634 Chapter 11. Dosimetry and Radiation Protection
whereI 0 andIare the incident and transmitted radiation intensities respectively.
The reason for defining optical density in this way is that the ratioI 0 /I has an
exponential dependence on doseD. Hence we can write
D∝OD. (11.3.4)This relation forms the basis for radiochromatic film dosimetry. With proper cal-
ibration, the above relation guarantees a linear relationship between the measured
optical density and the absorbed dose.
C.1 AdvantagesandDisadvantagesofFilmDosimeters
Following are the main advantages of using radiochromatic materials for dosimetry.
No Post Irradiation Processing:The radiochromatic films do not have to
be processed after irradiation as in the case of TL dosimeters.High Spatial Resolution: The cost to benefit ratio of film dosimetry is
much smaller than other types of position sensitive dosimetry techniques, such
as TLD arrays or electronic detectors. Spatial resolution of a fraction of a
millimeter is not uncommon for typical films.Good Spatial Uniformity:Radiochromatic films can be manufactured with
high degree (typically better than 95%) of spatial uniformity.Film dosimetry has some disadvantages as well, some of which are mentioned
below.
Fading:As with TL materials, radiochromatic films also show post irradiation
fading. This fading is highest immediately after irradiation and then slows
down after some time. It is therefore recommended to delay the optical density
for about two days to allow the film to get somewhat stabilized. However one
must ensure that this delay is the same as used during calibrations.Temperature Dependence:The optical density of radiochromatic films has
strong temperature dependence. Therefore the temperature must be kept as
uniform as possible during irradiation and optical scan.UV Sensitivity:Most of the radiochromatic materials are sensitive to ultra-
violet light.Film Orientation: Radiochromatic materials are non-isotropic crystals and
therefore their orientation matters. The optical density should be measured at
the same orientation at which the film was exposed to radiation.C.2 CommonRadiochromaticMaterials
The most commonly used radiochromatic film is calledGafchromic. These films are
made of sub-micron sized crystals of a monomer, which get polymerized after irra-
diation. Since this polymerization is highly localized and does not spread therefore
the spatial resolution from these films can be achieved up to the size of the crystal.