Physics and Engineering of Radiation Detection

5.3. Thermoluminescent Detectors 313

5.3.A PrincipleofThermoluminescence................

During the discussion on semiconductorsand diamond in the preceding sections we
saw that the crystal defects and impurities in those materials can produce energy
levels inside the forbidden gap. Since these levels can act as electron traps and
distort the signal, therefore they are not desirable in radiation detectors. Though
TL materials also have such levels but in this case they are advantageous since they
can be used to store information.

Eg

Radiation

Incident

Light

Emitted

Electron Hole

E

Heat

Valence Band

Conduction Band

Figure 5.3.1: A simple model of energy absorption in a TL material. The

saved energy is released in the form of a light photon when the material is

heated.

Fig.5.3.1 shows how the metastable electron traps in TL materials can be ex-
ploited to store and retrieve information. The TL material shown has two additional
energy levels inside the forbidden gap. The incident radiation elevates an electron
from the valence band to the conduction band and leaves behind a vacancy hole in
the valence band. The high level of impurities in the TL materials makes it highly
probable that this electron would quickly fall into the impurity level near the con-
duction band. Being a metastable site, this site retains the electron until energy is
provided to the material externally, such as in the form of heat. The heat elevates
the electron back to the conduction band. The electron can then fall into the im-
purity level near the valence band and emit a photon during this process. Since the
energy difference between the lower end of the conduction band and the impurity
level near the valence band corresponds to light photon energy therefore the emitted
photon is in the visible region of the electromagnetic spectrum. The electron in this
level eventually falls into the valence band and recombines with the hole.