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DAWN GRAND CALIBRATED MARS FLYBY COUNTS V1.0 (PDS)

NSSDCA ID: PSSB-00624

Availability: At NSSDC, Ready for Electronic Access

Description

This description was generated automatically using input from the Planetary Data System.

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 [PRETTYMANETAL2003]. GRaND measures the spectrum of planetary gamma rays and neutrons, which originate from cosmic ray interactions and radioactive decay within the surface, while the spacecraft (S/C) is in orbit around each body. The instrument, which is mounted on the +Z deck of the S/C, 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 the spacecraft. The nuclear spectroscopy data provided by GRaND will be analyzed to determine the abundance of major rock forming elements, such as O, Fe, Ti, Si, Al, Mg, Ca, Cl and radioactive elements, including K and Th, as well as light-elements such as H, C, and N, which are constituents of ices and the products of aqueous alteration of silicate minerals and ices. The GRaND Reduced Data Records (RDR) are a time-ordered collection of corrected gamma ray and neutron counting data and calibrated pulse height spectra acquired by GRaND during Mars Gravity Assist (MGA). Similar products will be developed for science mapping at 4-Vesta and 1-Ceres and for data acquired during cruise. The RDR is a calibrated data product (Level 1B) that is derived from Experiment Data Records (EDR, Level 1A). The objective of the RDR is to provide counting rates and ancillary pointing and ephemeris data needed for mapping. The ancillary data includes the subsatellite point, the distance and direction to body center, and the spacecraft velocity vector relative to Mars, which is needed to account for the motion of low energy (thermal and epithermal) neutrons relative to the spacecraft. In the current version of the data set, counting rates and fluxes corrected for altitude and spacecraft motion, which require knowledge of the instrument response, are not provided; however, future versions of the RDR, especially for Vesta and Ceres mapping will include counting rates corrected for solid angle and pointing as well as incident gamma ray and neutron fluxes. Processing steps for the RDR data set include: - Determination of the mid-point time of each science accumulation interval and the livetime, accounting for roll-over of the dead time counter; - Construction of pulse height spectra from the gamma ray and neutron event data; - Correction of pulse height spectra for ADC differential nonlinearity; - Calibration and gain/offset correction of pulse height spectra to provide a consistent energy scale for peak analysis; - Analysis of spectra to determine the net areas (counting rates) of selected spectral features, including: -- the 10B(n,alpha) peak area from the Category 1 (CAT1) spectrum, for the +Z phoswich, which is sensitive to epithermal neutrons from the target body; -- the 6Li(n,t) peak area from the +Z CAT1 phoswich, which is sensitive to thermal and epithermal neutrons from the target body; -- the net areas of the 478 keV gamma ray full energy peaks for each of the four CAT2 BGO pulse height spectra, resulting from coincidences between 93 keV deposited in the four boron-loaded-plastic (BLP) scintillators by the 10B(n,alpha)7Li* reaction and the associated gamma ray (7Li*->7Li+478keV): --- Because the +Z and -Z BLP scintillators are shielded by Gd and lithiated glass, their CAT2 counting rates are sensitive to epithermal neutrons; --- Because the side +Y and -Y scintillators are not shielded, their counting rates are sensitive to both thermal and epithermal neutrons. -- the fast neutron counting rates for each of the boron-loaded plastic scintillators; -- net counting rates for selected gamma rays. The data set consists of ASCII tables, divided by functionality into four categories: ephemeris and pointing data EPX (where X denotes the target: X='X' for cruise, X='M' for Mars, X='V' for Vesta, and X='C' for Ceres); neutron counting rates (NCR); gamma ray counting rates (GCR), and the BGO pulse height spectrum (BGO). The fast neutron flux spectrum (with units of neutrons/cm2/s/MeV), which can be unfolded from the fast neutron pulse height spectrum given the response function, and CZT composite spectrum will be included in future releases. The Level1b data are represented as a time series in which counting rates and spectra are averaged over a time-window, consisting of an odd number of science accumulation intervals (TELREADOUT) selected by the evaluator. Note that ephemeris and pointing data are not averaged. Rather, their instantaneous values at the mid-point of each time window is reported. Two types of time series are possible: a central moving average (CMA), in which the averaging window is centered on each point in the original time series, and a result is reported for each science accumulation interval; and a decimated time series (DTS), in which the averaging window is shifted forward in time by its width, resulting a data set that has fewer points than the original time series. The CMA is intended for mapping, for example, in cases where full sampling of rapidly changing counting rates and solid angles is needed. The DTS is useful for averaging over long periods of time, for example, during cruise, when counting rates are not changing rapidly. By averaging over long time intervals, the DTS can be used to produce high precision gamma ray spectra needed for accurate peak identification, analysis, and calibration.

