Answer to Question #7249 Submitted to "Ask the Experts"

What method is best for measuring ²²⁶Ra and ²²⁸Ra in the body? Is there a whole-body method, not a urine count?

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.