PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT INTERCHANGE_FORMAT = ASCII PUBLICATION_DATE = 2001-09-01 NOTE = "N/A" END_OBJECT = TEXT END 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