320 Chapter 6. Scintillation Detectors and Photodetectors
is around 20%. This implies that 80% of the scintillation photons are wasted away.
Although more efficient photocathode materials have been developed but this is still
one of the most problematic things with regard to operation of PMTs in low field
environment.
Another type of detector, called photodiode detector, can be used to detect pho-
tons produced by scintillation materials. A photodiode, as we saw in Chapter on
semiconductor detectors, is made of a semiconductor material that has been appro-
priately doped. In such a detector, the scintillation photon produces electron-hole
pairs instead of just electrons as in a PMT photocathode. The quantum efficiency
of a photodiode can be as high as 80%, which certainly makes it very attractive
for use in low level radiation environments. For highly sensitive measurements in
extremely low radiation fields, another special kind of photodiode detector, called
theavalanche photodiode, can be used. In such a device the electron-hole pairs pro-
duced by the incident radiation are multiplied through an avalanche process. The
process is analogous to the electron multiplication in a PMT except that in this case
there are no dynode-like mechanical structures involved, making the system less
susceptible to damage caused by mechanical shocks. The end result of the electron
hole multiplication is the transformation of a very low level signal into a pulse large
enough to be measured by the electronics.
Photodetectors are an integral part of scintillation detectors and therefore deserve
careful and detailed attention. Therefore later in this Chapter we will spend some
time discussing the working principles, structures, advantages, and disadvantages of
some commonly used photodetectors.
6.1 Scintillation Mechanism and Scintillator Properties
Although scintillation materials come in all forms and types but, broadly speaking,
they can be divided into two categories: organic and inorganic. Later in the Chapter
we will look at the properties of the materials belonging to these two categories. We
will see that how specific applications dictate the preference of a certain scintillator
over the others. But before we do that, let us have a general look at the impor-
tant properties of scintillation materials that are relevant to their use in radiation
detectors.
6.1.A Basic Scintillation Mechanism
Scintillators are insulators having wide gap between their valence and conduction
bands. Within this gap they also have the so called luminescence centers, which play
the central role in producing scintillation light. As shown in Fig6.1.1, a luminescence
center is generally composed of two energy levels with a difference that is equal to
the energy of photons in and around the visible region of electromagnetic spectrum.
If an electron jumps from the higher energy level of this center to the lower level, a
scintillation photon can be emitted.Can bebecause there is also a possibility of non-
radiation transfer in which the energy is dissipated by the heat carrying particles
calledphonons. If this happens, the process is said to have beenquenchedand the
information is lost.