PDS_VERSION_ID = PDS3
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PUBLICATION_DATE = 2001-09-01
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Written as part of the NEAR XGRS Planetary Data Systems
Submission. 9/21/01
GRS RESULTS
The results for the elemental ratios determined from
the gamma-ray measurements are given in Table 1. The
results for the iron to oxygen ratio using the Fe 7.631
MeV capture line and the O 6.129 MeV inelastic line as
derived for the four spectra are shown. The results for
the silicon to oxygen ratio using the Si 4.934 MeV capture
line and the O 6.129 MeV inelastic line are shown for
three spectra. The other composition results are derived
from the analysis of the anticoincidence spectrum. These
include the iron to silicon ratio from both inelastic lines,
the magnesium to silicon ratio from the inelastic lines, and
the potassium concentration from the 40K line. In addition
the iron inelastic to iron capture photon ratio and the silicon
inelastic to silicon capture photon ratio can be derived.
These do not contain any useful geochemical results, but are
an indication of the hydrogen content of the sample. The
gamma-ray calculations all assume zero hydrogen content and
derive the iron and silicon inelastic to capture photon ratios
on that basis. Comparison with measurements should indicate
the validity of that assumption. Smaller values of the
inelastic to capture ratios would indicate the presence of
hydrogen, even if it could not be detected by the neutron
capture line from hydrogen. Typical calculated values for
the ratio of the iron inelastic line (at 0.847 MeV) to the
iron capture line (at 7.631 MeV) for many meteorites are
around 1.8, though values can range to over 3 compared to
a derived value of 2.9. Typical calculated values for the
ratio of the silicon inelastic line (at 1.779 MeV) to the
silicon capture line (at 3.539 MeV) for many meteorites are
about 10.5, though values can range up to 13 compared to a
derived value of 11.9. The high values for these ratios
are typically for iron-nickel rich meteorites (e.g.,
mesosiderites). Because these ratios are derived from a
low energy gamma ray and a high energy gamma ray, these
ratios are dependent on the accuracy of the calculated
efficiencies. Considering these uncertainties, the two
derived values for the inelastic to capture ratios are both
in reasonable agreement with the calculated values and
indicate the assumption of zero hydrogen is valid. Similarly,
these ratios indicate that there are not significant
concentrations of elements such as Cl, Ti, Sm, and Gd that
depress the fluxes of thermal neutrons.
Realistic uncertainties for these derived compositions
are difficult to estimate. The statistical uncertainties
can be derived from the measurements and the background
subtraction. These statistical uncertainties tend to be
small, typically between 5 and 10 percent. Another source
of uncertainty is due to the response functions that may
not be linearly independent and the spectral shapes can
interfere with on another in fitting the data (Reedy et
al., 1973). Another uncertainty is in the calculation
of the detector efficiency. While efficiencies for
gamma ray ratios close in energy are probably valid, those
far apart in energy have an additional uncertainty associated
with the result. Perhaps the best measure of the uncertainty
in determining the elemental ratios is the spread in the
result for the same ratio. For the iron to oxygen ratio
there are four values calculated independently ranging
from 0.16 to 0.44 with a mean value of 0.28 and a standard
deviation of 0.12 (40 %). The other result that has multiple
values is the silicon to oxygen ratio with a mean value of
0.61 and a standard deviation of 0.05 (8%), but without a
value from the BGO spectrum. Because of the poor energy
resolution, the BGO spectrum is more difficult to analyze
and only the results for the highest energy lines were
determined. As a conservative approach, we assume that
this 40% uncertainty applies for all the GRS results.
Table 1. GRS Composition Results
Result
Anticoin First Escape Sec Escape BGO
Fe(7631)/O(6129)
Photon ratio 0.79 0.43 0.42 0.32
Composition ratio 0.44 0.27 0.26 0.16
Si(4934)/O(6129)
Photon ratio 0.24 0.28 0.30
Composition ratio 0.53 0.63 0.67
Fe(847)/Si(1779)
Photon ratio 0.65
Composition ratio 0.8
Mg(1369)/Si(1779)
Photon ratio 0.75
Composition ratio 0.75
K(1461)
Photons/s 0.11
Composition 0.07%
Fe(847)/Fe(7631)
Photon ratio 2.9
Si(1779)/Si(3539)
Photon ratio 11.9
Note: Energies of gamma-ray lines in parenthesis are
given in keV