PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT INTERCHANGE_FORMAT = ASCII PUBLICATION_DATE = 2001-09-01 NOTE = "N/A" END_OBJECT = TEXT END MAPSGRSFIG_10_11.TXT Written as part of the NEAR XGRS submission to the Planetary Data System - the surface calibration procedure. Gamma-ray Detector Efficiency ----------------------------- The intrinsic peak efficiency of a gamma-ray detector is defined as the number of counts in a peak per photon incident on the detector. Preflight calibrations of the NEAR GRS were made to determine the efficiency of the NaI and BGO detectors as a function of energy and angle (Evans et al. 2000). Radioactive sources ranging in energy from 320 keV to 6.129 MeV were used. In addition, a 252Cf neutron source was used to irradiate H, Si, and Fe targets that produce lines at 2.223, 4.934 and 7.631+7.646 MeV, respectively. Measurements were taken at angles ranging from 0 degrees to 90 degrees with respect to the long axis of the detector. For orbital measurements the efficiency at 0 degrees is adequate to describe the response of the detector. However, on the surface of Eros the GRS detects gamma rays from the asteroid at angles up to 90 degrees, corresponding to a nearly 2( geometry. With the spacecraft resting on the surface of Eros, the angle of the GRS with respect to the surface normal was determined to be 18 degrees. To calculate the detector response on the asteroid's surface we did a weighted average of the measured efficiencies as a function of angle and energy. The 18 degree offset from the surface normal is included, but in fact has only a small effect. Such a tilt angle would only become important if it exceed the half-opening angle of the NaI, which is about 25 degrees. Total efficiencies were calculated for the NaI photopeak, first escape, and second escape spectra and are shown in Figure 10 (Evans et al., 2001). The photopeak efficiency is significantly lower than that given in Evans et al. (2000) at 0 degrees, though the shapes of the curves are similar. The decrease is due to the impact of the BGO shield. As explained above, on the surface of Eros the viewing geometry is nearly 2( steradians. However, the opening angle of the BGO collimator corresponds to just 0.6 steradians, so about 90% of the surface gamma-ray flux passes through the shield before being detected by the NaI. On the surface of Eros, the NaI "sees" the asteroid mostly through the BGO shield, thus reducing the total efficiency. The BGO efficiency on the surface was calculated in a similar manner. Conversion of Photons to Composition ------------------------------------ To convert the measured gamma-ray-flux ratios to abundance ratios, a series of calculations were done. These gamma-ray- production calculations were done in the same way that gamma-ray fluxes were calculated for Mars (Masarik and Reedy, 1996) and Mercury (Br�ckner and Masarik, 1997). The first calculations used the LAHET Code System (LCS) to numerically simulate the interactions of galactic-cosmic-ray (GCR) particles with the surfaces of asteroids of various compositions and the subsequent production and transport of the neutrons and protons that make gamma rays. The flux of GCR particles averaged over a solar cycle was used. These particle fluxes then were used to calculate the rates for producing gamma rays as a function of depth. The cross sections for nuclear reactions producing gamma rays by inelastic-scattering reactions and intensities of gamma rays made by neutron-capture reactions were those used earlier for the Moon and Mars (Reedy, 1978). The gamma rays made in the asteroid's surface by these particles were then transported to the asteroid's surface. Only the fluxes of gamma-ray lines that escaped the surface without undergoing an interaction were considered. Calculations of gamma-ray productions were done for a variety of meteorite compositions, including compositions of individual meteorites that were chosen to span the range of compositions expected, particularly for iron the abundances. These compositions were taken from the meteorite database (Nittler et al., 2001a). An assumed density of the meteorites (about 3 g/cm3) was used, although several calculations done with a density of 1 g/cm3, which is about that expected for the surface of an asteroidal regolith, gave the same results. The averaged lunar composition of Reedy (1978) was also used for comparisons with previous gamma-ray-flux calculations (Masarik and Reedy, 1996). The surface was assumed to be of uniform composition both vertically and horizontally. The meteorite compositions used in the calculations assumed no H or C, which