Physics and Engineering of Radiation Detection

628 Chapter 11. Dosimetry and Radiation Protection

A.1 WorkingPrincipleandGlowCurve..............

The way TL materials save information, that is dose, is by trapping the electrons
and holes in the lattice defect sites. When these charges are released from those sites,
they give off energy in the form of light photons. The intensity of the emitted light is
proportional to the energy transferred by the incident radiation to the material. The
rate of release can be enhanced by providing uniform heat energy to the material.
It has been found that the probability of release increases with temperature. If we
increase the temperature of the material at a constant rate, the charges in different
defect levels will escape at different temperatures. Hence a plot of output light
intensity versus temperature (or time as it is proportional to the temperature for
linear temperature increase) should show different peaks corresponding to charges
stored at different levels.
The main building blocks of a TLD readout system are shown in Fig.11.3.1. The
material is slowly heated by a heater supply. The emitted light is filtered and then
detected by a photomultiplier tube. The temperature of the material is recorded
through the thermocouple.

HV

Optical
Filter

Heater
Thermocouple

TLD

Recorder

Power Supply

Amplifier

Light Photons

PMT

Figure 11.3.1: Block diagram of a

simple TLD readout system.

A typical plot of the output light intensity versus temperature is shown in Fig.11.3.2.
The same curve can also be drawn with time on the x-axis as explained above. Such
aplotisknownastheglow curve. Since peaks in the curve correspond to the de-
fect energy levels, therefore each TLD material has a specific shape of glow curve.
The height of each peak, however, can be different for different dose levels, types of
materials, and heat transfer rates.
The question as to whether the number of charges stored at each level is propor-
tional to the absorbed dose or not, depends on the radiation field. In case of single
radiation field, such as onlyγ-rays or only neutrons, each peak is approximately
proportional to the dose absorbed at the corresponding energy level. In a mixed