Mutation: A genetic change, which is transmitted to offspring and affects hereditary characteristics. Such changes in genes may be caused by radiation as well as chemical and thermal agents. Neutrino: A weightless, uncharged particle which carries off energy in the process of beta decay. Neutron: A neutral particle, one of the basic constituents of the nucleus. A neutron weighs slightly more than a proton, and when free, decays into a proton plus an electron and a neutrino. Noble gases: Helium, neon, argon, krypton, and xenon. They do not combine chemically with any elements including themselves. Nuclear bomb: A bomb which derives its energy from nuclear fission or fusion. Nuclear reactor: A machine for maintaining a controlled chain reaction. Nucleus: The core of an atom, consisting of neutrons and protons. Its charge is equal to the number of protons. Its weight is equal to the number of protons plus the number of neutrons. Periodic system: The chemical elements arranged in order of increasing atomic charge. Elements with similar chemical properties occur periodically. Plutonium: Element with charge 94, produced by capturing a neutron in uranium²³⁸ followed by two beta emissions. Like uranium²³⁵, plutonium is valuable as an atomic fuel. Positron: The positive counterpart of the electron. Potassium⁴⁰: A natural radioactive isotope. It has a half-life of one billion years and emits beta and gamma rays. Proton: A constituent of the nucleus. It has one unit of positive charge and weighs slightly less than a neutron. Radiation: Energetic charged particles, neutrons and gamma rays which cause ionization in matter. Radiation is produced in nuclear explosions but also occurs naturally from cosmic rays and from the decay of radioactive substances in our surroundings. Radioactivity: Spontaneous nuclear decay, releasing an alpha, beta, or gamma ray. Radioisotope: Short for radioactive isotope. Radiological bomb: A bomb designed to create radioactive contamination. Radium: Element with charge 88. The principal isotope has a weight of 226 and emits an alpha particle with a half-life of 1620 years. Range: Distance traveled by an energetic charged particle in matter before it stops. Heavy charged particles move in a straight line inside matter, but electrons frequently change their course. For this reason the range of electrons is only about one-half the total distance traveled. Reactor: Same as nuclear reactor. Roentgen: A measure of radiation dose—defined in terms of the amount of energy deposited per unit weight of irradiated material. A dose of 400,000 roentgens in living tissue deposits enough energy to raise the temperature by 1°C. A dose of only 400 roentgens in a human being will cause death fifty per cent of the time.