6.3. Inorganic Scintillators 353
Scintillation
LightRadiationIncidentEgElectron HoleEValence BandConduction BandDopant LevelFigure 6.3.2: Principle of dopant luminescence in an inorganic scin-
tillator.A common example of radiation vulnerable detector is cesium iodide. CsIhas
been extensively used in high energy physics experiments and its radiation damage
properties have been well observed. For example Fig.6.3.4 shows the decrease in
gain ofCsIbased detector after irradiation. Such a significant decrease in gain is
not specific to this material only as other materials such asPbWO 4 have also shown
similar behavior.
Lead tungstatePhWO 4 , as we will also see later, is one of the inorganic scintilla-
tors that have been found to possess significant radiation resistance. However even
with such a relatively high degree of radiation hardness,PhWO 4 is not completely
free from harmful effects of large integrated doses. For plastic scintillators we saw
that the dose integrated over a period of several months changes the transmission
properties of the detectors. This has also been observed withCWOunder high
irradiation (see Fig.6.3.5).
6.3.C Some Common Inorganic Scintillators
Some inorganic scintillators and their properties are listed in Table.6.3.1. The two
properties that attract most attention of detector developers are the wavelength
of maximum emissionλmax and the light yielddN/dE. The choice ofλmaxis
mainly driven by the efficiency of the photon counting detector, which is wavelength
dependent. for example in photomultiplier tubes the efficiency of the photocathode
depends on the wavelength of incident photons. This implies that a good match
between the wavelength of scintillation photons and efficiency of the photocathode is
necessary to ensure high overall efficiency and good signal-to-noise ratio. In practice,