What is the process by which a nucleus emits alpha, beta, or gamma radiation?

The process by which a nucleus emits alpha, beta, or gamma radiation is radioactive decay.

An unstable atomic nucleus releases energy and moves toward a more stable state during radioactive decay. In alpha decay, it emits a helium-4 nucleus containing two protons and two neutrons. In beta decay, a neutron or proton changes type through the weak interaction, producing an electron or positron and a neutrino-related particle. Gamma decay releases excess nuclear energy as a high-energy photon.

Radioactivity was discovered by Henri Becquerel in 1896 while studying uranium salts. Ernest Rutherford later identified and named alpha, beta, and gamma radiation, partly because the three emissions differed in penetrating power and their responses to electric and magnetic fields.

A common mix-up is confusing radioactive decay with nuclear fission or fusion. Fission splits a heavy nucleus, while fusion combines light nuclei; either can occur in nuclear reactions. Radioactive decay is spontaneous and random for individual atoms, although large samples follow predictable statistical patterns described by a half-life.