Parker XRF, Instruction Manual

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XRF Analyser Manual
33
The key elements of an X-ray tube.
Energy-dispersive XRF spectrometers usually incorporate either a silicon drift (SDD) or
silicon pin (Si-PIN) detector. The substantially greater cost of an SDD is justified in higher
end units, such as the Parker Kittiwake XRF Analyser, by better resolution and sensitivity.
An SDD is a semiconductor device built around high-resistivity silicon that absorbs X-ray
photons fluoresced by the sample under consideration. Electrons, ejected from the silicon
as a result of photon absorption, drift through the silicon under the influence of an electric
field applied by a series of concentric reverse-biased strips. The electrons are eventually
collected by a small anodic ring that sits at the centre of the strips and generate an
electrical current that can be measured. X-ray photons create a current pulse at
the anode that is proportional to the energy they carry, thus identifying the element
responsible for their birth. A relatively large number of pulses with the same height implies
a high concentration of that element within the sample. The high purity of the silicon
used within SDDs means that low noise levels can be achieved very simply using Peltier
cooling.