Functional principles
22
User Manual
An X-ray spectrum usually does not just contain the fluorescence lines of the sample material but
also excitation radiation, which has been scattered by the sample material.
3.4 Spectroscopic Resolution
The natural width of fluorescence lines is normally < 10 eV and variations depending on the type
of line series (K, L, M) and the chemical bond type of the atom are negligible.
Due to different influences from the detector and signal electronics a distinct line broadening of
the fluorescence lines is visible. This effect is caused by a physically determined error during the
registration of the X-ray quanta. Main contributions are the thermal noise of the detector and the
subsequent amplification electronics. Mono-energetic X-ray quanta are thus not displayed as
sharp lines in the displayed spectrum but spread to a Gaussian-type curve.
The most important parameter for the description of this essential detector feature is the Full
Width at Half Maximum (FWHM), which defines the peak width at the half peak maximum. The
FWHM value depends mainly on the following parameters:
design of the detector and the applied electronic components
temperature of the detector
configuration of the spectroscopic amplifier (shaping time)
input pulse rate
energy of the X-ray quanta
In order to cope with the fact that FWHM values increase with energy as well as to compare
different X-ray detection systems, it is agreed to use the Mn-Kα as a reference line of the FWHM.
This line is used because the radioactive isotope Fe55 emits this radiation. Fig. 4 shows the
fluorescence spectrum of a Mn sample and the definition of the FWHM for a SDD.
Fig. 4 Definition of FWHM
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