The microanalysis of light elements using transmitted energy loss electrons

The use of transmitted energy loss electrons is shown to hold considerable promise for the elemental analysis of light elements. In this technique, those electrons which have lost energy in exciting characteristic inner shell atomic levels are detected rather than X-rays resulting from the decay of the excited levels. The advantages of this technique are (1) a large fraction (0.1-1) of the information-carrying energy loss electrons can be detected, and (2) for each excited level, one energy loss electron is produced independent of the fluorescent yield. Thus the technique potentially offers higher sensitivity than X-ray analysis. We have begun a program to evaluate this technique both theoretically and experimentally for electron probe devices. First, we have developed the necessary theoretical framework to make predictions concerning relevant quantities of elemental analysis such as the minimum detectable mass (MDM) and mimimum detectable mass fraction (MMF). The results of these calculations for thin specimens indicate a potential reduction in the MDM by up to three orders of magnitude and in the MMF by up to 500 through the use of transmitted energy loss electrons rather than X-rays; the advantages over X-ray detection being greater for lower atomic number. Second, we have begun experimental measurements to verify our predictions. These experiments were performed in a field emission scanning microscope with known limitations in collection efficiency, but the results indicate the validity of the basic assumptions and also aid in the design of an instrument which can fully exploit this technique. The experimental results obtained indicate the ease of detection of characteristic K-shell energy levels in elements as light as lithium and indicate the mass detectability of less than 10(-18)g.