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

When we combine the constants in Eq. (6.35), we have
RADIATION DOSIMETRY 231
˙D∞(air) = 4.45 × 10 −7 × Ca × Ē mGy/h. (6.36)
Since the skin of a person in an infinite medium is irradiated from one side only,
and since soft tissue absorbs about 10% more energy per kilogram than air does, the
dose rate to the basal cells of the skin in a semi-infinite medium is
˙Db = 0.5 × 1.1 × ˙D∞(air) × e −(μβ,t×0.007) . (6.37)
If we combine Eqs. (6.36) and (6.37), we have the beta dose to the skin of a person
immersed in a large cloud of concentration C Bq/m 3 :
˙Db = 2.45 × 10 −7 × C × Ē × e −(μβ,t×0.007) mGy/h. (6.38)
Generally, if there are fi betas of average energy Ēi MeV whose absorption coefficient
is μβi,t each, then the beta dose rate is
˙Db = 2.45 × 10 −7 × C
i
and if we divide by the concentration C , we obtain DCF:
DCF (submersion) = 2.45 × 10 −7 ×
W Example 6.12
fi Ēi × e −(μβi ,t×0.007) mGy
, (6.39)
h
i
fi Ē i × e −(μβi ,t×0.007) mGy/h
. (6.40)
Bq/m3 Calculate the dose rate to the skin of a person immersed in a large cloud of 85 Kr at
a concentration of 37 kBq/m 3 (10 −6 μCi/mL).
Solution
Krypton-85 is a pure beta emitter that is transformed to 85 Rb by the emission of
a beta particle whose maximum energy is 0.672 MeV and whose average energy is
0.246 MeV. The tissue absorption coefficient is calculated with Eq. (6.21):
μβ,t = 18.6(0.672 − 0.036) −1.37 = 34.6 cm 2 /g,
and the skin dose is calculated with Eq. (6.38):
˙Db = 2.45 × 10 −7 × C × Ē × e −(μβ,t×0.007) mGy/h
˙Db = 2.45 × 10 −7 × 3.7 × 10 4 × 0.246 × e −(34.6×0.007)
˙Db = 1.8 × 10 −3 mGy/h (0.18 mrads/h) .