Physics and Engineering of Radiation Detection

11.3. Passive Dosimetry 633

TL Glow Curve

OSL Curve

Output Light Intensity

Time

Figure 11.3.3: Typical OSL

and glow curves obtained in

succession from a material

that exhibits both OSL and

TL properties.

B.2 CommonOSLMaterials....................

Although a number of materials have been identified with good OSL characteristics
but the most commonly used one is the carbon doped aluminum oxide orAl 2 O 3 :O.
This material is good forγ-ray and electron dosimetry but can not be very effectively
used for neutrons.
A big advantage of OSL materials is that they do not have to be heated and
hence do not have to possess high temperature stability. This one-less constraint
on the choice of material has been shown to be most advantageous for fast neutron
dosimetry specially in the field of medicine. In medical dosimetry of fast neutrons
one strives to use a material that is as close in content to the tissue as possible.
Since most of the fast neutrons are absorbed in the hydrogen of the tissue, the most
suitable dosimeter would have high hydrogen content. However the TL materials,
having high hydrogen content, have very low temperature stability and can not be
used as dosimeters. On the other hand, one can use an OSL material with a high
percentage of hydrogen without worrying about its temperature stability since the
material does not have to be heated. An example of such a material isNH 4 Br:Tl,
which has been shown to possess good dosimetry properties for fast neutrons.

11.3.CFilmDosimetry..........................

Film dosimetry is based on the so calledradiochromaticmaterials, which change
color when exposed to radiation. Since the amount of this coloration is proportional
to the delivered dose, therefore radiochromatic films provide a direct means of dose
measurement.
The coloration of the film becomes evident when white light is made to pass
through it after irradiation. Since white light is composed of photons of different
wavelengths, its transmission through the film is dependent on the absorption co-
efficient at each wavelength. Hence the film appears colored. The degree to which
the light is blocked by the film depends on itsoptical density, which is defined by

OD=log(I 0 /I), (11.3.3)