Why are bananas radioactive?

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 Why are bananas radioactive?.

So, if you saw the chart from xkcd (I had it up shortly after the quake), you, like many other people, wondered about why Bananas were radioactive. Now you know.

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