Delayed neutrons: Those released after a fraction of a second to a half-minute or so by the fission products. They comprise less than one per cent of the total number of neutrons released in the fission process but are useful for the purpose of control in reactors. Deuterium: Stable hydrogen isotope. Its nucleus (called a deuteron) consists of one proton and one neutron. Disintegration: Same as decay. Dose: A quantity of radiation—usually measured in roentgens. E = mc²: Einstein’s equation relating mass (m) and energy (E). The speed of light (c) enters as a proportionality constant. The equation asserts that one pound of mass is equivalent to ten megatons of energy. In the fission process only one-tenth of one per cent of the mass is converted. Therefore, to produce ten megatons of energy by fission 1000 pounds of uranium would be required. Electromagnetic radiation: Includes radio waves, visible, infrared, and ultraviolet waves; also X-rays and gamma rays. The latter two are energetic, penetrating forms of radiation. Electron: A particle having a unit negative charge and a weight equal to 1/1840 of the weight of the lightest atom (hydrogen). Electron capture: process in which an atomic electron unites with a proton in the nucleus producing a neutron and a neutrino. Electron-volt: The amount of energy acquired by an electron which is accelerated through an electric potential of one volt. Typically, the energy required to “knock” an electron out of an atom is a few electron-volts or so; particles ejected from radioactive nuclei have energies between a few hundred thousand and a few million electron-volts. Element: A collection of atoms whose nuclei all have the same charge. An element may consist of many isotopes. Enriched material: Uranium which contains a greater proportion of the 235-isotope than is found in the natural ore. Excited state: A state of an atom, molecule, or nucleus having excess energy. As soon as possible this excess energy is released and the system goes to the ground state. Fallout: Radioactive particles from an atomic explosion. They may be carried in the atomic cloud to large distances from ground zero and then “rained down” to the earth’s surface. Fireball: The luminous ball of hot air and bomb material which expands and cools as the shock wave races out. Fission: The breaking-up of a heavy nucleus into two or more fragments. A large amount of energy and some free neutrons are released in the process. Fissionable material: Isotopes which undergo fission when bombarded by _slow_ neutrons: uranium²³⁵ and plutonium²³⁹. Fission products: Fission fragments and their daughters, including hundreds of different radioactive species, among them strontium⁹⁰ and cesium¹³⁷.