Positrons from Bananas
Posted by Flip Tanedo on 21 Jul 2009 at 12:16 pm
I was recently preparing a “Physics of Angels & Demons” talk for a group of high school physics teachers who were visiting Cornell for a “Contemporary Physics for Teachers” workshop. While researching ‘natural sources of antimatter,’ I discovered a curious article about a naturally occurring potassium isotope that, some fraction of the time, decays via positron emission. The conclusion was that:
Tthe average banana (rich in potassium) produces a positron roughly once every 75 minutes.
Now any time you find something like this you have to remember that not everything on the Internet is true — not even Wikipedia, but I checked it out (e.g. the LBNL/Lunds table of isotopes) and indeed this seems to be correct!
Potassium-40 (40K) is a naturally occurring isotope that is unstable and decays, but it has a huge half life, about a billion years. These days only a small fraction (100 parts per million) of potassium atoms are actually 40K, but objects that are dense in potassium — such as bananas — are likely to have tens of micrograms of the stuff. If one crunches the numbers (as they do in the original article), it turns out that bananas pop out a positron every 75 minutes or so.
These positrons quickly annihilate with ambient electrons, perhaps undergoing some other interactions and releasing some photons beforehand. I’m sure the bloggers here who work on LHC calorimetry would have a better description of what happens to it! Advanced readers can read the “Passage of particles through matter” section of the PDG.
Potassium plays a necessary role in our biology, so yes, even you produce positrons every once in a while.
Yes, I was surprised to hear this too. Funnily enough, the fact was mentioned at a talk on the science of A&D here in Ireland!
This is common knowledge among people working with low background counting experiments, like dark matter searches, and who are concerned about even very small levels of contamination. Leading to jokes, of course, about banana-free diet for scientists, and about the radioactivity of people who eat table salt alternatives based on potassium.
And it was a part of an undergrad lab exercise I once taught — not bananas, but 40K in the environment (lots of it in windows, and some of it in people and other things) was visible in the spectrum when we were looking at the spectrum of some other isotope.
Fun to know!
[...] original blog and other LHC-related blogs at the US LHC Blog Web [...]
[...] (via US/LHC-Blogs) [...]
How much Chernobyl fallout does an average banana contain? Does any of that emit positrons? It seems to me that bananas contain a lot more than just potassium, and contamination from human sources might well be far more significant. Th original article assumes that all the antimatter in a banana is generated by decaying potassium atoms.
Oh hi, I wrote that! I was toying with the idea of bringing a “big bag of antimatter” to a talk as a prop, but any reasonably-sized bag would probably not actually have contained antimatter for the duration of the talk. I thought about potassium salt but it’s just not as funny. Didn’t know about window glass.
Tim J: The fallout in the air from fusion reactions (some from Chernobyl but more from the era of atmospheric nuclear weapons tests) tends to be neutron-rich. Unstable neutron-rich isotopes want to become less neutron-rich, so they decay by emitting negative electrons. So that doesn’t add positrons (though it adds antineutrinos).
The total amount of fallout in a banana would be a hard thing to calculate, but it’s been measured for wine.
Thanks for the response, Rob!