(226)Radium - (222)Radon decay data
The following question was answered by an expert in the appropriate field:
Q: Does anyone know what the grow-in time is for the progeny of 226Ra? I'm especially interested in 214Pb, 214Bi, and 210Pb. During work on an old radium purification site, I've been curious how close the 210Pb, 214Bi, and 214Pb would be to the 226Ra. The plant stopped operations in the mid-1920s. Analytical results have shown the 226Ra/214Bi/214Pb to be about 42%/29%/29%, respectively, of the reported activity. To date no 210Pb results have been reported. So the true question I want to ask is after almost 80 years, is the 29% 214Pb/Bi from original activity or is it from grow in? And might there actually be 210Pb present that hasn't been reported?
A: Radium-226 (226Ra) decays to a noble gas, radon-222 (222Rn). 226Ra has a 1,600 year half-life while 222Rn has a 3.8 day half-life. The latter decays to produce four short-lived decay products also known as progeny or daughters. These short-lived daughters are polonium-218 (218Po), lead-214 (214Pb), bismuth-214 (214Bi), and polonium-214 (214Po). Their effective half-life is approximately 30 minutes. 214Po decays to produce the long-lived 210Pb (ca. 20 year half-life) which then decays to 210Bi and 210Po. Gamma spectroscopy is the most common (and cheapest) method available for the identification and quantification of radioactive material. Of the radionuclides mentioned above, only 226Ra, 214Bi, and 214Pb emit gamma rays that permit analysis by gamma spectroscopy. 210Pb emits a weak low-energy gamma ray but it does not permit an "easy" analysis. It is a safe bet that the activities you report were determined by counting the 226Ra, 214Pb, and 214Bi gamma rays.Given the short half-life of the 214Pb and 214Bi, it is obvious that their presence after 80 years is due to grow in. Without going into the mathematics, it is safe to say that if we start with a pure parent, its shorter-lived decay products will achieve an activity equal to that of the parent after five or so half-lives of the decay products. For example, after one month, 222Rn's activity will equal the activity of its parent 226Ra. After, a few hours, the activity of 218Po, 214Pb, 214Bi, and 214Po will equal the activity of 222Rn. And after 80 years, the activity of 210Pb, 210Po, and 210Bi will be very similar to these. In general, I would expect, as a first approximation, that 226Ra and all of its decay products will have very similar activities.A catch is that 222Rn is a gas and some of it might escape the sample/material in question. While we would expect the concentration of 222Rn in unencapsulated material to be less than the activity of the 226Ra, the loss is usually not too great and the two activities are often very similar. While I might expect the activities of the 214Pb and 214Bi to be closer to the 226Ra activity than indicated in your question, the ratio you report is not unreasonable.However, there are at least two reasons why a gamma spectroscopy analysis of the 226Ra gamma ray might have overestimated its activity relative to that of 214Pb and 214Bi. The first is that the gamma emission rate (yield) from 226Ra is now believed to be higher than it was thought in the past (3.6% compared with 3.2%). If the current value is the more accurate, older analyses will have overestimated the 226Ra. The second reason is that uranium-235 (235U) emits a gamma ray at an energy almost identical to that of 226Ra (186 keV). This means that the presence of 235U in the sample can lead to an overestimate of the 226Ra. For these two reasons, it is almost standard practice to quantify 226Ra by measuring the activity of 214Pb and 214Bi rather than quantify the 226Ra directly via its 186 keV gamma.
Paul Frame, CHP, PhD
Answer posted on May 22, 2002. The