Radium in the body

Neither radionuclide is easily measured directly by
whole-body counting (WBC), because neither is a strong gamma-ray emitter. However,
both have strong gamma emitters among their progeny that are easily measured by
whole-body counting. For ²²⁶Ra, the gamma-emitting progeny are ²¹⁴Pb
and ²¹⁴Bi, the latter of which emits gammas at energies of 0.609,
1.12, and 1.76 MeV. The 1.76-MeV gamma ray, because it is higher in energy than
the 1.46-MeV gamma ray from naturally occurring ⁴⁰K, is usually used
for WBC. The WBC gives the body content of ²¹⁴Pb/²¹⁴Bi,
and to derive the body content of ²²⁶Ra, some measurement or
assumption must be made to determine the retention of ²²²Rn (the
first decay product of ²²⁶Ra, and the parent of ²¹⁴Bi/²¹⁴Pb)
by the body. This can be done by a measurement of ²²²Rn exhaled in
the breath, but this technique is not readily available. In the long-term
follow-up of radium workers, the average long-term ²²²Rn retention
was 37 percent (Toohey et al. 1983), but this factor could be different for
recent exposures. 

Similarly, the first decay product
of ²²⁸Ra, ²²⁸Ac emits gamma rays of energies around 0.9
MeV and can be measured directly; since the half-life of ²²⁸Ac is
only 6.15 hours, it can be assumed to be in equilibrium with ²²⁸Ra.
Another member of the decay chain, ²⁰⁸Tl emits a strong gamma ray at
2.62 MeV, and its relative equilibrium with ²²⁸Ra can be determined
by comparing the measured activities of ²⁰⁸Tl and ²²⁸Ac
in vivo. Research-quality whole-body counters, such as the one at Argonne
National Laboratory-East, which was specifically designed for the detection of ²²⁶Ra
and ²²⁸Ra in former radium workers, have detection limits of a few
nCi (or about 100 Bq) of ²¹⁴Bi or ²²⁸Ac. A commercial
whole-body counter would have detection limits several times higher, because of
higher background levels.

It should be noted that typical
environmental intakes of ²²⁶Ra and ²²⁸Ra, such as from
well waters that exceed the Environmental Protection Agency standard for
drinking water (5 pCi [185 Bq] per liter for each radionuclide) would be
unlikely to exceed the detection limits of WBC, and indoor levels that high of ²²²Rn
(10s of pCi or 100s of Bq per liter of air) will severely interfere with the
measurement of ²²⁶Ra via ²¹⁴Bi.

Richard E. Toohey, CHP, PhD

Reference
Toohey RE, Keane AT, Rundo J. Measurement techniques for radium
and the actinides in man at the center for human radiobiology. Health Phys 44 (Suppl. 1):323-341; 1983.