Thursday, 22 October 2009

Sharing your bed with radiation

So, last night I gave my "Field Guide to the Isotopes" talk at my home institution, the University of Surrey. It went, I think, pretty well, though the turn-out was on the low side for our evening lectures programme. I guess people must have been paying attention, as there were a few very pertinent questions along the way, and some interesting comments and questions at the end.

One of the comments was about the isotope potassium-40 (K-40). Potassium is an alkali metal, element number 19, which sits under sodium in the periodic table. It sits there because it has chemical properties similar to sodium, so it readily forms salts which are vital in the human body for the proper functioning of cells.

So, potassium is element number 19, which means that every nucleus of potassium has 19 protons in it but there are a few naturally-occurring isotopes which differ in the number of neutrons they have. The most common is K-39, next is K-41. They are both stable, but about 0.01% of potassium atoms has a K-40 nucleus at its centre. K-40 is radioactive (it will decay either to argon-40 or calcium-40), with a half life of around 1 billion years. Now, the earth is not so very much older than this (at around 4 billion years) so there is still some remaining K-40 that was created some time before 4 billion years ago to be found on the earth. Unlike the other main long-lived naturally-occurring radioactive isotopes (of Uranium and Thorium), Potassium is actually a biologically useful element, and so we all have some small amount of radioactive K-40 in us.

I expressed this in the talk by saying (and I don't take credit for it - it's been pointed out before) that if you sleep with someone else, you are getting an increased dose of radiation from their body by doing so - and vice versa. A questioner at the end was quite worried. Fortunately, humans have been sleeping with other humans for a long long time, and the doses are clearly not high enough to cause any trouble.

Still, all other things being equal, single people have a lower radiation dose!