Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 211

Characteristic X‐rays excited by high‐energy electrons can be recorded either by energy‐dispersive X‐ray spectroscopy (EDXS) or by wavelength‐dispersive X‐ray spectroscopy (WDXS). Stemming from electronic recombination, the X‐rays generated fingerprint the electronic structure of the excited element, thus making quantitative analyses possible. Soft X‐rays (e.g. emitted by light elements) are subject to absorption on their way out of the sample, however, so with decreasing atomic number, the emission of Auger electrons counteracts that of X‐rays. Hence, quantification for elements with atomic numbers smaller than 13 requires extraordinary diligence.

In EDXS, a semiconductor detector is used to count electron‐hole pairs generated by the impinging X‐rays. The spectral resolution is typically on the order of 130 eV, so in a few cases overlapping emission lines cannot be clearly separated: this will, for instance, apply to the Ti K_α1 and Ba L_α1 at about 4.51 and 4.47 keV, respectively. This problem is less serious with WDXS (with a spectral resolution of ≈10 eV), where Bragg’s law of selective reflection forms the basis of detection in a proportional counter. Whereas EDXS is a parallel‐registering technique, WDXS (which is very similar to electron microprobe analysis [EMPA]) is a serial technique, making it more time consuming and less commonly used than EDXS.

For a sufficient yield of X‐rays of a certain energy, as a rule of thumb, the primary electrons should have a kinetic energy of at least twice that amount. As an example, Si‐K_α X‐rays possess an energy of 1.74 keV, which is similar to the difference between the Si‐K and the Si‐L_2,3 energy levels in the electron shell of the Si atom. Thus, one needs to use at least 3.5 kV acceleration voltage to be able to detect Si in a glassy sample by means of EDXS or WDXS, let alone any heavier elements. Low‐voltage excitation in SEM might thus be advisable for imaging purposes, but not for chemical analysis.