Physics and Engineering of Radiation Detection

11.3. Passive Dosimetry 629

Photopeak

Temperature

Output Light Intensity

Figure 11.3.2: Typical glow curve

obtained by constant rate heating

of a TLD exposed to radiation. Dif-

ferent peaks in the curve corre-

spond to the defect energy levels of

the material. The total area under

the curve is proportional to the to-

tal absorbed dose.

field, however, this is generally not true since the probability of populating a certain
level is dependent on the type of incident radiation. This problem is generally solved
by using coupled TLDs, which we will describe later.

A.2 CommonTLMaterials.....................

There are quite a few TL dosimeters available commercially, some of which are listed
in Table.11.3.1. Each of them has its own characteristics, which should be taken into
consideration while deciding on a particular type. In some situations, choosing a
TLD can be hard, though. This can happen, for example, in a situation where one
type of TLD has good performance in high radiation fields but has very poor neutron
detection efficiency, while another one is good for neutrons but can not handle the
high radiation field. In such a situation one might even be tempted to use both
types of TLDs at the same time. There are no universally accepted guidelines for
choosing a TLD but the decision should normally be based on the following criteria.

Minimum Dose Rate and Total Expected Dose:It is always good to have

an idea about the expected dose rate since then it will be possible to estimate

the integrated dose over the course of deployment. Sometimes the length of

time the TLD has to stay in the area is determined by other considerations.

For example, in a particle accelerator, the TLDs can only be removed during

a shutdown period. On the other hand in a medical application the time may

be very limited. With the expected dose rate and the time as uncontrollable

parameters, one can calculate the expected integrated dose. This total dose can

then be used to decide on the best possible TL material based on signal-to-noise

ratio considerations.