Physics and Engineering of Radiation Detection

11.3. Passive Dosimetry 627

result in units ofMeV. Substituting the values of rest masses from atomic

data tables in the above equation gives

Ep =(14.003074 + 1008665− 14003242 − 1 .007825) 931. 48

=0. 626 MeV.

11.3PassiveDosimetry

Passive dosimetry involves the use of a material torecorddose and then take the
material out of the radiation environment to read the recorded value. This method is
not suitable for measuring instantaneous dose rates but is highly successful and con-
venient for measuring integrated dose. Some of the commonly used passive dosimetry
techniques are described below.

11.3.AThermoluminescentDosimetry

In the chapter on solid state detectors we briefly mentioned that there is a class of
solids which can store the energy they are exposed to. Upon heating they release the
stored energy in the form of photons. Since the released energy is proportional to the
stored energy therefore these materials can be used as dosimeters. Such materials
are known as thermoluminescent materials and the detectors based on them are
known as thermoluminescent dosimeters or somply TLDs.
TLDs can measure only integrated doses since they must be allowed to absorb
and store energy. Even though other more efficient detectors are now available, still
due to their simplicity of operation, TLDs are being used extensively in different
fields. Generally the intention is to determine the dose received by a person or
an equipment, such as a semiconductor detector, over a long period of time. For
example, radiation workers are required to wear badges made of some TL material
whenever they are working in an environment where radiation is higher than the
nominal background level. After a predefined period of time, usually several months,
the badge is read out to determine the integrated dose received by the person. The
dose level is then compared with the limit set by the organization to ensure that the
person has not received more than what she or he is supposed to.
TL dosimeters are also being routinely used in experiments at particle acceler-
ators, where the aim is to determine the integrated dose received by the radiation
vulnerable devices, such as silicon detectors. This approach has been highly success-
ful since these devices do not need any electronic circuitry for operation and can be
easily installed and retrieved. The drawback is that they can not be used to measure
instantaneous doses and are also not as accurate as electronic detectors.
Another area in which TL dosimeters are gaining interest is the clinical dosimetry,
which has traditionally been occupied by ionization chambers and semiconductor
detectors.