| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The Mars Odyssey Gamma-Ray Spectrometer (GRS) is a suite of
three instruments working together to collect data that will
permit the mapping of elemental concentrations on the surface
of Mars. The suite of three instruments, the gamma sensor head
(GS), the neutron spectrometer (NS) and the high-energy neutron
detector (HEND), are a complimentary set of instruments in that
the neutron instruments have better counting statistics and
sample to a greater depth than the GS, but the GS determines
the abundance of many more elements. A full description of the
Mars Odyssey Gamma-Ray Spectrometer instrument can be found in
[BOYNTONETAL2004].
The ODY MARS GAMMA RAY SPECTROMETER 5 AND data set is a table
of neutron data from the NS sub-system of the Mars Odyssey
Gamma-Ray Spectrometer that have been averaged (AND) over 5-
degree latitude by 5-degree longitude spatial grids and 15-
degrees of aerocentric solar longitude (Ls). The AND are an
intermediate data product. These data are calculated from the
derived neutron data (DND) and result in maps of thermal,
epithermal, and fast neutron counting rates.
The GRS collects a new spectrum (pixel) approximately every 20
seconds, 360 times per orbit. Approximately 4200 neutron
measurements are expected to be received every day. The data
are downloaded from the spacecraft by the Jet Propulsion
Laboratory (JPL) into the Telemetry Data System (TDS). The TDS
sends data to a process that translates data packets and
examines instrument health via messages. Data are output to a
spooler that passes it to the University of Arizona (UA)
database ingest process. The ingest process inputs raw data
into the UA database. Neutron data are processed by Los Alamos
National Laboratory through a number of programs to yield the
DND from which the AND are determined.
The AND is intended to be the second intermediate data product
available for the NS portion of the GRS data. This data should
be useful to those scientists who are experienced in neutron
spectroscopy.
Parameters
==========
The averaged neutron data set (AND) is composed of a single
data type. The objective of compiling the AND is to create a
table of neutron counting data from the DND time series. Each
AND product data file will contain a table of neutron counting
rates (counts per second) averaged over 5-degree latitude by
5-degree longitude spatial grids and 15-degrees of Ls, the
number of 20-second collection intervals that have been
averaged, and the standard deviation in counts per second of
the counting rate for each average. Each spatial grid
corresponds to one row in the table. The first 72 rows in the
table are the data for the 5 degree latitude band centered at
87.5 degrees north latitude, the second 72 rows are the data
for the band centered at 82.5 degrees north latitude, et
cetera. Within each latitude band, longitude increases eastward
from the cell centered at 2.5 degrees east longitude to the
cell centered at 357.5 degrees east longitude. AND data files
will be labeled by the beginning and ending Ls values.
Processing
==========
The products of the neutron data reduction algorithms are a
time-series data set (DND), corrected for variations in the
instrument response and the cosmic ray background, that
can be used to produce thermal, epithermal, and fast neutron
global maps of Mars. Tables of temporally and spatially
averaged counting rates (AND) are also provided. The AND data
set consists of maps of thermal, epithermal, and fast neutron
counting rates averaged over 15-degree Ls intervals. The maps
are 5-degree cylindrical projections. In addition to the
counting rates, maps of the number of measurements binned in
each pixel and the standard deviation of the pixel counting
rates are provided.
A summary of these algorithms adapted from Appendix A of
PRETTYMANETAL2004] is provided here. See [Neutron
Spectrometer Processing V.1.1] for details of the neutron data
reduction process. See [FELDMANETAL2002] and [BOYNTONETAL2004] for
details of the neutron spectrometer design and operation.
The neutron spectrometer consists of a block of boron-loaded
plastic that is diagonally segmented into four optically
decoupled prisms that are viewed by separate photomultiplier
tubes. During mapping, two of the prisms are aligned along the
axis of motion of the spacecraft: Prism 2 (P2) faces in the
forward direction and Prism 4 (P4) faces opposite to the
direction of motion. The remaining two prisms are oriented
along the nadir axis: Prism 1 (P1) faces downward towards Mars
and Prism 3 (P3) faces upwards towards the spacecraft. P1 is
covered with cadmium foil, which absorbs neutrons below roughly
0.5 eV. Consequently, P1 is sensitive to epithermal neutrons.
Raw (EDR) neutron data, including spectra, histogram, and
events arrays, are processed through several automated programs
to yield normalized neutron counting rates for all four of the
NS prisms, from which thermal, epithermal, and fast neutron
counting rates are determined.
The first step in the neutron processing is the determination
of the 10B(n,alpha) peak centroid and the after-pulse
threshold of each prism averaged over a course time window 720
collection intervals in length. This information is used
subsequently by algorithms that determine the area of the
10B(n, alpha) peak for each prism for each 19.75s interval The
10B(n,alpha) peak centroid is determined using double pulse
events (Category-2 events), which are primarily a result of
interactions of fast neutrons with the spectrometer. Fast
neutrons are identified by the detection of a characteristic
double pulse. The light output caused by proton recoils
produced by the prompt interaction of a fast neutron and the
hydrogen in the plastic provides a measure of the energy of
the incident neutron. A second pulse of light corresponding to
the absorption of the neutron at low energy by 10B follows a
delay in which the neutron is slowing down in the plastic, but
does not produce light. This double-pulse signature occurs for
neutrons above approximately 700 keV.
