How potassium-40 makes a positron
Ordinary bananas contain a small amount of potassium-40, a naturally occurring radioactive isotope of potassium. An isotope is a version of an element with a different number of neutrons in its atoms. Potassium-40 is unstable, so over time some of its atoms change into other elements.
In a rare kind of decay, a potassium-40 nucleus emits a positron—the positively charged counterpart of the negatively charged electron. That particle is the banana’s fleeting piece of antimatter. Most potassium-40 decays follow other paths, and the positrons are produced one at a time, not in a visible burst.
The antimatter disappears almost immediately
A positron does not last long in ordinary matter. As it moves through the banana and its surroundings, it can encounter an electron. The two particles then annihilate: their mass is converted into energy, typically carried away by gamma rays, a highly energetic form of light.
That does not mean the fruit is a meaningful source of radiation or energy. The decays are extremely sparse, and the energy from any one event is microscopic. The positron’s journey and disappearance happen at scales far too small to see or feel.
A real event, not a hazard
Potassium-40 is also present in people and many foods, so this kind of natural radioactivity is part of everyday life. The body regulates potassium levels, but it cannot distinguish potassium-40 from other potassium atoms; the isotope is simply included in the normal turnover of potassium.
The memorable twist is literal but modest: a banana can produce a positron, and that positron can meet an electron and vanish in a flash of particle-scale energy. Nothing about the fruit changes in a way a person could notice.