Parameters

The RDR data are derived from the EDR files, which cover arbitrary and irregular time periods, generally determined by the pattern of downlinks from the spacecraft. The RDR are evaluated for a single instrument state found in the EDR files, generally corresponding to the instrument fully configured for science data acquisition. Consequently, the value of TELREADOUT is the same for all records extracted from a particular EDR file. The EDR science records need not be contiguous; however, time-windows containing gaps are not included in the RDR. Generally, science data records with elevated counting rates associated with solar energetic particle (SEP) events will be excluded from the RDR; however, throughout MGA, solar activity was minimal and no records were lost. The RDR files are contained in a single directory. The name of the directory includes the type of time series (CMA or DTS): GRD-L1B-Y1M1D1-Y2M2D2_YCMCDC-CMA (directory name for a central moving average), or GRD-L1B-Y1M1D1-Y2M2D2_YCMCDC-DTS (directory name for a decimated time series). The top level directory name contains the SCET UTC dates for the first and last science data records (Y1M1D1 and Y2M2D2, respectively), and the creation date (YCMCDC) for the archive. For example, for GRD-L1B-090217-090218_090517, the first science data record was acquired on 17-Feb-2009. The last science data record was acquired on 18-Feb-2009. The archive was created by the processing code on 17-May-2009. The data files and formats are the same for both types of time series. All counting rates are reported as counts per second. Histograms have units of counts/second/channel. The processing parameters, including the window width and the value of TELREADOUT are given in the label files. The following files are included (note that 'TSI' is either 'CMA' or 'DTS', depending on the type of time series): GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-TSI-EPM.TAB - Contains a time series of ephemeris and pointing data, including the total livetime, the distance and direction to body center, the velocity of the spacecraft rotated into the reference frame of the instrument, the subsatellite position and altitude, and the fractional solid angle subtended by the body at the spacecraft. In addition, the average counting rate for the 'coincidence of three or more sensors' scaler (SCALER_SCI[15]) is provided as an indicator of the interaction rate of galactic cosmic rays. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-TSI-NCR.TAB - Contains a time series of neutron counting rates determined from CAT1, CAT2, and CAT4 pulse height spectra. Counting rates sensitive to neutrons in the thermal, epithermal, and fast energy ranges are provided. Propagated uncertainties (1-sigma) are provided. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-TSI-GCR.TAB - Contains a time series of net areas for selected full energy interaction peaks from the pulse height spectrum acquired by the BGO sensor. Propagated uncertainties (1-sigma) are provided. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-TSI-BGO.TAB - Contains a time series of BGO 1024-channel pulse height spectra that have been adjusted so that the offset is 0 keV with a slope of 8.9 keV/channel. Note that the method for determining the uncertainty in the counting rates for each channel is provided in the format file.

