counting rate) for each sensor and event category as a function of
    incident energy and direction;
  Data acquired during comprehensive performance tests (CPTs) following
  integration of GRaND with the spacecraft provide supplemental
  information needed to confirm the energy calibration.
  Following launch, GRaND was operated during Earth-Mars cruise to measure
  the response to galactic cosmic rays and energetic particles in the
  space environment.  The data are needed in order to characterize
  background sources (for example, prompt neutron and gamma production by
  galactic cosmic ray interactions with the bulk spacecraft and the
  buildup of induced radioactivity within the sensor).
  In addition, GRaND acquired data during Mars Closest Approach (MCA),
  which was compared directly to data acquired by 2001 Mars Odyssey,
  enabling cross calibration of GRaND during flight [PRETTYMANETAL2011].
  Selected calibration files will be archived along with the results of
  modeling. Analysis of calibration data is ongoing and will be subject to
  change as models of the instrument response are developed and improved.
  The relationship between particle energy and measured pulse height
  depends on bias voltage settings and environmental factors, such
  as the temperature of the scintillator, which can vary with time.
  During flight, prominent gamma ray and neutron spectral features with
  known energies are used to determine time-dependent, energy calibration
  Operational Considerations
  Science data will be acquired by GRaND during cruise, Mars Flyby, and
  mapping of Vesta and Ceres.  In order to acquire science data, GRaND
  must be in NORMAL mode with high voltages turned on and adjusted to
  nominal settings.  Large gaps in the data are expected during cruise,
  when the instrument is off.  For science mapping, only data acquired
  when the instrument bore-sight is pointed to within 5-deg of body center
  are used.  In addition, solar energetic particle events are reported
  separately from data acquired during quiet conditions. Contamination
  from other instruments and spacecraft subsystems appears to be
  negligible, but will be evaluated throughout the mission.
  GRaND uses scintillator- and semiconductor-based radiation sensors to
  detect neutrons and gamma rays as well as energetic particles from the
  space environment. A scintillator is a transparent material that
  converts the kinetic energy of charged particles (such as electrons
  produced by gamma ray interactions or alpha particles and recoil protons
  produced by neutron reactions) into flashes of light detectable by a
  photomultiplier tube or photodiode. Semiconductors can be used to detect
  gamma rays.  Swift electrons produced by Compton and photoelectric
  interactions ionize the semiconductor, producing electron-hole pairs.
  The electrons and holes drift under the influence of an applied electric
  field to electrical contacts.  As they drift, the electrons and holes
  induce charge on contacts, which can be measured by a charge-sensitive
  preamplifier.  The amplitude of the charge pulse is proportional to the
  energy deposited by the gamma ray, which enables semiconductors to be
  used for spectroscopy.
  The sensors and shielding/structural materials were arranged in order to
  separately measure gamma rays and neutrons originating from the target
  body from background sources, including neutrons and gamma rays produced
  by cosmic rays in the bulk spacecraft, and energetic particle
  interactions with the instrument.  The sensors on GRaND were selected to
  operate between -20C and 30C and do not require active cooling.
  GRaND uses four types of radiation sensors:
  1. Bismuth germinate(BGO) scintillator: A 7.6 (X) cm x 7.6 (Y) cm x 5.08
  (Z) cm BGO crystal (approximately 300 cm3 volume) is located in the
  center of the scintillator subassembly.  The scintillator is coupled to
  a 5.08-cm diameter photomultiplier tube.  BGO has high density and high
  atomic number and is sensitive to gamma rays over a wide energy range
  (up to 10 MeV).  The pulse height resolution at room temperature is
  approximately 10% full-width-at-half-maximum (FWHM) at 662 keV.
