This glossary lists terms in common use in the nuclear sciences. Fifteen terms used in radiation protection are listed in Table 2.2 and properties of nuclear particles are summarised in Table 1.2.

Absorption (1) The retention, in a material, of energy removed from radiation passing through it. (2) The process whereby neutrons (or other elementary particles) are captured by nuclei.

Activation analysis A method of analysis based on the identification and measurement of characteristic radiation from radio nuclides formed by neutron irradiation in a sample of the irradiated material.

Activity (of a substance) Short for radioactivity. The number of disintegrations per unit time taking place in a radioactive material. The unit of activity is the becquerel (Bq), one disintegration per second.

Alpha particles Energetic helium nuclei (4He) emitted from the nuclei of higher Z number atoms (Z > 60) during alpha particle decays.

Annihilation radiation The electromagnetic radiation resulting from the mutual annihilation of the mass of negatively charged electrons (negatrons) and positrons (β− − β+). The annihilation radiation consists of two photons, each 0.51 MeV, emitted in opposite directions.

Atomic mass unit One-twelfth of the mass of a carbon-12 atom and approximately equal to the mass of a single proton or neutron. It is abbreviated amu.

Atomic number The number (Z) of protons in the nuclei of the atoms of an element, indicating the position of that element in the periodic table.

Atomic weight For a given specimen of an element, the mean weight of its atoms, expressed in either atomic mass units (physical scale) or atomic weight units (chemical scale).