Processing

The RDR data from each EDR data file are processed in several steps: 1. Specification of processing parameters, including the target instrument state, the time series type and window width, and parameters for spectrum processing and analysis; 2. Extracting science data records from the EDR for the selected state; 3. Generation of the time series structure; 4. Construction of pulse height spectra from event data; 5. Correction of pulse height spectra in preparation for peak analysis (including livetime, ADC differential nonlinearity and gain and offset corrections). 6. Determining peak areas and uncertainties from time-averaged neutron and gamma ray spectra; 7. Writing the data and labels. Details of the processing are described in Prettyman, T.H. and W.C. Feldman (2009), PDS data processing for the Dawn gamma ray and neutron detector, GRD_L1B_Processing_V2.0.pdf, available in the document directory of this volume. A summary of the processing algorithms and parameters is provided here. The RDR time series is constructed by averaging corrected counting rates over a window using one of two methods: Central Moving Average (CMA) or Decimated Time Series (DTS). The CMA method provides a result for each science data record by averaging over records on either side of the current point as illustrated here: Science records XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX ... | 1 | | 2 | | 3 | | 4 | | 5 | .... where each of the time-ordered science records is indicated by an X and the window boundaries are indicated by the vertical bars. The middle of each time series is labeled with an index (1, 2, ...). The DTS method marches the window through the science data records, resulting in a decimated time series as follows: Science records XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX ...| 1 || 2 || 3 || 4 || 5 |... The number of science data records averaged for each point in the time series is given by WINDOW_WIDTH, which is an odd integer. For both methods, all time, ephemeris, and pointing information is determined at the midpoint of the window. A variety of spectrum analysis methods are used to extract peak areas recorded in the neutron and gamma ray files. These are described in detail in the data processing document. For Mars flyby, time-averaged background data were determined during a period of time for which the instrument was far from Mars and sensitive only to neutrons produced on board the spacecraft by galactic cosmic ray interactions. In some cases (e.g., analysis of the CAT1 spectrum), background continuum functions were determined using a coarse time window (CMA), under the assumption that the background shape does not change as rapidly as the peak. The window width for Mars flyby background subtraction was 11 science accumulation intervals.

Ancillary Data

The Level 1B data include ancillary data in the form of SCET UTC strings reported in each row of the time series tables. The UTC strings are determined from the spacecraft clock ticks recorded in each state-of-health packet and for the first packet in each science data record using NAIF SPICE (leap seconds kernel). This information is used in Level 1B processing to accurately determine the mid-point of each science accumulation interval, which is used for mapping. In addition, pointing and ephemeris data are provided in the Level 1B file at the center of each time interval: - the distance and direction of body center in the instrument coordinate system; - the velocity of the spacecraft rotated into the reference frame of the instrument - the subsatellite longitude, latitude, and altitude; - the fractional solid angle subtended by Mars at the spacecraft. The target was 'IAU_MARS'.

Coordinate System

The instrument coordinate system (Fig. 1) determines the naming convention of the sensors and orientation of the instrument relative to the spacecraft. The use of MZ indicates a sensor on the -Z (zenith-facing during mapping) side of GRaND; PZ indicates the sensor is on the +Z (spacecraft) side; MY indicates the sensor is on the -Y side (inboard) side of the instrument; and PY indicates the sensor is on the +Y side (outboard, towards the +Y solar panel) side of the instrument. The phototube assembly, marked 'P' on the diagram in Fig. 1 points along the +X axis (towards the high gain antenna). ................. . ooooooooooooo . . o o . . o o . . o +Z o . . o (PZ) o . . o o .---> +Y (PY) . ooo ooo . . P P . . P P . . PPPPPPPPP . . . ................. | v +X (PX) Figure 1. The coordinate system for GRaND is the same as that of the S/C. For the diagram above, the observer is looking in the -Z (MZ) direction and can see the outline of the phoswich assembly (o) on the +Z side of GRaND. The phototubes are on the +X side and the scintillators are on the -X side. During mapping at Vesta and Ceres, the planetary surface is in the +Z direction.

Software

The RDR were generated using an IDL-based processing code compatible with IDL Version 7.0, distributed by ITT Visual Information Solutions, Boulder, CO (2008). Program name: GRD_L1B_Pipeline.pro. Version: 1.0. Ancillary data were generated using the NAIF/ICY IDL toolkit.

Media/Format

The RDR label and data files are delivered by electronic transmission to the PDS.

These data are available on-line from the Planetary Data System (PDS) at:

http://sbn.psi.edu/archive/dawn/grand/DWNMGRD_1B/

Alternate Names

  • DAWN-M-GRAND-3-RDR-MARS-COUNTS-V1.0

Discipline

  • Planetary Science: Small Bodies

Additional Information

Spacecraft

Experiments

Questions and comments about this data collection can be directed to: Dr. David R. Williams

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. William C. FeldmanData ProviderLos Alamos National Laboratorywfeldman@lanl.gov
Dr. Thomas H. PrettymanGeneral ContactPlanetary Science Instituteprettyman@psi.edu
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