PIXE tomography of samples with inhomogeneous elemental composition

Abstract

Proton induced X-ray emission tomography (PIXET) can provide quantitative, three-dimensional maps of elemental composition in small (less than a few mm) samples with fine (in principle micron scale) spatial resolution. The concept of PIXET is similar to single photon emission tomography which produces cross-sectional concentration maps of photon emitting radioactive elements within a sample. In PIXET, the photon emitting “sources” are elements along the trajectories (rays) of the incident beam as it slows down in the sample. The number of X-rays detected from a particular element at a given location in a sample depends on the local proton energy dependent X-ray production cross-section and the attenuation of the X-rays from that location to the detector. X-ray mass attenuation coefficients and proton stopping powers are weighted by the local elemental fractions along the detected X-ray path or incident beam direction. Local areal densities needed to complete the calculation of ion energy loss or X-ray attenuation are determined using the complementary density map obtained from ion microtomography (IMT) data. A software package is described which reconstructs cross-sectional images of density and element concentration of data obtained from samples having inhomogeneous elemental composition. A code has also been developed which generates simulated IMT and PIXET sinograms from user-specified density and composition maps of test objects. Simulated data from test objects having inhomogeneous elemental composition have been used to study the quality of images produced by the reconstruction code. Limitations of the PIXET technique are addressed. PIXET and IMT reconstructions from measured experimental data are discussed.