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D.2.2.1 Dose Conversion Factors for Carbon-14
D.10
The assumptions that the specific activity of 14 C per gram of carbon in man even-
tually reaches equilibrium with that in the atmosphere and that there are 16.1 kg of carbon
in the 70-kg body of Reference Man (ICRP 1959) lead to the derivation of dose and dose com-
mitment factors as discussed in the following paragraphs.
At a release rate of 1 Ci/yr over 30 years the accumulated quantity of 14 C in the
environment will be 30 Ci. At the end of an additional 40 years there will still be 30 Ci
in the environment. Diluting 30 Ci in the 6.15 x 1017 g of carbon in the atmosphere
(Killough 1977) yields a specific activity of
(30 Ci x 1012 pCi/Ci)/(6.16 x 1017g) = 4.87 x 10 -pCi/g
The dose rate (DR) factor after 30 years of release can be calculated from the following
equation (Soldat 1976):
where
DR = 0.0187 CE rem/yr
C = concentration in body (pCi of 14 C per g of body tissue)
= (4.87 x 10 - 5 pCi of 14C per g of C) (1.61 x 104 g of C)/(7 x 10 4 g total body)
= 1.12 x 10 - 5 pCi/g
E = 0.0538 (MeV/dis) . (rem/rad),
The factor 0.0187 is derived from the product of (0.037 dis/sec per pCi) (3.156 x
107 sec/yr) (1.602 x 10-8 g - rad/MeV).
Therefore
DR = (0.0187) (1.12 x 10 -5 ) (0.0538)
= 1.13 x 10-8 rem/yr per person
For 6.4 x 109 persons, the worldwide dose rate factor thus becomes 72.1 man-rem/yr after
the release of 1 Ci/yr for 30 years.
The 70-year dose commitment (DC) factor, which is the sum of the dose during release
and the dose after release has stopped, is calculated as follows:
DC = [(0 + 72.1 man-rem/yr)/2] (30 yr) + [(72.1 man-rem/yr) (40 yr)]
= 3970 man-rem per 1 Ci/yr released for 30 years.
These dose factors are summarized in Table D.2.1.