Secondary Fluorescence Issues in Electron Probe Microanalysis

EPMA is recognized as a mature microanalytical technique and an essential tool in characterizing a range of materials, particularly of relevance to geological or material science investigations. Primary high energy electrons impact the target material, with inner shell ionizations yielding characteristic x-rays, with additional continuum x-rays generated (creating "the background").

An "interaction" volume defining where these primary x-rays are generated can be easily calculated using a Monte Carlo program or equation found in many texts. The size is a function of (1) accelerating voltage (e.g. 15 or 20 kV) and (2) the atomic number and density of the material. For many relevant geological materials, say for 15 kV, the depth/diameter is in the 1-3 micron range.

What is less understood is the phenomenon of "secondary fluorescence" -- that the primary x-rays generated in the primary interaction volume by the primary electrons, have "lives of their own" outside of the original interaction volume. Some of the characteristic and continuum x-rays will be captured by the WDS or EDS detector, but a large number are free (1) to impact the inner walls of the chamber and (2) others to travel through the target material for up to hundreds of microns. If other phases are present in the sample near the beam impact spot, and happen to contain elements of interest to the researcher, false conclusions can result from the creation of a second generation of x-rays. This is most troublesome when trace elements are the object of interest.

One way to understand the extent of this problem, is to create a couple (two different materials intimately touching each other) and experimentally measuring the secondary fluorescence away from the boundary. Creating these couples is not trivial. Another method is now available, a Monte Carlo program -- PENEPMA -- which explicitly tracks all x-rays and determines which are due to secondary fluorescence. Here I present results from several simulations that students in my EPMA class and I have run, demonstrating the level of secondary fluorescence in several materials of interest to geologists and material scientists.

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