  2. Cadmium Zinc Telluride (CZT) semiconductor: A planar array of 4x4 CZT
  crystals is positioned on the +Z side of the BGO crystal (Fig. 1), which
  faces towards the target body center during science mapping. Each
  crystal is 10 mm x 10 mm x 7 mm. Consequently, the array has a sensitive
  volume of 11.2 cm3.  Coplanar grids are used to mitigate the effects of
  hole trapping, resulting in excellent peak shape and pulse height
  resolution over a wide range of energies.  The pulse height resolution
  is better than 3% FWHM at 662 keV.  The array was designed to measure
  gamma rays with energies up to 3MeV.  The relatively high energy
  resolution of the CZT array enables accurate measurement of gamma rays
  in the densely-populated, low energy region of the spectrum, which
  contains gamma rays from radioactive decay and cosmic-ray induced
  reactions within the surface of the target planetary body.
  3. B-loaded plastic scintillator: Two L-shaped boron-loaded plastic
  (BLP) scintillators (each 193 cm3) are located on the -Y and +Y sides,
  surrounding the sides of the BGO crystal and CZT array.  The
  scintillators act as anticoincidence shields to reject cosmic ray
  interactions.  In addition, the scintillators are sensitive to neutrons.
  Fast neutrons (with energies greater than 700 keV) can undergo elastic
  scattering with H within the plastic to produce knock-on protons, which
  ionize the scintillator, resulting in the production of detectable
  light.  In addition, thermal and epithermal neutrons can be captured via
  the 10B(n,alpha)7Li* to produce 93 keVee light output. Note that the
  subscript ee indicates an electron-equivalent energy, corresponding to
  the energy a swift electron would need in order to produce the same
  light output as the reaction products. The reaction product, 7Li*,
  produces a 478 keV prompt gamma ray.  Fast neutrons with energies
  greater than 700 keV produce a characteristic double pulse signature,
  corresponding to light output from fast-neutron proton recoils followed
  later by neutron capture with 10B after the neutron has thermalized.
  The amplitude of the first pulse is related to the energy of the
  incident neutron. Thermal and epithermal neutrons also produce a unique
  coincidence signature, corresponding to 93 keV of light produced in the
  plastic in coincidence with 478 keV deposited in the BGO crystal.
  4. Li-glass, B-loaded-plastic phosphor sandwich (phoswich): Two BLP
  scintillators are located on the nadir (-Z) and spacecraft (+Z) sides of
  the instrument, centered on the CZT array and BGO crystal.  Each BLP
  scintillator is approximately 10.16 cm x 10.16 cm x 2.54 cm (264 cm3)
  and is read out by a 2.54 cm diameter phototube. With the exception of
  the outward facing side, each scintillator is covered with a sheet of Gd
  foil, which absorbs thermal neutrons. The outward facing side is covered
  by a plate of lithiated glass, 0.2 cm thick. The lithiated glass is
  optically-coupled to the BLP such that the phototube measures light
  produced in both the glass and the plastic. 6Li is a strong thermal
  neutron absorber. Consequently, the BLP is shielded from thermal
  neutrons.  Epithermal neutrons that undergo capture via the 10B(n,alpha)
  reaction in the BLP produce 93 keVee light output.  Thermal and
  epithermal neutrons can undergo neutron capture via the 6Li(n,triton)
  reaction, which produces approximately 340 keVee, which is seen as a
  separate peak in the pulse height spectrum.  Consequently, the thermal
  neutron signature can be determined by weighted difference between the
  counting rates observed for the two reactions.  The spectrum of fast
  neutrons is measured using the double pulse signature in the BLP.  In
  addition, the (n,gamma) BLP-BGO coincidence signature provides an
  independent, low background measurement of epithermal neutrons.
  | xxxxxxxxxxxxx |
  | x           x |
  | x           x |
  | x    +Z     x |
  | x   (PZ)    x |
  | x           x |---> +Y (PY)
  | xxx       xxx |
  |   x       x   |
  |   x       x   |
  |   xxxxxxxxx   |
  |               |
       +X (PX)
  Figure 1. The coordinate system for GRaND is the same as that of the
  spacecraft (Fig. 2). The observer is looking in the +Z (PZ) direction
  and can see the outline of the phoswich assembly (x) on the +Z side of
  GRaND.  The phototubes are on the +X (PX) side and the scintillators are
  on the -X (MX) side.  During science mapping, the center of the target
  (Vesta or Ceres) will be in the +Z direction.
