Biomedical Engineering Reference
In-Depth Information
Fig. 15.11
Schematic plan view of an X-ray facility.
From Fig. 15.9 we find that the needed thickness of lead for 125 kVp is about 1.5 mm.
From Fig. 15.10, we estimate the needed thickness of concrete to be about 5.0 in.
2)
Lead is a most effective and practical material for X-ray shielding. On a weight
basis, it is considerably lighter than concrete, as the last example illustrates. The
lead thickness, expressed as the mass per unit area (density
= 11.4
gcm
-3
), is
0.15
cm
× 11.4
gcm
-3
= 1.7
gcm
-2
. The concrete thickness (density
= 2.35
gcm
-3
)
is
5.0
in
× 2.54
cm in
-1
× 2.35
gcm
-3
= 30
gcm
-2
. The ratio of concrete-to-lead
thicknesses is
30/1.7
18
. A concrete shield covering the same wall area as lead
would thus weigh 18 times as much. The principal physical reason for this dif-
ference is the much larger photoelectric attenuation coefficient of lead (and other
materials of high atomic number) for low-energy photons (cf. Figs. 8.8 and 8.9).
The peak X-ray energy in the last example was 125 kVp. At higher peak energies,
the difference between lead and concrete, though substantial, becomes less pro-
nounced.
A primary protective barrier can either be erected when a structure is built or it
can be provided by adding shielding to an existing structure. The attenuation of
many common building materials per g cm
-2
is approximately the same as that of
=
2 Note. For the computation we have used the
distance
d
from the source to the opposite
side of the wall. NCRP Report No. 147
considers that the nearest likely approach of
the sensitive organs of a person is not less
than 0.3 m beyond the wall.
