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

224 CHAPTER 6
For reasons discussed in Chapter 7, for health physics purposes, the air kerma is
called a sievert (Sv) in SI units and a rem in traditional units (1 Sv = 100 rems).
Source Strength: Specific Gamma-Ray Emission
The radiation intensity from any given gamma-ray source is used as a measure of
the strength of the source. The gamma-radiation exposure rate from a point source
of unit activity at unit distance is called the specific gamma-ray constant and is given
in units of sieverts per hour at 1 m from a 1-MBq point source (or, in the traditional
system, roentgen per hour at 1 m from a 1-Ci point source). The source
strength may be calculated if the decay scheme of the isotope is known. In the
case of 131 I, for example, whose gamma rays are shown in Figure 4-7 and whose
corresponding true absorption coefficients are found in Figure 5-20, we have the
following:
MeV/PHOTON PHOTONS/TRANSFORMATION μ (ENERGY ABSORPTION), m −1
0.723 0.016 3.8 × 10 −3
0.673 0.069 3.9 × 10 −3
0.503 0.003 3.8 × 10 −3
0.326 0.002 3.8 × 10 −3
0.177 0.002 3.4 × 10 −3
0.365 0.853 3.8 × 10 −3
0.284 0.051 3.7 × 10 −3
0.080 0.051 3.2 × 10 −3
0.164 0.006 3.3 × 10 −3
The gamma-radiation exposure level, in Sv/h (Gy/h air kerma), is calculated
by considering the energy absorbed per unit mass of air at the specified distance
from the 1-MBq point source due to the photon flux at that distance, as shown in
Eq. (6.15). For a distance of 1 m from the point source, we have
˙
X =
f photons
t
× E MeV
photon ×1.6 × 10−13 J
tps
s μ
× 1 × 106 ×3.6 × 103 ×
MeV MBq h m
4π (1 m) 2 × ρ kg J/kg
× 34
m3 C/kg
(6.15)
where
˙X = exposure rate, Sv/h (air kerma),
f = fraction of transformations that result in a photon of energy E ,
E = photon energy, MeV,
μ = linear energy absorption coefficient, m−1 ,
ρ = density of air, kg/m3 , and
d = distance from the source, m.