Biomedical Engineering Reference
In-Depth Information
clear track emulsions are employed. The tracks of individual recoil protons that
neutrons produce in the emulsion are then observed and counted under a micro-
scope. The work is tedious, and the technique is limited by the fact that the ranges
of recoil protons with energies less than about 2.5 MeV are too short to produce
recognizable tracks.
Film calibration and the use of densitometer readings to obtain dose would ap-
pear, in principle, to be straightforward. In practice, however, the procedure is com-
plicated by a number of factors. First, the density produced in film from a given
dose of radiation depends on the emulsion type and the particular lot of the man-
ufacturer. Second, firm is affected by environmental conditions, such as exposure
to moisture, and by general aging. Elevated temperatures contribute to base fog
in an emulsion before development. Third, significant variations in density are
introduced by the steps inherent in the film-development process itself. These in-
clude the type, concentration, and age of the developing solution as well as the
development time and handling through agitation, rinsing, and fixing. Variations
from these sources are significantly reduced by applying the following procedure
to both the film dosimeters worn by workers and those used for the calibration of
the dosimeters. All units should be from the same manufacturer's production lot,
stored and handled in similar fashion, developed at the same time under the same
conditions, and read with a single densitometer, and even by a single operator. Ex-
perience shows that an acceptable degree of reproducibility can be thus attained.
A serious problem of a different nature for dose determination is presented by
the strong response of film to low-energy photons. The upper curve in Fig. 10.35
illustrates the relative response (darkening) of film enclosed in thin plastic to a fixed
dose of monoenergetic photons as a function of their energy. From about 5 MeV
down to 200 keV, the relative response, set at unity in the figure, is flat. Below about
200 keV, the rising photoelectric absorption cross section of the silver in the film
leads increasingly to more blackening at the fixed dose than would occur if film
were tissue- or air-equivalent. The relative response peaks at around 40 keV and
then drops off at still lower energies because of absorption of the photons in the
packaging material around the film.
The lower curve in Fig. 10.35 shows the relative response when the incident
radiation passes through a cadmium absorber of suitable thickness placed over the
film. The absorption of photons in the cadmium filter tends to compensate for the
over-response of the film at low energies, while having little effect at high energies,
thus extending the usefulness of the badge to lower-energy photons.
Film badges are also used for personnel monitoring of beta radiation, for which
there is usually negligible energy dependence of the response. For mixed beta-
gamma radiation exposures, the separate contribution of the beta particles is as-
sessed by comparing (1) the optical density behind a suitable filter that absorbs
them and (2) the density through a neighboring “open window.” The latter consists
only of the structural material enclosing the film. Since beta particles have short
ranges, a badge that has been exposed to them alone will be darkened behind the
open window, but not behind the absorbing filter. Such a finding would also result
from exposure to low-energy photons. To distinguish these from beta particles, one
