319Chapter 6
Scintillation Detectors and Photodetectors
In Chapter 3 we saw that the passage of radiation through some materials causes
them to emit photons. The basic process is very simple: when atoms of incident
radiation interacts with atoms of these so called scintillation materials, they transfer
some of their energy to the atoms. As a result these excited atoms go into short
lived excited states. When they return to ground state they emit photons, mostly
in visible and ultraviolet regions of the spectrum. This provides an alternate to the
ionization mechanism to detect and measure radiation.
The basic steps involved in scintillation detection of radiation are:
Interaction of radiation with scintillation material.
Transfer of energy to the bound states of the material.
Relaxation of the excited states to the ground state resulting in the emission
of light photons.
Collection of photons by the photodetector.
Detection of the photodetector signal by associated electronics.
The efficiency of a typical scintillation material to emit light after absorbing
radiation ranges from 10% to 15%, implying that scintillation is not a very efficient
process. However, as we will see later, even with such aninefficientmaterial, one can
develop a highly efficient radiation detector. The trick is to use a photon detector
that has high photon collection and counting efficiency. In this chapter we will also
visit two distinct types of detectors, namely photomultiplier tubes and photodiodes,
that are used to detect scintillation photons.
The basic working principle of a photomultiplier tube or PMT involves conversion
of the scintillation photon into an electron and then multiplication of this electron
into a very large number of electrons. The photon-electron conversion, which is basi-
cally the photoelectric effect we studied in Chapter 2, takes place in a thin material
calledphotocathode. The electrons produced in the photocathode are accelerated to-
wards a metallic structure calleddynodewhich releases a large number of electrons
due to the impact. These secondary electrons are then accelerated towards another
dynode, which also multiplies their number. The process is repeated several times
by letting the electrons pass through a series of dynodes. The end result of this
process is an output pulse with an amplitude large enough to be easily measured by
the associated electronics. The gain of a PMT can be higher than 10^5 ,whichmakes
it very attractive for sensitive measurements. The downside is the low quantum effi-
ciency of the photon-electron conversion process, which for a typical photoelectrode