Physics and Engineering of Radiation Detection

630 Chapter 11. Dosimetry and Radiation Protection

Energy Response:Not all TL materials respond equally to the energy de-

posited by the incident radiation. It is therefore recommended to match the

expected energy spectrum of the incident radiation with the energy response

parameters supplied by the TLD manufacturer.

Fade Characteristic: Every TL material fades with time, i.e., the trapped

electrons in the defect levels escape even at room temperatures. However most

of the materials are designed such that this fading is not significant. Still it is

a good practice to compare the fade characteristics of the material to the total

length of time from its deployment to the eventual read out.

Type of Incident Radiation: As stated earlier, not every TL material is

suitable for all types of radiation. One should therefore be careful in making

a choice for a particular application. In particular, thermal and epithermal

neutron dosimetry can be done only with special TLDs, most of which have

either lithium-6 or boron-10 as the neutron-active medium. The neutrons react

with these materials according to

n+^63 Li →^31 H+α (11.3.1)

and n+^105 B →^73 Li+α. (11.3.2)

Theα-particles produced then deposit all their energy into the material. Al-

though these materials can be and are used for neutron dosimetry, there are

two problems associated with them. First of all, these materials have low cross

sections for fast neutrons and therefore the TLDs made with them have low

fast-neutron-efficiencies. Secondly it has been observed that the response of

these materials toγ-rays decreases with neutron exposure. Hence they can

yield inaccurate results if used in situations where neutrons are also accompa-

nied byγ-rays, which actually is the case for fast neutron fields.

A.3 AdvantagesandDisadvantagesofTLDosimeters......

Following are some of the reasons for preferring TL dosimeters over other types of
detectors.

No Electronics Circuitry:A TL dosimeter does not require any high voltage

or readout circuitry. It essentially acts as a memory device from which the

information can be retrieved offlineusing a TL reading device. Hence its

deployment in a radiation environment does not require any cable connections.

Small Size: A typical TL dosimeter is less than 5mmlong and 2mmwide.

This makes it perfect for installation in very narrow spaces where other detec-

torswouldbehardtofitin.

Wide Dynamic Range: General TL dosimeters can record doses in a wide

range.

TL dosimeters also has some undesirable characteristics that do not allow them
to be used in certain situations.