6.1. Scintillation Mechanism and Scintillator Properties 331
in the previous Chapter we would expect to see some variation. Fig.6.1.7 shows
the behavior ofCsIcrystal at different temperatures. The two curves represent the
scintillator with and without addition of a wavelength shifter. The difference can be
attributed to the change in the energy band structure after addition of the second
scintillator.
Since the charge pair yield changes with temperature we would expect the light
yield to also show temperature dependence. In fact, the temperature dependence of
light yield has been found to be very profound in most of the scintillation materials.
As shown in Fig.6.1.8, generally the light output is seen to decrease with increase in
temperatures.
Figure 6.1.8: Variation of light yield of pureCsIwith
respect to temperature. The data was obtained with two
different photodetectors (2).Another important parameter that has strong temperature dependence is the
decay constant. In most scintillation materials a time plot of the pulse decay at
different temperatures produces significantly different decay rates, as can be seen
in Fig.6.1.9. Since the decay of scintillation pulse depends on the thermal energy
therefore one would expect the decay constant to depend on the thermal energy of
the particles in the material through the relation
τd=τd 0 eE/kBT, (6.1.5)whereτd 0 is a material dependent constant,Eis the mean activation energy,kBis
the Boltzmann’s constant, andTis the absolute temperature.
As we discussed earlier, every scintillator has more than one decay constant and
therefore generally an average of these values is given in literature. Although this