Radioactive decay modes

An unstable nucleus rids itself of excess energy through radioactive decay, and different nuclei do this in different characteristic ways. Distinguishing these decay modes matters because each changes the nucleus differently and involves a different underlying force.

Alpha decay ejects an alpha particle — a helium-4 nucleus consisting of two protons and two neutrons bound tightly together — from the parent nucleus. This reduces the mass number by 4 and the atomic number by 2, transforming the nucleus into a different element two places lower on the periodic table. Alpha decay occurs mainly in heavy nuclei, where ejecting a tightly bound alpha particle is energetically favourable compared to the alternative configurations.

Beta decay operates through the weak nuclear force rather than the strong force that governs alpha decay, and converts one type of nucleon into another inside the nucleus. In beta-minus decay, a neutron converts into a proton, an electron, and an antineutrino, with the electron and antineutrino ejected from the nucleus; the atomic number increases by 1 while the mass number stays the same, since a neutron has simply become a proton. Beta-plus decay is the mirror process: a proton converts into a neutron, a positron, and a neutrino, decreasing the atomic number by 1. In neither case is the emitted electron or positron a pre-existing particle that was "inside" the nucleus beforehand — it is created at the moment of decay, along with its accompanying neutrino, out of the energy released by the conversion.

Gamma decay is different in kind from both. It does not transform the nucleus's identity at all — no protons or neutrons are gained, lost, or converted. Instead, it occurs when a nucleus is left in an excited energy state, typically as an aftermath of a prior alpha or beta decay that left the daughter nucleus with excess internal energy, and that excited nucleus drops to a lower energy state by emitting a high-energy photon (a gamma ray) — directly analogous to how an excited atomic electron drops to a lower orbital by emitting a lower-energy photon, but here the transition occurs between energy levels of the nucleus itself rather than of an orbiting electron. Because neither the mass number nor the atomic number changes, gamma decay leaves the nucleus as the same isotope, just at a lower, more stable internal energy.