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the elements and the continuous spectral background. That means that indi-
vidual X-ray photons emitted by the sample within a finite period of time have
to be registered by the detector. As far as possible, the energy of these photons
should also be determined. Generally, the detector gives an electronic pulse as
a response to the incoming photon, which is counted. If the pulse height is
proportional to the photon energy, it can be used as a measure for this energy
after calibration.
Two main types of detectors can be used for WDS, and both of them are
based on the principle that atoms of a particular medium will be ionized by
photoelectric processes [47,49,62]. The gas-filled detectors use an inert gas; the
scintillation detectors use a solid phosphor material. A gas-filled detector is
represented in Figure 3.21. X-ray photons are directed into the detector
through the secondary collimator and enter it through a thin window. They
can hit now individual atoms of the gas and produce several photoelectrons and
gas ions. The photoelectrons are accelerated to the central thin wire held at a
high positive potential serving as the anode. The heavy ions are collected by the
Figure3.21.
Gas-filled detector with an insulated thin wire tightened along the axis of a metal
cylinder. The wire with about 80
μ
m diameter is held at a high positive voltage of about 2 kV and
used as anode. The metal cylinder filled with argon or xenon gas is grounded and is used as cathode.
X-ray photons enter the detector through a collimator and a thin window. They ionize gas atoms
and produce photoelectrons. The electrons are accelerated toward the wire producing further
electron-ion pairs, the ions are discharged at the casing. The gas amplification can be controlled by
the variable ohmic resistor. The electron-ion pairs produce a short voltage drop at the capacitor,
which is proportionally amplified and passed to the counting circuitry.
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