INSTRUMENTATION FOR TXRF AND GI-XRF - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 187

Figure3.24. Efficiency of typical Si(Li) ( ————— ) and HPGe detectors (—————) calculated as
dependent on the energy of the indicated photons. The Si(Li) detector was assumed to have a 3 mm
thick Si intrinsic region and the HPGe detector to have a 5 mm high-purity Ge crystal. Both
detectors are assumed to be provided with a 20 nm thin gold contact and a 7.5 μ m thin Be window.
Figure from Ref. [2], reproduced with permission. Copyright1996, John Wiley and Sons.
50 keV. 5 On the other hand, the replacement of the gold layer by an aluminum
layer leads to a somewhat better efficiency for low photon energies < 8 keV.
The low efficiency at photon energies below 1 keV is caused by the beryllium
window even if it is only 7.5 μ m thick. This window, however, can be replaced
by an ultrathin polymer foil, for example, a 0.5 μ m thin polyimide foil [53,54].
Figure 3.25 indicates a shifting of the 10% efficiency from 0.7 keV photons
down to 0.2 keV photons (see also Ref. [58]). The steps in the curves can be
assigned to the M edges of gold, to the K edges of aluminum and of silicon, and
to the K edges of C, N, and O of the window foil.
If the thin polymer foil has to be strengthened by a silicon grid the efficiency
may be reduced to about 80% in total. At photon energies below 5 keV, the low
efficiency can be diminished still further by an air path between the sample and
the detector. For a typical distance of about 5 mm, the overall efficiency is
reduced to 72% for 2 keV photons and to 13% for 1 keV photons. However,
this reduction can be avoided by applying a medium vacuum (100 Pa) by a
simple water-jet pump or by a helium-flush.
5 New detectors have been developed with an intrinsic region of 1 mm thickness so that the
efficiency for high-energy photons is extended to about 35 keV.