Instrument Information


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 Document            
  (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 energy calibration is dependent on high voltage settings and            
  temperature, which are subject to change.  During flight, common gamma      
  ray and neutron spectral features are used to determine the energy          
  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  for the     
  center of ths spacecraft in the (+Xsc,+Ysc) quadrant (Fig. 2). The          
  location of the instrument on the +Z deck is given in the GRaND NAIF        
  Spice cks.                                                                  
                             |             |                                  
                             |             |                                  
                             |  +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 (adapted from the GRaND       
  NAIF Spice tf.                                                              
  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.