Biomedical Engineering Reference
In-Depth Information
15.2
Gamma-Ray Shielding
In Section 8.7 we discussed attenuation coefficients and described the transmis-
sion of photons through matter under conditions of “good” geometry. The relative
intensity
I
/
I
0
of monoenergetic photons transmitted without interaction through a
shield of thickness
x
is, from Eq. (8.43),
I
0
e
-
µx
,
I
=
(15.1)
where
µ
is the linear attenuation coefficient. If the incident beam is broad, as
in Fig. 8.10, then the measured intensity will be greater than that described by
Eq. (15.1) because scattered photons will also be detected. Such conditions usually
apply to the shields required for protection from gamma-ray sources. The increased
transmission of photon intensity over that measured in good geometry can be taken
into account by writing
BI
0
e
-
µx
,
(15.2)
I
=
where
B
is called the buildup factor (
B
1). For a given shielding material, thick-
ness, photon energy, and source geometry,
B
can be obtained from measurements
or calculations.
Figures 15.1-15.5 show exposure buildup factors for five materials for mono-
energetic photons with energies up to 10 MeV from point isotropic sources. The
≥
Fig. 15.1
Exposure buildup factors,
B
, in lead for point sources
of monoenergetic photons of energies from 0.1 MeV to 10 MeV
as functions of the number of relaxation lengths,
µ
x
.
