Computer Science – Databases
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...50r&link_type=abstract
Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 50
Computer Science
Databases
Abundance, Chemical Composition, Fast Neutrons, Moon, Lunar Composition, Lunar Environment, Lunar Geology, Gamma Ray Spectra, Gamma Ray Spectrometers, Lunar Prospector, Neutron Spectrometers, Thermal Neutrons
Scientific paper
The determination of elemental abundances is one of the highest science objectives of most lunar missions. Such multi-element abundances, ratios, or maps should include results for elements that are diagnostic or important in lunar processes, including heat-producing elements (such as K and Th), important incompatible elements (Th and rare earth elements), H (for polar deposits and regolith maturity), and key variable elements in major lunar provinces (such as Fe and Ti in the maria). Both neutron and gamma-ray spectroscopy can be used to infer elemental abundances; the two complement each other. These elemental abundances need to be determined with high accuracy and precision from measurements such as those made by the gamma-ray spectrometer (GRS) and neutron spectrometers (NS) on Lunar Prospector. As presented here, a series of steps, computer codes, and nuclear databases are needed to properly convert the raw gamma-ray and neutron measurements into good elemental abundances, ratios, and/or maps. Lunar Prospector (LP) is the first planetary mission that has measured neutrons escaping from a planet other than the Earth. The neutron spectrometers on Lunar Prospector measured a wide range of neutron energies. The ability to measure neutrons with thermal (E < 0.1 eV), epithermal (E about equal 0.1 - 1000 eV), and fast (E about 0.1-10 MeV) energies maximizes the scientific return, being especially sensitive to both H (using epithermal neutrons) and thermal-neutron-absorbing elements. Neutrons are made in the lunar surface by the interaction of galactic-cosmic-ray (GCR) particles with the atomic nuclei in the surface. Most neutrons are produced with energies above about 0.1 MeV. The flux of fast neutrons in and escaping from the Moon depends on es the intensity of the cosmic rays (which vary with solar activity) and the elemental composition of the surface. Variations in the elemental composition of the lunar surface can affect the flux of fast neutrons by about 25% , with Ti and Fe emitting more fast neutrons than light elements like O and Si. Most elements moderate neutrons to thermal energies at similar rates. The main exception is when neutrons scatter from H, in which case neutrons can be rapidly thermalized. The cross sections for the absorption of thermal neutrons can vary widely among elements, with major elements like Ti and Fe having high-capture cross sections. Some trace elements, such as Sm and Gd, have such large neutron-absorption cross sections that, despite their low abundances, can absorb significant amounts of thermal neutrons in the Moon. Because the processes affecting neutrons are complicated, good modeling is needed to properly extract elemental information from measured neutron fluxes. The LAHET Code System (LCS) can be use to calculate neutron fluxes from GCR interactions in the Moon. Lunar Gamma-Ray Spectroscopy: The main sources of planetary gamma-rays are the decay of the naturally occurring radioactive isotopes of K, Th, and U and the interactions of GCRs with atomic nuclei in the planet's surface. Most "cosmogenic" gamma-rays are produced by fast and thermal neutrons made in the planet's surface by GCRs, and their production rates can vary with time. Over 300 gamma-ray lines have been identified that can be emitted from planetary surfaces by a variety of production mechanisms. There exist nuclear databases that can be used to identify and quantify other gamma-ray lines. Use will be made of gamma-rays from major elements, particularly those from Si and O, that have not been routinely used in the past. The fluxes of gamma-rays from a given element can vary depending on many factors besides the concentration of that element. For example, the fluxes of neutron-capture gamma-rays in the planetary region of interest depend on (1) the total cross section for elements to absorb thermalized neutrons and (2) the H content of the top meter of the surface. The fluxes of the fast neutrons that induce inelastic-scattering and other nonelastic-scattering reactions can vary with the composition of the surface There are several key steps in preparing gamma-ray data into a form from which accurate elemental abundances can be determined. One needs to identify, quantify, and remove or correct for all backgrounds in the gamma-ray spectra. Among the more important of these backgrounds are features made by the decay of radioactivities made in the GRS by cosmic-ray particles and the prompt and decay gamma-rays emitted from the material surrounding the active elements of the LPGRS and from the LP spacecraft. Gamma-ray spectra obtained during the cruise to the Moon or those measured while LP was at various distances from the Moon can be used to distinguish features in the gamma-ray spectra that are from the Moon and those that are made on the LP spacecraft. Each background-corrected spectrum will be analyzed with existing gamma-ray spectral-unfolding codes to identify the energies and intensities of all peaks. These peaks will be examined when there are potential interferences in the analysis of a given gamma-ray line. Such interferences could be a problem for determining Mg and Al using some of their inelastic-scattering gamma-rays such as the 1.369-MeV gamma-ray from Mg that is also readily made from Al and Si. The key data needed to get elemental abundances from the fluxes of gamma-rays in the processed spectra are good values for the fluxes of gamma-rays that should be emitted from a given region for known or nominal elemental abundances. Such flux determinations were done for analysis of the Apollo gamma-ray data. The codes to do such calculations and the nuclear data used in such calculations have been improved much since then. Additional information contained in original.
Reedy Robert C.
Vaniman David T.
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