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

SECTION II
ENERGY DISPERSIVE X-RAY FLUORESCENCE
(ED-XRF)
I. Introduction
In Energy Dispersive X-Ray Fluorescence spectrometry (ED-XRF), the identification of
characteristic lines is performed using detectors that directly measure the energy of the
photons. In the simplest case an electron is ejected from an atom of the detector material by
photoabsorption. The loss of energy of this just created primary electron results in a shower of
electron-ion pairs in the case of a proportional counter, optical excitations in the case of
scintillation counter, or showers of electron-hole pairs in a semiconductor detector. The
resulting detector signal is proportional to the energy of the incident photon, in contrast to
wavelength dispersion in which the Bragg reflecting properties of a crystal are used to
disperse X-rays at different reflection angles according to their wavelengths. Although energy
dispersive detectors generally exhibit poorer energy resolution than wavelength dispersive
analyzers, they are capable of detecting simultaneously a wide range of energies.
The most frequently used detector in EDXRF is the silicon semiconductor detector,
which nowadays can have excellent energy resolution. The two other types of detectors,
mentioned above, with their poorer energy resolution are limited to special cases where
certain features of semiconductors are not acceptable. Also the germanium semiconductor
detector with its comparable characteristics has a major drawback for conventional XRF:
inherently the escape peaks of intense lines can obscure other lines of interest.
II. Instrumentation
An ED-XRF system consists of several basic functional components, as shown in
Figure II.1: an –ray excitation source, sample chamber, Si(Li) detector, preamplifier, main
amplifier and mutlichannel pulse height analyzer. The properties and performances of an ED-
XRF system differ upon the electronics and the enhancements from the computer.
Figure II.1: Typical ED-XRF detection arrangement.