  GRaND derives power from the S/C 28Vdc power bus.  The instrument low
  voltage power supply provides +/-5V to the digital and analog circuits
  and +12V to the high voltage power supply, which supplies 0 to +1500V to
  the photomultiplier tubes and -1500V/+70V to the CZT sensors.  The
  instrument transmits and receives data through an RS-422 interface.  The
  instrument is controlled by a UTMC micro-controller, which manages
  instrument subsystems, processes commands, monitors state of health
  (SOH), and processes the science data.   Each of the radiation sensors
  is read out by analog front end electronics, which provides shaped
  pulses, which are digitized by analog-to-digital-converters (ADC) to
  determine pulse amplitude, and timing signals for analysis of coincident
  events. Signals from the FEE are processed by an Actel field-
  programmable-gate-array (FPGA).  The FPGA categorizes signals from the
  sensors, identifying patterns that correspond to important events (for
  example, the fast neutron double-pulse signature).  The event categories
  are described in the Measured Parameters section.  SOH data are recorded
  in the engineering telemetry, including high voltage values and
  temperatures.  Commandable parameters include instrument high voltage
  settings, parameters used to classify coincidence events, and
  measurement intervals.
  GRaND is mounted on the +Z deck of the spacecraft (SC), offset from the
  center of the spacecraft in the (+Xsc,+Ysc) quadrant (Fig. 2). See
  PRETTYMANETAL2011 for a photograph of the instrument as installed on the
                             |             |
                             |             |
                             |  +Zsc    +Ysc
   o==/ /==================o |      o----->|o==================/ /==o
     -Y Solar Array          |      |      |        +Ysc Solar Array
                             |      |  +Z o-----> +Y
                                 .--V+Xsc |
                               .'       `.|
                                  `.|.'    +X            +Zsc and +Z
                                      HGA                   are out
                                                          of the page
  Figure 2. Location of GRaND on the spacecraft.
  Operational Modes
  GRaND has three operational modes: 1) STANDBY; 2) NORMAL; and ANNEAL.
  The instrument starts in STANDBY mode.  In STANDBY mode, the radiation
  sensors are not operational (all commands are accepted except high
  voltage enable commands).  Only SOH data are generated in standby mode.
  Data from the temperature sensors are recorded in STANDBY if the +/-5V
  low voltage supply is activated.  From STANDBY, the instrument can be
  commanded to NORMAL mode for which all commands are accepted.  In NORMAL
  mode, the instrument can be configured for science data acquisition,
  including enabling and setting the high voltage level for each sensor.
  Both SOH and science data are included in the telemetry.  From STANDBY,
  the instrument can also be commanded to ANNEAL mode, which is designed
  to anneal radiation damage accrued by the CZT crystals
  [PRETTYMANETAL2003B, PRETTYMANETAL2011].  Only SOH data are generated in
  ANNEAL mode.
  Measured Parameters
  Each science record sent by GRaND contains counting data acquired during
  a collection interval, which is set by the commandable parameter
  TELREADOUT.  The collection intervals are successive, forming a time
  series that can be analyzed to map elemental abundances. The records are
  time-tagged with the spacecraft clock (SCLK) value, which can be merged
  with NAIF SPICE ephemeris data for mapping.  Each science record
  includes scaler data, event data, and histograms.  The pattern of pulses
  recorded by the sensors for each radiation interaction is processed by
  the FPGA, which categorizes the events.  The events are scaled and
  binned into histograms.  In addition, a subset of neutron and gamma ray
  events are recorded in a fixed length list-mode buffer.  At the end of
  each collection interval, the data are compressed, packetized, and
  transmitted.  The event categories recorded by GRaND are as follows
  (Note that event categories 3, 5, 6, and 8 were deleted during
  instrument development):
  Category 1 (CAT1):  A single pulse from the -Z or +Z phoswich.  CAT1
  data are binned into a histogram (256 channels) which can be analyzed to
  determine the areas of peaks corresponding to the 93 keVee 10B(n,alpha)
  and the 340 keVee 6Li(n,triton) reactions.
