EPMA using low-energy (0.1-1.0 keV) x-rays - an historical perspective

Studies with the electron-probe microanalyser using low-energy (<1 keV) X-r ays are reviewed with particular reference to analysis of the ultra-light e lements (atomic number Z < 11). The paper begins by describing the historic al development of methods for measuring low-energy X-ray spectra and goes o n to show that minimum detection levels achieved for ultra-light elements a re now comparable to those for heavier elements, in ideal circumstances abo ut 100 ppm using wavelength-dispersive spectrometry and about 1000 ppm with energy-dispersive spectrometry. Regarding quantitative analysis, the corre ction models developed during the 1960s worked fairly well in most cases bu t, because they were based upon an X-ray depth distribution function of lim ited validity, were unsatisfactory for the ultra-light elements. Since then , advances made using empirical curve-fitting methods to derive this functi on have led to accuracies of a few per cent. relative, close to figures for heavier elements, any limitations being most likely associated with the ac curacy of the mass absorption coefficients used rather than the models them selves. The merit of utilising low-energy radiation for surface analysis is discussed and it is shown that, by employing a low-voltage electron probe to restrict X-ray excitation to surface layers and recording low-energy X-r ay emission, surface films down to about 1 nm thickness may be investigated . Finally, examples are given to show the value of studying the shape of lo w-energy spectra as a means of identifying the phase of microstructural fea tures.