Biomedical Engineering Reference
In-Depth Information
Fig. 10.35
Relative response of film to a fixed dose of
monoenergetic photons as a function of energy. Upper curve is
for film covered only with thin layer of plastic. Lower curve is for
film covered with a cadmium filter to compensate for the
over-response to low-energy photons.
can employ two additional filters, one of high and the other of low atomic number,
such as silver and aluminum. They should have the same density thickness, so as
to be equivalent beta-particle absorbers. The high-
Z
filter will strongly absorb low-
energy photons, which are attenuated less by the low-
Z
material. The presence of
low-energy photons will contribute to a difference in darkening behind the two.
Figure 10.36 shows an exploded view of a multi-purpose film badge used at sev-
eral sites from about 1960 to 1980. It was designed for routine beta-gamma and
neutron personnel monitoring; criticality applications; assessment of a large, acci-
dental gamma dose; and for personal security identification. The laminated picture
front, identifying the wearer, was of low-
Z
material. An assembly, comprised of fil-
ters and other units, was placed behind the picture and in front of the film packs,
which included both X-ray and nuclear track emulsions. There were four filter ar-
eas in the assembly through which radiation could pass to reach the firm. (1) At
the window position, the only material traversed in addition to the laminated pic-
ture (52 mg cm
-2
) was the paper wrapper around the film (28 mg cm
-2
), giving
a total density thickness of 80 mg cm
-2
.(2)The(low-
Z
) plastic filter had a thick-
ness of about 215 mg cm
-2
. (3) A gold foil was sandwiched between two pieces of
cadmium, each 0.042 cm thick. The combination presented a Cd-Au-Cd absorber
thickness of about 1000 mg cm
-2
. (4) The aluminum filter (275 mg cm
-2
)waspro-
visionally included at the time for eventual help in determining the effective energy
of photon exposures.