  Category 2 (CAT2):  A prompt coincidence between the BGO and any one of
  the phoswich or BLP scintillators.  The pulse heights of the coincidence
  event must occur within windows, which are set to bracket the 93 keVee
  peak from the BLP/phoswich and the 478 keVee full energy peak from the
  BGO.  The upper and lower bounds of the windows are commandable.  The
  CAT2 events are binned into histograms (64 channels), which can be
  analyzed to determine the flux of epithermal and thermal neutrons.
  Category 4 (CAT4): A double-pulse occurring in any one of the phoswich
  or BLP scintillators.  To reduce after-pulsing, events for which the
  second pulse occurs within 400 ns of the first pulse are rejected. The
  maximum time to the second pulse (TTSP) recorded by GRaND is 25.6
  microseconds. The amplitudes of the first and second pulse and the TTSP
  are recorded as event data in a fixed length buffer.  The total number
  of CAT4 events processed by the FPGA during the collection interval is
  recorded in the scaler data.  The CAT4 data can be analyzed to determine
  the flux and energy distribution of fast neutrons.
  Category 7 (Cat7): A coincidence between a single CZT sensor and the BGO
  scintillator.  The CZT pulse height (digitized by a 12-bit ADC) and CZT-
  sensor-ID are recorded as event data in the gamma event buffer.  The BGO
  pulse height is recorded as a 9 bit unsigned integer.  The portion of
  the gamma event buffer reserved for CAT7 events is commandable.  The
  CAT7 data can be used to discriminate gamma rays originating from the
  target body and the spacecraft.  For example, gamma rays originating
  from the target body (from nadir) can undergo low angle Compton
  scattering in a CZT sensor prior to entering the BGO crystal, where they
  may deposit the rest of their energy.  The energy of the gamma ray can
  be determined by summing the pulse heights measured by the CZT and BGO
  sensors.  Gamma rays originating from the spacecraft are shielded from
  the CZT array by the BGO crystal.  In addition, those originating from
  the spacecraft  that interact with a CZT sensor must scatter through a
  large angle, depositing a relatively large amount of energy in the CZT
  sensor before reaching the BGO crystal.  Consequently, summing the
  energy deposited in the CZT and BGO sensors for events in which the
  energy deposited in the BGO sensor is greater than the energy deposited
  in the CZT sensor tends to reject gamma rays originating from the
  Category 9 (CAT9): A single pulse from the BGO scintillator.  The CAT9
  events are binned into a 1024 bin histogram.
  Category 10 (CAT10):  A single interaction with a CZT sensor.  The pulse
  height (digitized by a 12-bit ADC) and CZT-sensor-ID are recorded as
  event data in a fixed length buffer.  The total number of CAT0 events
  processed by the FPGA during the collection interval is recorded in the
  scaler data.  The CAT10 event data can be processed, given the known
  energy calibration for each of the sensors, to form a composite pulse
  height spectrum.
  During mapping, the CAT9 histogram and CAT10 composite spectrum contain
  full energy peaks corresponding to radioactive decay and nuclear
  reactions occurring within the planetary surface, which can be analyzed
  to determine elemental abundances.
  The scaler data provide additional information needed to analyze the
  histograms and event data, including a dead time counter.  A scaler for
  events occurring in coincidence with three or more sensors (BGO and
  multiple BLP/phoswich) can be used as a galactic cosmic ray monitor.
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