Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 266
provide information regarding chemical composition [5]. The different types of elements
present (qualitative) can be identified by the energy (and hence wavelength) of the X-rays
emitted, and the amount of each element present (quantitative) can be obtained by the
number of X-rays of a particular energy detected per unit time [5].
Kα1 and Kα2 X-rays (the K-doublet) are around eight times more intense than Kβ X-
rays, and are therefore used to identify lighter elements. For atoms heavier than tin (Z =
50), electrons with an energy greater than 25 keV are needed to excite K lines, therefore
L lines (or M lines for very heavy elements) provide a suitable signal for identification
[5]. All elements have at least one strong X-ray line of energy < 10 keV. The most
efficient production of X-rays occurs when the bombarding electrons have around three
times the energy. Therefore an incident electron beam of energy 25-30 keV should
produce suitable X-rays from the sample for chemical analysis [5].
The relative amounts of characteristic X-rays emitted from elements of similar atomic
number may be affected by a phenomenon known as fluorescence. This occurs when X-
rays travelling through the sample excite other atoms, which emit X-rays of slightly
lower energy. Even though the process is not particularly efficient, it can lead to
complications when trying to accurately quantify the amount of each element present in a
sample which contains elements of similar atomic number [5].
The size of the X-ray sampling volume is virtually identical to the interaction volume.
Therefore, the smallest volume which can be analysed is typically 1 µm 3 . Reducing this
volume would require reducing the beam energy, which in turn would result in an
insufficient amount of X-rays emitted for analysis [5]. Therefore, accurate quantitative
analysis is complex [5].