Overlap corrected on-line PIXE imaging using the proton microprobe

Abstract

Conventional methods for PIXE imaging have used the counts recorded in a simple energy window set on an X-ray peak to approximate the image of an element. This method produces image artefacts due to the overlap of the X-ray lines of interfering elements, detector artefacts, such as peak-tails, pile-up and escape peaks, and continuum background. This paper reports on continuing development to solve this problem, based on the construction of a matrix transformation that transforms directly from PIXE spectrum vector to elemental concentration vector. This transformation provides a fast method for extracting on-line quantitative estimates of the concentration of a sample while still under the proton beam; the method has been called Dynamic Analysis to reflect this capability. If the spectrum is replaced by a single count in a channel, an event recorded at a point in a raster scan of the proton beam over a target, then the resultant concentration vector is the set of increments to make to all elemental images at that pixel. PIXE images accumulated in this way are inherently (i) quantitative (accumulated in ppm μC), (ii) overlap-resolved and background-subtracted, and (iii) can be formed directly on-line. The method is under development at the CSIRO and in routine use at the NAC using a simple procedure involving the fitting of a preliminary scan spectrum to build the transform matrix; spectrum fitting and matrix construction are part of the GeoPIXE software package. This paper outlines the method, reports various tests of it, and presents recent examples of its application to major and trace element imaging in geology.