During beta plus decay, a proton inside an unstable nucleus converts into a neutron. This releases a positron and a neutrino at the same time. Because a positron is antimatter, it cannot exist near ordinary matter for long.
The instant a positron collides with a regular electron, both particles annihilate each other completely. Their entire combined mass converts into pure energy, following Einstein’s famous equation, energy equals mass times the speed of light squared. This burst of energy comes out as two gamma ray photons flying off in opposite directions.
Doctors put this reaction to practical use in a medical scan called PET, short for positron emission tomography. Patients receive a small amount of a radioactive tracer that emits positrons. As those positrons annihilate with electrons inside the body, the resulting gamma rays are detected and used to build a picture of metabolic activity.
This makes PET scans especially useful for spotting cancer, since fast-growing tumor cells often use far more sugar than normal, healthy tissue does. Physicist Paul Dirac first predicted the positron mathematically in 1928. Carl Anderson then confirmed it experimentally in 1932, giving scientists their first real proof that antimatter exists.
Outside of a lab or a PET scanner, positrons rarely last long in the everyday world, since ordinary matter is everywhere.