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

522 CHAPTER 10
Shielding
the specific gamma-ray constant Ɣ = 0.33 R-m 2 /Ci-h (7.82 × 10 −8 Sv-m 2 /MBq-h).
Substituting the respective values into Eq. (10.16), we have
H ˙
R · m2
= π × 0.33
Ci · h
× ln (0.25 m)2 + (0.305 m) 2
(0.305 m) 2
0.1 Ci/m3
× × 1 − e −1 3.2m −3.2m−1 × 0.79 m
× 1 rem
R
˙
H = 1.5 × 10 −2 rem/h = 15.3 mrem/h (0.15 mSv/h).
In Chapter 5, Eq. (5.27), we saw that, under conditions of good geometry, the attenuation
of a beam of gamma radiation is given by
I = I0 e −μt .
However, under conditions of poor geometry, that is, for a broad beam or for a very
thick shield, Eq. (5.27) underestimates the required shield thickness because it assumes
that every photon that interacts with the shield will be removed from the
beam and thus will not be available for counting by the detector. Under conditions
of broad-beam geometry (Fig. 10-5), this assumption is not valid; a significant number
of photons may be scattered by the shield into the detector, or photons that
had been scattered out of the beam may be scattered back in after a second collision.
This effect may be illustrated by Figure 10-6, which shows the broad-beam
and narrow-beam attenuation of 60 Co gamma rays by concrete. According to Eq.
(5.27), about 7 in. of concrete shielding is required to transmit 10% of the incident
60 Co radiation, under conditions of good geometry. For a broad beam, on
the other hand (Fig. 10-6) shows that this thickness of concrete will transmit about
25% of the radiation incident on it. To transmit only 10% of a broad beam requires
about 11 in. of concrete. In designing a shield against a broad beam of radiation,
experimentally determined shielding data for the radiation in question should
be used whenever they are available. (Broad-beam attenuation curves for radium,
60 Co, and 137 Cs for concrete, iron, and lead are given in Figures 10-6, 10-7, and
10-8). When such data are not available, a shield thickness for conditions of poor
Figure 10-5. Gamma-ray absorption under conditions of broad-beam geometry, showing the effect
of photons scattered into the detector.