330 Chapter 6. Scintillation Detectors and Photodetectors
In the process, they can emit light such as shown in Fig.6.1.6. Since production
of delayed light with a prominent peak involves providing thermal energy to the
material, the phenomenon is calledthermoluminescence. There are many materials
that are exceptionally good in producing this kind of light with minimal prompt
scintillation. The phenomenon of thermoluminescence is exploited in the detectors
called Thermo-Luminescent Detectors or TLDs, which are memory devices used to
monitor absorbed dose over a period of time. We learned a little bit about such
detectors in the Chapter on solid state detectors and will discuss them in detail in
the Chapter on dosimetry.
In common scintillation detectors thermoluminescence is not of much concern
provided the temperature of the sample is not allowed to increase with time. However
saturation of metastable traps can sometimes cause undesirable results including
broadening of the prompt pulse.
Let us now turn our attention to the long decay time of scintillation that is
observed in most scintillators. The longer the pulse takes to decay the more afterglow
the material is said to have. Intuitively we can think that the cause of this afterglow
is the slow transitions of electrons trapped in impurity levels. However since we know
that elevating electrons from the traps is more favorable at higher temperatures
therefore we would expect the afterglow to increase at with temperature. In the
next section we will see that this is actually what is observed.
6.1.I TemperatureDependence
Almost all of the parameters we studied in the preceding sections depend on the
temperature of the material. That is why the values given in literature always
correspond to operation at a certain temperature or in a temperature range.
Figure 6.1.7: Relative
yield of electron hole pairs
in CsI with and with-
out addition of a secondary
fluor (2).Let us first see how the production of electron hole pairs by the incident radiation
is affected by a change in temperature. Based on our discussion on semiconductors