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Jeff Vanhoy

Ion Beam Analysis

Elemental and Isotopic Analysis of Meteorites

Chemical and accelerator-based elemental analysis techniques are used to determine the chemical composition of materials. Chemical techniques are time consuming and require much handling of the sample. Chemical techniques also destroy the sample, and this is a great disadvantage for rare materials. The popularity of accelerator-based techniques such as PIXE (proton induced X-ray emission) is currently growing rapidly because the sample remains intact and undamaged. Both these techniques determine elemental compositions of samples.

In some instances, it is useful to also know the isotopic composition of materials. Any variations in isotopic abundance which deviates from "standard" abundances may Control console and accelerator. be due to 1) a sensitivity of chemical reaction rates to the isotopic mass of the reactants, 2) sensitivity of physical processes (transport, crystallization, etc) to the isotopic composition, or 3) differences in radiation environment.

The ground work for this project was laid during the Spring 1993 semester with SP490 research student 1/C Matt LaBonte. We wish to compare the isotopic composition of elements found in meteorites to the "standard" isotopic composition of elements found on Earth. The use of meteorites is unique because it allows one to search for variations throughout the solar system and from different depths (core/mantle) inside their parent planetary body. We are focusing on a particular class of meteorites, the carbonaceous chondrites, because they are thought to contain large amounts of primitive material predating the formation of the solar system (~4 billion years old). Such primitive material has the potential providing a window to the early solar system and contains a radiation record of the distant past.

sample chamber and X-ray detector We are exploring various techniques for extracting the desired abundance information. First, we are using the PIXE technique to determine the elemental composition of the sample. Nuclear reaction techniques are required to extract the isotopic information. We are considering neutron activation (with the assistance of Prof Nelson, NAOME), inelastic neutron scattering, and proton capture. Each technique has its merits and draw-backs. The neutron activation technique must battle the orders-of-magnitude variation in neutron capture cross sections between neighboring nuclei, but is not confined to a particular region of the Periodic Table. The proton capture technique probably does not suffer from order-of-magnitude variation in capture, but is confined to elements lighter than ~ Zinc. The inelastic neutron scattering technique, does not suffer from either of these effects, and may be the most promising.

In either case, isotopes are identified by their unique gamma ray de-excitation signatures. The intensity of the g-rays will be unfolded to yield composition of the sample. The ultimate goal would be to develop a portable method for making such measurements -- using a PuBe or AmBe neutron source.

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