M spectra of the rare earth elements measured with an ultra-thin window Si(Li) detector

The oxides of the rare earth elements 57≤Z≤71 were excited by electrons with an energy between 3 and 15 keV. X-rays were detected by an energy dispersive Si(Li) spectrometer, with an ultra-thin polymer entrance window. Due to the limited resolution of this type of spectrometer, the M spectra of the rare earths appear to consist of only four peaks: Mζ, Mαβ, Mγ, and M2N4. The net height of these peaks relative to that of Mαβ was used as a measure of the relative intensity. For 3 keV electrons, a continuous decrease of the relative intensity of Mζ with increasing Z was observed, ranging from approximately 90 % for 57 La to less than 10 % for 71 Lu. This behavior is in agreement with a model involving a gradual filling up of the levels N6 and N7 with increasing Z. The relative intensity of Mγ is lower than that of Mζ by about a factor of 5, whereas M2N4 is approximately half as intense as Mγ. (author)