INSTRUMENTATION FOR TXRF AND GI-XRF - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 188

Figure3.25. Efficiency of a new SDD in comparison to that of a Si(Li) detector (see Figure 3.24)
dependent on the energy of the indicated photons. The SDD is made of a 0.5 mm thick high-purity
Si crystal. Its front side may be metallized by a 30 nm thin Al layer. The whole sensor is usually
protected by a 7.5 μ m thin Be window (- - - - - -) or by a 0.5 μ m thin polyimide foil (————). The Al-K
edge at 1.559 keV of the front layer and the Si-K edge at 1.839 keV of the dead layer can clearly be
recognized. The edges at 0.284 keV, at 0.410 keV, and at 0.544 keV come from the elements C, N,
and O of the polyimide foil.
The efficiency of gas-filled and scintillation detectors is essentially unity or
100% in the energy range between 20 and 100 keV. For gas-filled detectors it
remains > 10% for lower energies down to 0.1 keV. For a typical WDS the path
length may be about 20 cm and the transmission of X-rays in such an air path is
< 4% for energies below 3 keV but can distinctly be increased by applying a
high vacuum of 1 Pa to > 90% for energies > 0.1 keV.
3.8.2.2SpectralResolution
The different characteristic X-ray peaks of an EDS are not infinitely small but
cover an energy range of some 100 eV. The histograms usually recorded show
peaks that span about 10-20 channels, with nearly a Poisson or Gaussian
distribution. Each peak can be characterized by a width defined as FWHM.
The peak width is mainly caused by the production of electron-hole pairs
because of the incident photons. This process is not the only one possible;
rather, it competes with the generation of lattice vibrations or the emission of
phonons (heat radiation), for example. Without such a competing process, the
necessary energy for the generation of an electron-hole pair would amount to
the band gap energy of 1.1 eV for silicon and about 0.7 eV for germanium.
Because of the competing processes, however, the average energy consumed
per electron-hole pair is greatly increased: to about 3.8 eV for silicon and