6.1. Scintillation Mechanism and Scintillator Properties 329
of information and therefore can induce non-linearities in the detector response.
Hence the optical properties of the material play a very important role in the overall
efficiency of the system. The parameter of interest here is the refractive index of the
material for the wavelength of the light emitted by the scintillation material.
Unfortunately most of the material that have very good scintillation efficiencies
produce ultraviolet photons. These photons have very short attenuation lengths and
therefore quickly get absorbed in the material, resulting in the loss of information.
The trick that is employed to solve this problem is to mix homogeneously another
scintillator in the bulk of the material that absorbs these photons and emits visible
light photons. Since the visible photons have longer attenuation lengths, their life-
time in the material increases and consequently the overall efficiency of the detector
is considerably increased. The second scintillators are generally known aswavelength
shiftersorsecondary fluors.
6.1.H PhosphorescenceorAfterglow..................
Figure 6.1.6: Fluorescence and phosphorescence from
Lu 2 O 3 :Euafter 16 minute irradiation (solid line). The
curve also shows the variation of sample temperature (28).Scintillation materials produce not only prompt scintillation light but also delayed
light. The delayed light, as we discussed earlier, is emitted by transitions made by
electrons trapped in the long lived energy states. This is obviously not a very
desirable trait of a scintillation material since it can not only stretch the decaying
part of the scintillation pulse but could also produce a significant afterglow if the
temperature is raised. Fig.6.1.6 shows the time profile of light emission after 16
minute irradiation of a ceramic scintillatorLu 2 O 3 :Eu. The second peak shown in
the figure occurred after the sample was heated. Providing thermal energy to the
electrons in the long lived states elevate them to the conduction band. From there
they can follow the scintillation levels and eventually end up in the valence band.