Atomic cloud: The cloud remaining after the energy of the explosion has been carried off by the shock wave and the thermal radiation. It consists of condensed water vapor, ground material, and bomb debris including the radioactivity. Atomic energy: Energy released in nuclear reactions, for example in fission. Atomic energy and nuclear energy mean the same thing, but the latter name is more appropriate. Atomic reactor: Same as nuclear reactor. Background radiation: Natural radiation due to cosmic rays, and due to radioactive substances in the earth, in the atmosphere, and in our own bodies. Beta ray (particle): An energetic electron or positron emitted by some radioactive nuclei. Practically all of the fission products are beta (electron) emitters. Blast wave: Same as shock wave. Cesium¹³⁷: A radioactive fission product. It emits a 0.5 million volt beta ray and a 0.7 million volt gamma ray with a half-life of 30 years. The daughter nucleus is stable barium¹³⁷. Chain reaction: Self-maintained sequence of fissions. Neutrons released by the fission of one nucleus are used to induce fission in another nucleus. Chromosome: A small irregularly shaped body found in cells. Chromosomes carry the genes, which are responsible for heredity. Clean bomb: A nuclear bomb which produces heat and blast, but only a negligible amount of radioactivity. The energy of such a bomb is derived almost entirely from the fusion process. Cobalt⁶⁰: Radioisotope—decays into nickel⁶⁰ with the emission of a weak beta ray. The half-life for this decay is 5.3 years. The nickel⁶⁰ immediately ejects two gamma rays with a total energy of 2.5 million electron-volts. Cobalt bomb: A radiological bomb which produces a large quantity of cobalt⁶⁰. Control rod: A rod of neutron-absorbing material used to control the power level of a nuclear reactor. Cosmic rays: Energetic particles from outer space. They induce nuclear reactions in the earth’s atmosphere and thus contribute to the background radiation. This cosmic radiation is more intense at high altitudes than at sea level. Counter: A device which detects nuclear radiation. Critical mass: The amount of fissionable material required to sustain a steady chain reaction. With less than the critical amount, the reaction stops because too many neutrons are lost. Cyclotron: A machine that accelerates charged particles to high energy. Energetic charged particles can be used to induce nuclear reactions. Daughter: The nucleus which remains after decay of a radioisotope. