Radioactivity is the spontaneous decay of an unstable atom through the emission, from the atomic nucleus, of a particle of ionising radiation. The different types of radiation can be identified by their ability to pass through matter. In 1899 Ernest Rutherford named alpha, α, beta, β, and gamma, γ, radiation, after the first three letters of the Greek alphabet.
A diagram showing an alpha particle (α) being ejected from the nucleus of an atom. Protons are red and neutrons are blue.
α-decay – when an atomic nucleus transforms into a different element by emitting an α-particle. An α-particle is the nucleus of a helium atom, helium-4, which consists of two protons and two neutrons. The atomic number of an α-particle is Z=2, so the atomic number of the decaying nucleus is decreased by two during an α decay and a different element is created. α-radiation can be stopped by a sheet of paper.
β decay – when a nucleus decays spontaneously by emitting an electron or a positron (the electrons positive antimatter partner). An electron is emitted when a neutron is converted to a proton inside the decaying nucleus. This tends to occur in proton deficient nuclei, where the neutron number is much greater than the proton number. A positron is emitted when a proton is converted into a neutron. During a β-decay a proton is either created or lost, so the atomic number of the decaying nucleus is changed by one. Most β-particles can be stopped by six millimetres of aluminium.
γ-decay – when a nucleus emits electromagnetic radiation in the form of a high energy photon or γ-ray. It can be caused by the redistribution of protons and neutrons in a nucleus, or by the de-excitation of an energetic nucleus. The atomic number is not changed during a γ-decay so the chemical element of the decaying nucleus does not change. γ-rays can be stopped by several millimetres of lead.