Category-2 event mode data (EDR EVENTS_ARRAY) are accumulated
over a coarse time window containing 720 19.75s intervals. The
overload counts (EDR GCR_CNT) are monitored and events are
discarded for intervals for which the overload counts exceed a
user-defined threshold, currently set at 7000. For each time
window, the second event spectrum is constructed from the
accumulated event-mode data. If the number of accepted
intervals is less than 360, then the time window is not
analyzed and the window is assigned the results for centroid
and threshold from the analysis of the previous window.
The second event spectrum acquired for each window is analyzed
by simultaneously fitting two normal distributions (one for the
10B peak and one for the after-pulse peak) and a constant
background (to represent the continuum) to the data. The
fitting algorithm minimizes the sum of the squares of the
difference between the model and the data weighted by the
statistical uncertainty in the counting data i.e., a weighted
least squares fit to the data. The weighted least squares fit
yields the peak centroids, the highest of which is taken as the
10B peak, the full width at half maximum, and the after-pulse
threshold, which is taken to be the channel for which the model
is minimum between the two centroids. For a more complete
description see Neutron Spectrometer Processing V1.1 in the
documents directory that accompanies this release. The peak
centroid determined by this process is used subsequently to
make gain corrections in the next processing step, which is
evaluation of the Category 1 single event spectrum.
Thermal and epithermal neutrons are detected through the
10B(n,alpha)7Li* reaction, which produces a distinct peak in
the pulse-height spectrum caused by the deposition of the
recoil energy of the reaction products in the scintillator.
This peak corresponds to an equivalent electron energy of 93
keV. For each prism, the net counting rate for this peak was
determined from the Category-1 pulse-height spectrum, a
spectrum of single-interaction events that is recorded every
19.75 s. During this measurement interval, the spacecraft
traverses approximately one degree of arc length (roughly 60
km). Determination of peak areas for each prism from the
Category-1 spectra involved several steps summarized here:
1) A correction was applied to each spectrum to remove
artifacts of the differential nonlinearity of the analog to
digital converter from the spectrum.
2) The spectra were corrected in gain so that the 93 keV peak
always fell in Channel 11. Implementation of this step
required knowledge of the original peak location determined
from the second interaction spectrum. The gain correction
enabled the same region of interest (range of energy) to be
used for the peak and background for all spectra.
3) For each prism, selected channels surrounding the peak
above and below were fitted to a power law. The power law was
determined for spectra in a sliding window 5 x 19.75 s in
length. The length of the window was selected empirically to
minimize the uncertainty in the background parameters because
of statistical variations in counting rate while avoiding the
introduction of bias resulting from integration over surface
features.
4) The peak areas (counts in the peaks for each prism) were
determined for each 19.75 s spectrum by subtracting the
background predicted by the power law from the total counts in
the peak region of interest.
The thermal-neutron counting rate was determined by subtracting
the peak area for the backward-looking prism (P4) from that of
the forward-looking prism (P2). During the mapping phase, the
spacecraft traveled in a circular polar orbit at an average
altitude of 400 km and speed of 3380 meters per second. A
significant portion of the population of thermal neutrons at
orbital altitudes had speeds much less than that of the
spacecraft and the flux of low-energy neutrons incident on the
exposed face of P2 was much higher than that of P4. If the
return of neutrons from the spacecraft is neglected, both
prisms should receive the same contribution from epithermal
neutrons. Consequently, subtraction of P4 from P2 yielded a
response that was representative of the thermal-neutron
population. The epithermal-neutron counting rate was taken to
be the counting rate for P1, which was shielded from thermal
neutrons by a Cd foil. Note that based on an analysis of cruise
data, the cosmic-ray-induced background for Category-1 counting
rates for P1, P2, and P4 was found to be consistent with zero.
For fast neutrons, two data products are provided in the raw
data set: Event-mode data for which the magnitude of the first
pulse, time to the second pulse, and magnitude of the second
pulse of a selected number of events (84 per 19.75 s
acquisition period) were recorded; and histogram data for which
first pulses with magnitude above a preset, fixed threshold
were binned into histograms for early and late time windows.
The late time interval (nominally chosen to be between 20 and
25 microseconds) was selected so that it is well beyond the
die-away time of neutrons in the spectrometer and thus contains
accidental events only. The early time interval (nominally
chosen to be between 0.4 and 5.4 microseconds) was selected so
that it is sensitive to neutrons slowing down in the
spectrometer. Subtraction of the late from early time
histograms yields a pulse-height spectrum that is sensitive to
the energy spectrum of fast neutrons.
