11.5 Scintillation Detector
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11.5 Scintillation Detector
Scintillation counters do not include any wearing parts and their
service life is not limited, provided they are used under normal op-
erating conditions. Malfunctions in the scintillation detector can
only be caused by excessive mechanical or thermal stress. The
transfer of heavy vibrations or shocks to the detector has to be
prevented by suitable measures, such as separate installation or
shock absorbers. Moreover, with temperatures of above 50°C at
the installation site, heat shields or water cooling devices have to
be foreseen for the detector.
11.5.1 Working Principle
The scintillation counter comprises:
a) sodium iodide crystal doped with Thallium
b) photomultiplier
c) electronics unit.
As the gamma radiation hits the detector, it triggers minute
flashes of light in the crystal which are invisible to the naked eye.
These flashes of light release electrons from the light-sensitive
cathode (photo cathode) of the photomultiplier. This electron cur-
rent is amplified via a dynode system resulting in an electrical im-
pulse at the photomultiplier output for each impinging light flash.
In the electronic unit connected to the photomultiplier output these
pulses are amplified and counted and the result is transmitted as
digital information via two-wire cable.
The high voltage for operating the photomultiplier is generated in
the electronic unit. The connecting cable between evaluation unit
and scintillation counter only carries a direct voltage and the
measuring signal.
The scintillation counter is equipped with a drift compensation
which compensates for the effects of aging and temperature-
related gain variations, thus ensuring high long-term stability.
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