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biological effects of radiation--are discussed briefly to provide some necessary basic knowledge For further study, see the com prehensive reference list at the end of the chapter Meanings of "Atomic Terms" Before discussing the characteristics of ionizing radiation, it would be best to define many of the terms used A word printed m italics is defined elsewhere m this glossary Accelerator A device for imparting very high velocity to charged particles such as electrons or protons These fast particles can penetrate matter and are types of radiation Fast particles of this type are used in research or to study the structure of the atom itself Some accelerators are called atom smashers Activation Making a substance artifically radioactive in an accelerator such as a cyclotron or by bombarding it with neutrons in a reactor Alpha particle (alpha ray, alpha radiation) A small electrically charged particle of very high velocity thrown off by many radioactive materials, including ura nium and radium It is identical with the nu cleus of a helium atom and is made up of two neutrons and two protons Its electric charge is positive and twice as great as that of an electron Anti-particle A particle which interacts with its counterpart of the same mass but op posite electric charge and magnetic properties (e g, proton and anti-proton or neutron and anti-neutron), with complete annihilation of both and production of an equivalent amount of radiation energy The positron and its anti-particle, the electron, annihilate each other upon interaction and produce gamma rays Atom The tiny "building block" ofnature All materials are made of atoms The ele ments, such as iron, lead and sulfur, differ from each other because they contain differ ent atoms Atoms are unbelievably small No one has ever seen one There are six sextillion (6 followed by 21 zeros) atoms in an ordinary drop of water The word "atom" comes from the Greek work meaning indivisi ble Now we know it can be split and con sists of an inner core (nucleus) surrounded by electrons which rotate around the nucleus like the planets around the sun Atomic energy Energy released m nu clear reactions Of particular interest is the energy released when a neutron splits an atom's nucleus into smaller pieces (fission) or when two nuclei are joined together under millions of degrees of heat (fusion) "Atomic energy" is really a popular misnomer It is more correctly called "nuclear energy " Atomic number The number of protons (positively charged particles) found m the nucleus of an atom All elements have differ ent atomic numbers The atomic number of hydrogen is I, that ofoxygen 8, iron 26, lead 82, uranium 92 The atomic number is also called charge number and is usually denoted by Z Atomic theory Since the time of the ancient Greeks man has held the theory that all matter is composed of tiny, invisible par ticles called atoms It remained for the chem ists and physicists of the 19th and 20th cen turies to verify the existence of the atom and the validity of the atomic theory Atomic weight The atomic weight is approximately the sum of the number of protons and neutrons found in the nucleus of an atom This sum is also called mass number The atomic weight of oxygen, for example is approximately 16, with most oxy gen atoms containing 8 neutrons plus 8 protons Aluminum is 27--it contains 14 neutrons and 13 protons ATOM SMASHER. A machine (an accelera tor) that speeds up atomic and sub-atomic par ticles so that they can be used as projectiles to literally blast apart the nuclei of other atoms Autoradiography Self-portraits ofradio active sources made by placing the radio active material next to photographic film The radiations fog the film leaving an image of the source It was such self-portraits that led to the discovery of radioactivity This glossary is adapted from one originally published as "101 Atomic Terms and What They Mean," by the Esso Research and Engineering Company, and is reproduced here by permission 1433