Why are bananas radioactive?

The chart of relative doses of radioactivity that appeared on io9 yesterday set many minds at ease, but also raised questions. Questions like, "Why do you get dosed with radioation when eating a banana?"

The chart, created by XKCD's Randall Munroe, was intended to show people the realistic danger of the recent leaks at the Fukushima nuclear plant in Japan. Contact with radioactive substances is just a part of everyday activities like taking a flight, sleeping next to someone, or eating a banana. But wait. Sure, airplanes and x-rays expose people to radiation, but eating a banana? Why a banana? And how? Sure, the dose is low - one banana will only expose someone to one ten millionth of a sievert - but there must be something about them to get them on that chart.

It turns out that using bananas to measure doses of radiation has precedent. There's even a name for it, a Banana Equivalent Dose or BED. The BED was invented pretty much exactly for charts like that. It helps put danger in perspective, especially when it comes to food and radiation, but the BED is not random. There are quite a few foods that are naturally radioactive, and the banana is an extreme example.

The element that makes bananas official radioactive food is an isotope of potassium. Potassium-40 (K-40) is about .01 percent of all potassium. It has a half-life of around 1.25 billion years, which means it's not going anywhere. K-40 decays two different ways. About 89% of the time, one of its neutrons decays to a proton, turning it from potassium to calcium. When it does this, it emits a beta particle - an electron. About 11% of the time, potassium decays by capturing an electron and turning one of its protons into a neutron. When it does this it emits gamma rays - very high intensity radiation. Exposure to enough beta radiation or gamma radiation can cause radiation sickness and high rates of cancer.

Bananas experience about 14 decays per second, a rate measurable by commercial radiation sensors. This may sound like a lot, but it would take the consumption of around five million bananas in a sitting to give anyone radiation sickness. (And the body itself is full of potassium, since it needs the element to stay alive.) Any food that has a lot of potassium has the same percentage of the radioactive K-40. Commercially-made salts have K-40 in them, and bags of such salts are used in classrooms to do experiments measuring radiation.

Other radioactive foods include potatoes, nuts (especially Brazil nuts), and kidney beans. So the perfect radioactive meal would be a potato and kidney bean stew with banana nut bread for dessert.

Via About.com, The Energy Collective, Argonne National Laboratory, and Today I Found Out.