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29 August 2008

Answer to Question #36 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation — Food, Plants, Animals

The following question was answered by an expert in the appropriate field:

What is the normal C-14 content for pine trees in California?

My naive answer would be that the specific activity of carbon (i.e., the activity of C-14 per unit mass of total carbon) would follow the specific activity in the atmosphere. There would be some changes over time, due to increases of anthropogenic sources of carbon over the last two centuries and due to atmospheric testing of nuclear weapons and fuel reprocessing over the last few decades. Thus, in looking at pine needles (say), the specific activity should be more or less the same as currently occurs in the atmosphere. In the tree trunks, however, the specific activity could vary with the age of the tree ring to follow the time history of the specific activity in the atmosphere. According to Killough and Rohwer [Health Phys. 34, 141 (1978)], the specific activity in the biosphere is about 6 pCi/g. This should be about the average value in pine trees in California. David Kocher ORNL

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