radioactivity, the process by which an unstable nucleus emits one or more particles or energy in the form of electro-magnetic radiation, nuclear radiation, the particles that are released from the nucleus during radioactive deca, alpha particle, a positively charged atom that is released in the disintegration of radioactive elements and that consists of two protons and two neutrons, beta particle, a charged electron emitted during certain types of radioactive decay, such as beta decay, gamma ray, the high-energy photon emitted by a nucleus during fission and radioactive decay, half-life, the time required for half of a sample of a radioactive substance to disintegrate by radioactive decay or by natural processes, fission, the process by which a nucleus splits into two or more fragments and releases neutrons and energy, nuclear chain reaction, a continuous series of nuclear fission reactions, critical mass, the minimum mass of a fissionable isotope that provides the number of neutrons needed to sustain a chain reaction, fusion, the process in which light nuclei combine at extremely high temperatures, forming heavier nuclei and releasing energy, background radiation, the nuclear radiation that arises naturally from cosmic rays and from radioactive iso-topes in the soil and air, rem, the quantity of ionizing radiation that does as much damage to human tissue as 1 roentgen of high-voltage X rays does, radioactive tracer, a radioactive material that is added to a substance so that its distribution can be detected later

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