Instrument Information |
|
IDENTIFIER | urn:nasa:pds:context:instrument:dawn.grand::1.0 |
NAME |
GAMMA-RAY AND NEUTRON DETECTOR |
TYPE |
SPECTROMETER |
DESCRIPTION |
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 development; - 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 =========== 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 parameters. 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. Detectors ========= 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 | | | ._______________. | v +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. Electronics =========== 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. Location ======== 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 spacecraft. ._____________. | | | | | +Zsc +Ysc o==/ /==================o | o----->|o==================/ /==o -Y Solar Array | | | +Ysc Solar Array | | +Z o-----> +Y .______|_____|. .--V+Xsc | .' `.| /___________V `.|.' +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 spacecraft. 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. |
MODEL IDENTIFIER | |
NAIF INSTRUMENT IDENTIFIER |
not applicable |
SERIAL NUMBER |
not applicable |
REFERENCES |
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Soldner, S.A. Storms, C. Szeles, and R.L. Tokar, Gamma-ray and neutron
spectrometer for the Dawn mission to 1 Ceres and 4 Vesta, IEEE Trans. Nucl.
Sci., Volume 50, Issue 4, pp. 1190-1197, August 2003,
doi:10.1109/TNS.2003.815156 Prettyman, T.H., W.C. Feldman, H.Y. McSween, Jr., R.D. Dingler, D.C. Enemark, D.E. Patrick, S.A. Storms, J.S. Hendricks, J.P. Morgenthaler, K.M. Pitman, R.C. Reedy, Dawn's Gamma Ray and Neutron Detector, Space Sci. Rev. (2011) 163:371-459, doi:10.1007/s11214-011-9862-0 Prettyman, T.H., D.W. Mittlefehldt, N. Yamashita, D.J. Lawrence, A.W. Beck, W.C. Feldman, T.J. McCoy, H.Y. McSween, M.J. Toplis, T.N. Titus, P. Tricarico, R.C. Reedy, J.S. Hendricks, O. Forni, L. Le Corre, J.-Y. Li, H. Mizzon, V. Reddy, C.A. Raymond, C.T. Russell, Elemental Mapping by Dawn Reveals Exogenic H in Vesta's Regolith, Science (2012) 338(6104):242-246, DOI 10.1126/science.1225354 |