can rapidly thermalize energetic neutrons and significantly increase the fluxes of gamma rays made by neutron-capture reactions (Masarik and Reedy, 1996). The assumed compositions also excluded such neutron-absorbing elements as Cl, Sm, and Gd that can reduce the fluxes of neutron-capture gamma rays from other elements (Reedy, 1978). In some cases (e.g., C in ureilites), these abundances were included in the data of Nittler et al. (2001a) and intentionally excluded from the calculations. In other case (e.g., Sm, Gd), abundances were not known. The major neutron-capture gamma rays expected from Eros are the 7.646 and 7.631 MeV gamma rays from Fe and the 4.934 and 3.539 MeV gamma rays from Si. The fluxes of gamma rays made by inelastic-scattering reactions, such as those at 0.847 MeV from Fe, 1.779 and 6.878 MeV from Si, 1.369 and 2.754 MeV from Mg, and 6.129, 6.917, and 7.117 MeV from oxygen are only weakly affected by the bulk composition. High concentrations of elements with larger atomic masses, such as Fe, result in higher fluxes of the fast neutrons that make such inelastic-scattering gamma rays (Gasnault et al., 2001). The flux of the 1.461 MeV gamma ray made by the decay of naturally radioactive 40K was also calculated. The agreement with previous calculations done using LCS is good, and these calculations are consistent with earlier work. As noted in Masarik and Reedy (1996), gamma-ray-flux calculations done with LCS differ slightly from those done using other models for GCR interactions, such as those of Reedy (1978). As the flux of GCR particles at Eros was low and not well known during the NEAR-Shoemaker mission, ratios of gamma-ray fluxes to ratios of elemental abundances were calculated for the cosmogenic gamma rays. For two gamma rays that are both made by the same type of nuclear reactions (inelastic scattering or neutron capture), e.g., the ratio of the fluxes of the 0.847 and 1.779 MeV inelastic-scattering gamma rays to the Fe/Si abundance ratio, such trend lines are very linear. There is some scatter about the trend line for the ratio of the 7.631 MeV Fe capture gamma rays to the 6.129 MeV oxygen inelastic- scattering gamma ray as a function of the iron to oxygen abundances in the meteorite models. The compositions that were the farthest from the trend line were those for the Moon that had significant amounts of the neutron-absorbing elements Ti, Sm, and Gd. In those cases, the flux ratios were lower than the trend line. There are two examples of results from these calculations. Figure 11(a) (Evans et al, 2001) gives the relationship between the ratio of the Fe capture line at 7.631 MeV to the oxygen inelastic line at 6.129 MeV compared to the ratio of iron to silicon in the meteorite models and a fit to the data. This ratio is particularly important as it can be determined independently from each of the four spectra. Figure 11(b) (Evans et al, 2001) gives the relationship between the ratio of the Fe inelastic line at 0.847 MeV to the Si inelastic line at 1.779 MeV and the ratio of iron to silicon in the meteorite models and a fit to the data. Calculations for one lunar (Reedy, 1978) and two Mercury compositions (Br�ckner and Masarik, 1997) are included for comparison. These conversion curves are used to convert the measured photon ratios to composition ratios. References ----------- Br�ckner, J. and Masarik, J., (1997) Planetary gamma-ray spectroscopy of the surface of Mercury. Planet. Space Sci. 45, 39-48. Evans L. G., Starr R. D., Trombka J. I., McClanahan T. P., Bailey S. H., Mikheeva I., Bhangoo J., Br�ckner J., and Goldsten J. O. (2000) Calibration of the NEAR gamma-ray spectrometer. Icarus, 148,95-117. Evans L. G., Starr, R. D., Br�ckner J., Reedy, R. C., Boynton, W. V., Trobmka J. I., Goldsten, J. O., Masarik, J., Nittler, L. R., McCoy T. J., (2001) Elemental composition from gamma-ray spectroscopy of the NEAR- Shoemaker landing site on 433 Eros, submitted to Meteorit. Planet. Sci. Gasnault O., Feldman W. C., Maurice S., Genetay I. and d'Uston C. (2001) The first lunar map of the average soil atomic mass (abstract). Lunar Planet. Sci. 32, #1963. Lunar and Planetary Institute, Houston, Texas, USA (CD-ROM). Masarik, J. and Reedy, R. C. (1996) Gamma ray production and transport in Mars. J. Geophys. Res. 101, 18,891-18,912. Nittler, L. R., McCoy T. J., Clark P. E., Murphy M. E., Trombka J. I., and Jarosewich E. (2001a) Bulk element compositions of meteorites: Linking asteroids and meteorites and understanding their formation, submitted to Meteorit. Planet. Sci. Reedy R. C., (1978) Planetary gamma-ray spectroscopy. Proc. Lunar Planet. Sci. 9th, 2961-2984.