Development of Techniques for 3D Imaging with Focused Ion Beams - Hydrogen Depth Profiling and Microtomography with Applications in Geology

University dissertation from Division of Nuclear Physics Department of Physics Lund University Box 118 SE-221 00 Lund Sweden

Abstract: In nuclear microprobe experiments, high resolution maps of the measured parameter, e.g. element- or mass distribution, in a sample can be produced with a variety of ion beam analytical techniques. The principle underlying all the techniques is that ions at MeV energies are used as projectiles to cause interactions with the target material. Because of the limited range of ions in matter, there is a maximum analytical depth for every sample. This means that either the samples must be restricted to micrometre thickness, or that information can be obtained from only the outer few micrometres of a thick sample. For samples with complex structure, standard, two-dimensional maps do not always deliver full and satisfactory information. Then the possibility of three-dimensional analysis can offer valuable additional information. This work focuses on techniques for obtaining depth profiles and three-dimensional information, both of element distribution and mass distribution, mainly in geological samples. A method for depth profiling of hydrogen, with the elastic proton-proton scattering ?technique? is described. Due to the detection of protons in coincidence at certain angles, the method is highly specific for hydrogen and offers sensitivity in the ppm region. This makes the method especially suited for the analysis of hydrogen in anhydrous minerals, which contain ppm levels of hydrogen in the bulk, and are difficult to analyse due to the care one has to take not to damage or alterate the sample due to the volatility of hydrogen. The depth profiling capacity of the method is used to study the bulk hydrogen content away from the influence of the ever present surface contamination. Also for the study of zonations in minerals the method is of great importance and benefit. The second part of the work describes the development of a system for microtomography at the Lund nuclear microprobe. This system is dedicated to the analysis of microscopic samples with complex structures, and can provide information of the mass distribution via scanning transmission ion microscopy technique (STIM) and is well suited for future particle-induced X-ray emisson (PIXE) tomography for qualitative and/or quantitative imaging of the element distribution. Tomographic experiments have been performed on test samples, to determine and optimise experimental parameters and test the reconstruction technique on real experimental data. Also, the tomography system has been used in a study of the porosity in the clay material bentonite. Here it is demonstrated that, in a combination, PIXE, hydrogen analysis and microtomography with the STIM technique, can provide unique information on the internal structure and element distribution in a microscopic sample.

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