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:

Q

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.

A

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

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 4 December 2013. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.