Atom
The smallest particle of matter which determines the chemical and physical properties of an element. Atoms have a central nucleus (made up of protons and neutrons surrounded by a cloud of electrons. Elements are identified by the number of protons in the nucleus of their atoms as this number is unique to each element.

Beta particle
An electron released at high speed by some radioisotopes. They don't travel very far -- only a few millimetres in human tissue -- and are easily stopped by aluminium foil.

Control rods
Control rods regulate the number of neutrons in the core of a nuclear reactor and so control the rate of the reaction. They are plates or tubes holding material, such as cadmium or boron, that absorbs neutrons.

Electron
A negatively charged particle that whizzes around the nucleus of an atom.

Gamma rays
Part of the electromagnetic spectrum similar to X-rays but with shorter wavelengths and greater penetrative power (they can only be stopped by layers of lead or thick concrete).

Heavy Water
Water with a high proportion of deuterium (one proton and one neutron) instead of hydrogen (with one proton, but no neutron in the nucleus).

Isotope
Atoms of the same element (that is, having the same number of protons) but with different numbers of neutrons. Some are stable, others are unstable and radioactive (see radioisotope).

Light Water
Ordinary water. Water molecules are made of two atoms of hydrogen and one of oxygen.

Neutron
A particle in the nucleus of an atom weighing about the same as a proton, but carrying no charge.

Neutron Scattering
A technique for 'seeing' fine details of the structure of a substance. It involves firing a beam of neutrons at a sample and watching how they scatter off it. Neutrons pass in between atoms, unless they collide with the nucleus. When they do, they don't bounce off randomly, but deflect down a specific path and different structures create different pathways.

Nuclear Magnetic Resonance (NMR)
An imaging technique which relies on the way some atoms have a nucleus that spins.

When a sample is put in a magnetic field, some of its nuclei align themselves with that field. Radio waves tuned to the frequency at which those nuclei spin (or resonate) are used to knock them out of alignment. When the radio is turned off, the nuclei fall back into line, releasing pulses of radio waves that the imaging machine uses to build up a picture.

Periodic Table
The chart where all the elements are arranged so that those with similar chemical properties are in the same column.

Proton
A particle in the nucleus of an atom weighing about the same as a neutron but carrying a positive charge.

Radioisotope
Unstable isotopes that release radiation when they break down. 'Unstable' is a relative term: some radioisotopes last only for seconds, others for millions of years. Some occur naturally, for example carbon-14 used in radiocarbon dating; others are made artificially, such as americium-241 used in smoke detectors.

Radiopharmaceutical
A compound, some of whose atoms are radioisotopes, used in a medical procedure.

X-rays
Part of the electromagnetic spectrum with a wavelength shorter than visible light, but longer than gamma rays. They are able to penetrate solids, but not to the extent that gamma rays can.