Glossary — Atomic structure

Activity — the rate at which the unstable nuclei in a source decay, measured in becquerels (Bq). One becquerel is one decay each second.

Alpha particle — a particle of two protons and two neutrons, emitted from the nucleus of an unstable atom. It is the same as a helium nucleus.

Atomic number — the number of protons in the nucleus of an atom. It decides which element the atom is.

Background radiation — the ionising radiation that is present all the time, from natural sources such as rocks and cosmic rays and from man-made sources.

Beta particle — a fast electron ejected from the nucleus when a neutron turns into a proton.

Chain reaction — a reaction in which the neutrons released by one fission cause further fissions, so the reaction keeps going.

Contamination — the unwanted presence of radioactive atoms on or in another material.

Energy level — a distance from the nucleus at which electrons are arranged. An electron moves to a higher or lower level by absorbing or emitting electromagnetic radiation.

Fission — the splitting of a large and unstable nucleus into two smaller nuclei, releasing energy.

Fusion — the joining of two light nuclei to form a heavier nucleus, releasing energy. Some of the mass is converted into the energy of radiation.

Gamma ray — electromagnetic radiation emitted from the nucleus of an unstable atom. It has no mass and no charge.

Half-life — the time it takes for the number of nuclei of a radioactive isotope to halve, or for the count rate from a sample to fall to half its starting level.

Ion — a charged particle formed when an atom loses or gains one or more electrons.

Irradiation — exposing an object to nuclear radiation. An irradiated object does not become radioactive.

Isotopes — atoms of the same element, with the same number of protons but different numbers of neutrons.

Mass number — the total number of protons and neutrons in the nucleus of an atom.

Neutron — a particle in the nucleus with no electric charge.

Nuclear radiation — the particles or electromagnetic radiation emitted by an unstable nucleus as it decays.

Proton — a particle in the nucleus with a relative charge of +1.

Radiation dose — a measure of the energy that ionising radiation transfers to a person, measured in sieverts (Sv).

Radioactive decay — the random process in which an unstable nucleus gives out radiation as it changes to become more stable.

Tracer — a small amount of a radioactive isotope taken into the body so that its movement through an organ can be followed with a detector.