Biomedical Engineering Reference
In-Depth Information
undisturbed air that would ultimately be released from the particular mixture of
short-lived daughters in their decay to
210
Pb. The PAEC can be expressed in J m
-3
or MeV m
-3
. For a given PAEC, the equilibrium-equivalent decay-product concen-
tration (EEDC) is defined as the concentration of each decay product that would
be present if secular equilibrium existed. The ratio of the EEDC and the concen-
tration of radon is called the
equilibrium factor
. By definition, this factor is equal to
unity if the radon and all of its short-lived daughters are in secular equilibrium.
Equilibrium factors for most indoor atmospheres are in the range of 0.2 to 0.6, a
factor of 0.5 often being assumed as a rule of thumb. A limitation of the quantities
described in this paragraph is that they do not distinguish between the attached
and unattached fractions.
Until now, we have discussed only
222
Rn, which is a member of the uranium se-
ries. Radon is also generated in the other two series of naturally occurring radionu-
clides. However, these isotopes of radon are of lesser radiological importance. The
thorium series generates
220
Rn, which is also called thoron. The parent nuclide,
232
Th, is somewhat more abundant than
238
U, but has a longer half-life. As a re-
sult, the average rate of production of
220
Rn in the ground is about the same as that
of
222
Rn. However, the shorter half-life of
220
Rn, 56 s, as compared with 3.82 d for
222
Rn, gives it a much greater chance to decay before becoming airborne. The con-
tributions of the daughters of
220
Rn to lung dose are usually negligible compared
with
222
Rn. The third (actinium) series produces
219
Rn, also called actinon, after
several transformations from the relatively rare original nuclide
235
U. Its half-life
is only 4 s, and its contribution to airborne radon is insignificant.
Example
Measurements of room air show the nuclide activity concentrations given in Table 4.1.
Calculate the PAEC for this case.
Solution
The PAEC (and EEDC) pertain to the short-lived decay products and do not involve
the radon itself, which is not retained by the lungs. To obtain the PAEC, we need to
calculate the number of daughter atoms of each type per unit volume of air; multiply
these numbers by the potential alpha-particle energy associated with each type of
Tab l e 4 . 1
Activity
Concentration
(Bqm
-3
)
Nuclide
222
Rn
120
218
Po
93
214
Pb
90
2
l
4
Bi
76
214
Po
76
