PIXE TOMOGRAPHY OF SAMPLES WITH INHOMOGENEOUS ELEMENTAL COMPOSITION

Proton induced X-ray emission tomography (PIXET) can provide quantitat ive, three-dimensional maps of elemental composition in small (less th an a few mm) samples with fine (in principle micron scale) spatial res olution. The concept of PIXET is similar to single photon emission tom ography which produces cross-sectional concentration maps of photon em itting radioactive elements within a sample. In PIXET, the photon emit ting ''sources'' are elements along the trajectories (rays) of the inc ident beam as it slows down in the sample. The number of X-rays detect ed from a particular element at a given location in a sample depends o n the local proton energy dependent X-ray production cross-section and the attenuation of the X-rays from that location to the detector. X-r ay mass attenuation coefficients and proton stopping powers are weight ed by the local elemental fractions along the detected X-ray path or i ncident beam direction. Local areal densities needed to complete the c alculation of ion energy loss or X-ray attenuation are determined usin g the complementary density map obtained from ion microtomography (IMT ) data. A software package is described which reconstructs cross-secti onal images of density and element concentration of data obtained from samples having inhomogeneous elemental composition. A code has also b een developed which generates simulated IMT and PIXET sinograms from u ser-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 reconstructi on code. Limitations of the PIXET technique are addressed. PIXET and I MT reconstructions from measured experimental data are discussed.