Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

526 CHAPTER 10
in units of relaxation lengths. One relaxation length is that thickness of shield that
will attenuate a narrow beam to 1/e of its original intensity. One relaxation length,
therefore, is numerically equal to the reciprocal of the absorption coefficient. The
use of the buildup factor in the calculation of a shield thickness may be illustrated
with the following examples:
W Example 10.4
A 3.7 × 10 4 MBq (1-Ci) source of 137 Cs is to be stored in a spherical lead container
when not in use. How thick must the lead be if the air kerma rate at a distance of 1 m
from the source is not to exceed 25 μSv/h (2.5 mrems/h)? Assume the source to be
sufficiently small to be considered a “point.”
Solution
In Table 6-3, we find the specific gamma-ray emission of 137Cs to be 7.82 × 10−8 Sv/h/MBq at 1 m (0.33 R-m2 /Ci-h). The exposure rate at 1 m from the unshielded
source, therefore, is
3.7 × 10
K ˙ =
4 −8 Sv · m2
MBq × 7.82 × 10
MBq · h
1m2 = 2.89 × 10 −3 Sv/h.
If there were no buildup, the required thickness of lead would be calculated from
Eq. (5.23), using the value for the attenuation coefficient for lead for 0.661-MeV
gamma rays from Table 5-2, μ = 1.24 cm −1 :
I = I0 × e −μt
25 = 2890 × e −1.24t
t = 3.8cm.
This is an underestimate of the required shield thickness, since it does not include
the additional thickness to account for buildup.
In Figures 10-9 and 10-10, we see that the buildup factor is a function of the
shield thickness. Since the shield thickness is not yet known, Eq. (10.17) has two
unknowns, the buildup factor B and the shield thickness t. To determine the proper
shield thickness, we estimate a thickness, then substitute this estimated value into
Eq. (10.17) to determine whether it will satisfy the dose-rate reduction requirement.
The minimum shield thickness can be estimated by assuming narrow-beam
attenuation and then increasing the thickness thus calculated by one half value