Functional principles
20
User Manual
Depending on the ionized shell, the radiation is labelled K-, L- or M-radiation and, depending
on the refilling shell, an additional indication, e.g. Kα1,2, Kβ1, Lα1, describes the relative
intensity of the lines of one line series and depends on the level where the electrons are
coming from.
Fluorescence lines can only be excited with energy lines that are higher than the fluorescence
line itself. Thus, e.g. the Au Lα -lines of about 9.5 V to 13 keV can only be excited with
radiation of higher energies than the Au-L absorption edge.
Another component of radiation in case of excitation with electrons is the continuous brems-
strahlung which is generated due to the stopping of highly accelerated electrons. Their energy will
then be emitted in steps of different sizes as electro-magnetic radiation i.e. a continuous radiation
is emitted.
This continuous radiation is emitted by the tube. The tube radiation, both characteristic radiation
of the tube material (for the M4 TORNADO typically Rhodium) and the bremsstrahlung, excites
the sample to emit only characteristic radiation. This reduces the spectral background of the
excited spectrum. The background in X-ray excited spectra is mainly a result of scattering of tube
radiation on the sample. Therefore tube excited spectra have a better peak-to-background ratio
and allow for better sensitivity.
3.2 Detection of X-ray Quanta
The emitted radiation of the sample hits the detector. There it is absorbed and generates charges.
In the SDD, electrical charges are generated as holes and excited electrons. These charges are
collected by the electrical field and then amplified and filtered to reduce the different noise
contributions. A more detailed description of the detection process is given in the reference
manual Physical Principles of Micro-XRF.
3.3 Digitalization and Channel Allocation
In order to facilitate element identification and quantification, the amount of X-ray quanta
measured by the detector must be converted into a digital spectrum (see Fig. 3). For this purpose,
the measured energy range is divided into sections, the so-called channels. Concerning a
measured range of 40 keV and 4096 channels, one channel corresponds to an energy range of
about 10 eV.
For better spectra evaluation the electronics generates a peak at exact energy zero. This peak
can be used for energy calibration, but also for the calibration of energy resolution and of dead
time determination.
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