INSTRUMENTATION FOR TXRF AND GI-XRF - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 193

TABLE3.9.CharacteristicDataofDifferentEnergyDispersiveX-RayDetectors

Detector e pair or e ion (eV) Fano Factor F Δ E electronic (eV)

Ar gas-filled 26.4 0.8 100

NaI scintillation 200 1.0 200

Si(Li) 3.86 0.115 55

SDD 3.86 0.115 25

HPGe 2.96 0.10 60

differ for each individual detector on a 10% level. The solid-state detectors Si
(Li), SDD, and HPGe show very similar characteristics. Note that the HPGe
detector exhibits a better resolution than the Si(Li) or the SDD detector but the
differences are on the 10% level. While the HPGe is better suited for the
detection of higher photon energies, the SDD can even detect photons with
energies down to 0.2 keV. For comparison, a gas-filled or a scintillation detector
has a much poorer spectral resolution. For that reason, they normally are not
used alone but only in combination with a Bragg crystal of a WDS.
The characteristics of five Bragg crystals are also illustrated in Figure 3.29.
Their peak or line width is only between 0.3 and 100 eV for photons with
energies between 0.1 and 10 keV. In this spectral region, their spectral resolu-
tion is very good. The energy-dispersive detectors are much poorer in resolu-
tion than a WDS with Bragg reflectors; however, their resolution is amply
sufficient to separate the K α peaks of neighboring elements in the periodic
system. A good spectral resolution is important for the separation of neigh-
boring peaks (see Section 4.3.2). It is also recommendable for a reduction of the
spectral background with respect to low detection limits (see Section 6.1.2).
Conventionally, the spectral resolution 6 is only specified by the Mn-K α peak
with a photon energy of 5.9 keV. This quantity can simply be measured by
means of the radioactive isotope iron-55 or 55 Fe, which decays to 55 Mn (half-
life 2.7 years) emitting the Mn-K α peak. Of course this is a simplification of the
complex relationship represented by Figure 3.29. However, the single value of
the peak width of Mn-K α is well suited to characterize the influence of the size
of the detector crystal. The size of the frontal area determines its capacitance
and influences the noise component Δ E electronic [57,68]. Table 3.10 presents a
survey of crystals of different sizes and their influence on the resolution for Si
(Li) and HPGe detectors [68].
The spectral resolution is usually determined at a low photon flux or count
rate below 1000 cps. However, the resolution of an EDS is also dependent on
this flux of the incident X-ray photons. Section 3.8.2.3 considers this

dependence.

6 The smaller the peak width, the better is the spectral resolution. For that reason, the peak width
may not be suited as an ideal measure of the spectral resolution. The ratio of photon energy and
peak width, defined as a relative measure, R ( E ) = E / Δ E , may be better suitable.