The fast-neutron counting rate was taken to be the first two
channels of the histogram spectrum for P1, which is primarily
sensitive to neutrons coming directly from Mars. The lower
channels were selected because they give the greatest count
rates and are also least susceptible to contamination by
after-pulsing. However, because considerable drift in gain
occurred, the threshold selected for after-pulse suppression
used to construct the histogram data was not always effective.
During times of high gain, the threshold was too low and
after-pulsing events contaminated the histogram spectrum.
Variations caused by shifts in gain were corrected using the
BellyBand algorithm described below.
Neutron counting rates are sensitive to a number of factors
unrelated to the surface and atmosphere of Mars. These
include, for example, instrument drift and variations in the
galactic cosmic-ray flux. To eliminate these variations from
our data set, we developed an algorithm to adjust our
time-series counting data so that counting rates at equatorial
latitudes did not change with time and were equal to the
average counting rates observed over a normalization interval,
which was arbitrarily selected to be from LS equals 329 degrees
to LS equals 131 degrees. The region between latitudes plus to
minus 30 degrees (the belly band) was used for normalization.
Minor variations in counting rate due to seasonal changes in
atmospheric mass were initially ignored, but were later
restored to the time series.
The belly-band normalization algorithm removes gain and offset
variations caused by a number of effects, including variations
in the galactic cosmic-ray background and errors in the
Category 1 background subtraction algorithm, with a linear
correction (CAT1_PRISMX_NORM_SLOPE, CAT1_PRISMX_NORM_OFFSET)
However, the normalization algorithm also removes variations
that would otherwise be present as a result of seasonal changes
in atmospheric mass at the equator. To restore the effect of
atmospheric mass, we combined maps of atmospheric mass at
equatorial latitudes (derived from the ARC-GCM) with
expressions for counting rate as a function of atmospheric
thickness [PRETTYMANETAL2004] to determine the relative
variation of belly-band-averaged neutron-counting rates
as a function of time. The normalized counting rates were
then multiplied by the relative variation to restore the
atmospheric effect. Use of pressures from the ARC-GCM is
justified because the ARC-GCM fits the Viking 1 and 2
landing-site pressure data that are representative of
equatorial to midlatitude pressure variations and were found
to be reproducible year after year. This procedure was applied
to the fast and epithermal counting data that show the largest
effect. Thermal neutrons vary negligibly with atmospheric mass
when the water abundance of the surface is less than about 15
percent, which is representative of the belly band. The fast
neutrons, which have the largest variation, show a 5 percent
peak-to-peak variation in counting rate over an entire Martian
year. Therefore, the correction is relatively minor.
Averaged neutron counting rates are produced from the derived
(background subtracted, normalized) neutron counting rates. A
summing algorithm is used to bin and combine thermal,
epithermal, and fast neutron counting data both spatially and
temporally. The spatial constraint is variable, but will be set
at 5-degrees latitude by 5-degrees longitude bins for the AND
records. In order to be grouped within a particular 5-degree
bin, the latitude and longitude must be greater than or equal
to the lower limit and less than the upper limit. The thermal
data are assigned latitudes and longitudes that are shifted by
4.5 degrees along the spacecraft trajectory from the
sub-satellite point. Similarly, the epithermal data are
assigned locations that are shifted by 1 degree along the
trajectory. The temporal constraint on the AND product is
variable, but will be constrained for the PDS to be 15-degrees
of Mars Ls. The 15-degree periods are called seasons by the GRS
Team.
Data
====
The AND data set is composed of a 24 data tables per Mars year.
Each data table will contain 2592 rows of data.
Averaged Neutron Data
---------------------
Averaged Neutron Data are composed of the averaged neutron
data and the associated timing, and spatial information for
each 5-degree latitude by 5-degree longitude spatial and
15-degree Ls temporal interval. The averaged neutron data
consists of 11 columns of data that are output from the AND
processing for each spatial and temporal cell. The timing and
spatial data provided with the AND includes center points of
the spatial cell and the beginning and ending times that
data was averaged over.
Ancillary Data
==============
None at this time.
Coordinate System
=================
The coordinate system used for all GRS data is a Mars
aerocentric system following the IAU convention
[SEIDELMANNETAL2002], with east longitudes from 0 to 360.
Software
========
A library of source code to parse the AND data product files
is included in the software directory. This library allows a
programmer to build applications that display or manipulate
AND data. This source is written in the Java language, and
requires version 1.3 of the Java Runtime Environment (JRE) or
Java Software Development Kit (SDK).
Documentation for the code is located in the software directory
in the file GRS_CODE_DOC.ZIP. The contents of this file are
described in the label GRS_CODE_DOC.LBL and the source and
the binary classes that make up the library are in the file
DR_CODE.JAR.
Media/Format
============
The AND will be delivered using CDROM media. Formats will be
based on standards for such products established by the
Planetary Data System (PDS) [PDSSR2001].
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