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Measurement of Radiation, Sodium Iodide Detector
The second most common type of radiation detecting instrument is the
scintillation detector. The basic principle behind this instrument is the
use of a special material which glows or “scintillates” when radiation
interacts with it. The most common type of material is a type of salt called
sodium-iodide. The light produced from the scintillation process is reflected
through a clear window where it interacts with device called a photomultiplier
tube.
The first part of the photomultiplier tube is made of another special
material called a photocathode. The photocathode has the unique characteristic
of producing electrons when light strikes its surface. These electrons are
then pulled towards a series of plates called dynodes through the application
of a positive high voltage. When electrons from the photocathode hit the
first dynode, several electrons are produced for each initial electron hitting
its surface. This “bunch” of electrons is then pulled towards
the next dynode, where more electron “multiplication” occurs.
The sequence continues until the last dynode is reached, where the electron
pulse is now millions of times larger then it was at the beginning of the
tube. At this point the electrons are collected by an anode at the end of
the tube forming an electronic pulse. The pulse is then detected and displayed
by a special instrument.
Scintillation detectors are very sensitive radiation instruments and
are used for special environmental surveys and as laboratory instruments.
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