Answer to Question #9281 Submitted to "Ask the Experts"
Category: Radiation Basics
The following question was answered by an expert in the appropriate field:
Will uranium be in secular equilibrium with radium in vegetation samples after one month? I would like to measure 214Bi by gamma spectrometry as a means of estimating the uranium content of the plant.
Unfortunately, one month is not sufficient time to allow radium to come into equilibrium with the uranium. The decay of 238U proceeds initially through a long series, first to 234Th, which has a 24.1 day half-life, next 234mPa, with a 1.17 minute half-life, and thirdly 234U, which has a half-life of 2.74 × 105 years. This third member of the chain is the major block in the decay chain to radium and subsequent progeny. In fact, if 238U were taken up and distributed in plant leaves, with no significant initial uptake of uranium progeny, more than 1.2 × 106 years (five half-lives of the 234U) would be necessary for the subsequent progeny to achieve equilibrium with the 238U.
If the plants were growing in soil containing aged uranium in which the progeny were already in equilibrium with the 238U, then an equilibrium quantity of 234U would probably be taken up by the plant since it is unlikely the plant would distinguish isotopically between 238U and 234U. Even so, however, the daughter of 234U is 230Th, which has a half-life of 8.0 × 104 years, which means that almost half a million years would be required for the 230Th to achieve equilibrium with the 234U and 238U, assuming that the plants did not also take up the thorium in equilibrium amounts. Of course, the plants of interest may also take up 226Ra directly, and if this is the case, you would have to have additional information to ascertain the relationship between the 238U and 226Ra activities.
The bottom line is that radium would not achieve equilibrium with the uranium in any reasonable amount of time unless the plant took up equilibrium quantities of other uranium progeny, including 226Ra. We should also point out that the 214Pb and 214Bi that emit gamma radiation, often used to assess 226Ra content in some environmental samples, are produced by decay through 222Rn, the daughter of 226Ra. As you know, radon is a gas and is often released to varying degrees through plant leaves. Thus, if you had a situation in which you wanted to assess the radium content of the plant by measuring the 214Pb and/or 214Bi, you would have to determine whether radon was being released from the plant and, if so, to what extent.
I’m sorry I cannot offer more encouragement for the analytical method you are considering.
George Chabot, PhD, CHP