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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
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.