Answer to Question #10849 Submitted to "Ask the Experts"
Category: Radiation Basics — Alpha Radiation
The following question was answered by an expert in the appropriate field:
I'm currently working on ecological risk assessment for one naturally enhanced radiation area especially rich in thorium. To be able to run a model for a dose calculation, I need levels of 228Th in biota. I have results 232Th, 228Ra but not for 228Th. People who conducted gamma spectrometry suggest I use 228Ra for 228Th as they are in equilibrium. I'm wondering if they are in equilibrium in plant samples.
You have posed a most interesting question, which unfortunately is not simply answered. However, the suggestion to assume secular equilbrium between 228Ra and 228Th is a good one and will likely give an upper limit value on the amount of, and hence the calculated dose contribution from, 228Th and thus is a conservative answer to your question. But take note of this caveat: inasmuch as the fractionation, uptake, and retention of thorium and radium in plants is not well known and is certainly variable among different plant species, soils, and availability of trace minerals, scenarios could be postulated that will yield different values. This is clearly demonstrated by the known high variability of radium in plant-based foodstuffs, with concentrations ranging over several orders of magnitude, ranging from about 0.4 mBq kg-1 in fruits to a high of about 14 Bq kg-1 in Brazil nuts. Wheat flours, peanuts, and chocolate typically have concentrations of 226Ra of about 0.4 Bq kg-1.
Ron Kathren, CHP