An X-Ray Microprobe Beam Line for Trace Element Analysis

Abstract

The application of synchrotron radiation to a x-ray microprobe for trace element analysis is a complementary and natural extension of existing microprobe techniques using electrons, protons, and heavier ions as excitation sources for x-ray fluorescence. This was first recognized by HOROWITZ and HOWELL [1] in their development of the first synchrotron radiation microprobe at the Cambridge Electron Accelerator. SPARKS, et al. [2] used a miniprobe beam at the Stanford Synchrotron Radiation Laboratory in an attempt to find natural occurring superheavy elements by x-ray fluorescence of characteristic L-lines. The ability to focus charged particles leads to electron microprobes with spatial resolutions in the sub-micrometer range and down to 100 ppm detection limits and proton microprobes with micrometer resolution and ppm detection limits. The characteristics of synchrotron radiation that prove useful for microprobe analysis include a broad and continuous energy spectrum, a relatively small amount of radiation damage compared to that deposited by charged particles, a highly polarized source which reduces background scattered radiation in an appropriate counting geometry, and a small vertical divergence angle of ~ 0.2 mrad which allows for focussing of the light beam into a small spot with high flux. The features of a dedicated x-ray microprobe beam line developed at the National Synchrotron Light Source (NSLS) are described.