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

RADIATION DOSIMETRY 225
This calculation is made for each different quantum energy and the results are
summed to obtain the source strength. For the 0.080-MeV gamma ray, at a distance
of 1 m, we have
X ˙ = 5.1 × 10−2 × 8 × 10−2 × 1.6 × 10−13 × 1 × 106 × 3.6 × 103 × 3.2 × 10−3 4π (1) 2 × 1.293 × 1 × 1
−10 Sv
= 4.628 × 10 (air kerma).
h
The exposure rate for each of the other quanta emitted by 131 I is calculated in a
similar manner, except that the corresponding frequency and absorption coefficient
is used for each of the quanta of different energy. The results of this calculation are
tabulated below:
PHOTON ENERGY (MeV) Sv/h at 1m
0.723 1.558 × 10−9 0.637 6.048 × 10−9 0.503 0.203 × 10−9 0.326 0.088 × 10−9 0.177 0.043 × 10−9 0.365 41.960 × 10−9 0.284 18.930 × 10−9 0.080 46.280 × 10−9 0.164 0.116 × 10−9
= 1.152 × 10−7 Sv-m2
MBq - h
Equation (6.15) contains several constants: 1 × 10 6 tps/MBq, 3.6 × 10 3 s/h, 1.6 ×
10 −13 J/MeV, 4π (1 m) 2 , and 1.293 kg/m 3 . If all these constants are combined, the
source strength Ɣ, in Sv air kerma per MBq per hour at 1 m, is given by
−5
Ɣ = 3.54 × 10
where
i
fi × E i × μi
Sv-m2 . (6.16a)
MBq-h
fi = fraction of the transformations that yield a photon of the ith energy,
E i = energy of the ith photon, MeV, and
μi = linear energy absorption coefficient in air of the ith photon.
For many practical purposes, Eq. (6.16a) may be simplified. For quantum energies
from about 60 keV to about 2 MeV, Figure 5-20 shows that the linear energy
absorption coefficient varies little with energy; over this range, μ is about 3.5 × 10 −3
m −1 . With this value, Eq. (6.16) may be approximated as
Ɣ = 1.24 × 10 −7 fi × E i
Sv-m2 . (6.16b)
MBq-h