X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

II.4.2 Scintillation Counters
The scintillation counter, “SC”, used in XRF comprises a sodium iodide crystal in
which thallium atoms are homogeneously distributed ‘NaI(Tl)’. The density of the crystal is
sufficiently high to absorb all the XRF high energy quants. The energy of the pervading X-ray
quants is transferred step by step to the crystal atoms that then radiate light and cumulatively
produce a flash. The amount of light in this scintillation flash is proportional to the energy that
the X-ray quant has passed to the crystal. The resulting light strikes a photocathode from
which electrons can be detached very easily. These electrons are accelerated in a
photomultiplier and, within an arrangement of dynodes, produce so-called secondary
electrons giving a measurable signal once they have become a veritable avalanche (Figure
IV.11). The height of the pulse of voltage produced is, as in the case of the gas proportional
counter, proportional to the energy of the detected X-ray quant.
Figure IV.11: Scintillation counter including photomultiplier.
II.4.3 Pulse Height Analysis (PHA), Pulse Height Distribution
If the number of the measured pulses (intensity) dependent on the pulse height is
displayed in a graph, we have the ‘pulse height spectrum’. Synonymous terms are: ‘pulse
height analysis’ or ‘pulse height distribution’. As the height of the pulses of voltage is
proportional to the X-ray quants energy, it is also referred to as the energy spectrum of the
counter (Figure IV.12a and IV.12b). The pulse height is given in volts, scale divisions or in
‘%’ (and could be started in keV after appropriate calibration). The “%”-scale is defined in
such a way that the peak to be to be analyzed appears at 100 %.