Most of us spend a good portion of our summers in our backyards enjoying the shade of our trees. In a world where many people are misinformed about radioactivity and concerned about it, have you ever regarded your trees as radioactivity sources?
Ever since our planet has had an atmosphere, cosmic rays from space have been producing neutrons when they collide with the atmosphere’s atomic nuclei. These neutrons in turn strike nitrogen atoms and occasionally produce carbon-14 (C-14), a radioactive isotope with a half-life of 5730 years.
C-14 mixes in the atmosphere and makes up just over 1 part in a trillion of all the carbon atoms in it. (The measured ratio of carbon 14 to atmospheric carbon is 1.33 x 10-12.) Trees incorporate atmospheric carbon as they grow and hence are sinks for C-14.
A reasonably-sized tree in a backyard weighs approximately 1000 kg. Of that material typically 50% is carbon. Wikipedia conveniently provides us with the information that “a gram of carbon containing 1 atom of carbon-14 per 1012 atoms will emit ~0.2 beta particles (C-14 decays) per second.”
For our tree (500 kg carbon), this translates to an impressive-sounding number of 100,000 disintegrations of C-14 per second, with each decay emitting a low-energy electron. (No gamma rays are associated with this decay.) The range of these electrons in wood or similar material is a fraction of a millimetre, so that even with the most sensitive radiation detector you are highly unlikely to be able to measure any radioactivity emitted by your tree while you relax in your outdoor hammock in the shade
Radioactivity has always been and will continue to be part of the natural world and not only the man-made “hazard” often associated with mushroom clouds, nuclear weapons or reactor accidents.
The Canadian Nuclear Heritage Museum houses an extensive library of books. Visitors are welcome to visit and browse the collection to learn more about the many aspects of radioactivity/radiation and our environment. You might want to learn how carefully measuring the amount of C-14 in ancient artifacts such as the Shroud of Turin leads to a determination of its age. To arrange a tour, send an email to info@nuclearheritage.com.
