DON’T BE FOOLED BY LOW RA-226 AND MUCH HIGHER PB-214 AND BI-214 ACTIVITIES IN WATER AND LEACHATE SAMPLES

Gamma spectroscopy results for lead-214 (Pb-214) and bismuth-214 (Bi-214) are sometimes used as proxies for radium (Ra-226) activity in water or leachate samples. This practice is justified if users understand which of two forms of secular equilibrium is predominant in the sealed sample container. Except for the parent member, these decay chains are the same. Radon (Rn) and polonium (Po) appear in both:

Ra-226 → Rn-222 → Po-218 → Pb-214 → Bi-214

Rn-222 → Po-218 → Pb-214 → Bi-214

The former chain, applicable when a sample’s Ra-226 activity exceeds or equals its Rn-222 activity—such that Ra-226 is able to replenish Rn-222—reaches secular equilibrium after several half-lives of Rn-222, or about 20 days. The 1600-year half-life of Ra-226 then sets the pace of its own declining activity and the decay rate of progeny.

The latter chain, applicable when Ra-226 activity is less than Rn-222 activity—such that Ra-226 is insufficient to replenish Rn-222—reaches secular equilibrium after several half-lives of Pb-214, or about 5 hours. The 3.82-day half-life of Rn-222 then sets the pace of its own declining activity and the decay rate of progeny. Results indicative of latter-chain decay, not always recognized as such, are evident in samples from several sites ranging from New York to Oregon. Part of the explanation for the two decay chains is that Rn-222, as a gas, may drift away from its parent Ra-226, so either may be present in a sample at a greater activity than the other. Further explanation comes from the fact that neither Pb-214 nor Bi-214 lasts more than a few hours without replenishment from Rn-222.

A crucial aspect of the latter decay chain is that Rn-222 activity at the time of sample testing, as shown by Pb-214 and Bi-214 results, needs to be multiplied by a factor F that depends on the holding period (such as F = 2 for a 3.82-day holding time) in order to account for unreplenished Rn-222 decay within the sample. Specifically,

F = exp( −ln(2) Δt / T_1/2 )

where the holding period Δt is expressed as a negative number because it runs backward in time from sample testing to sample collection. The value thus obtained is the Rn-222 activity at the time of sample collection. Such a value provides an informed basis for radon management. It may be supplemented by follow-up testing for radon but should not be dismissed due to unfamiliarity with the latter decay chain.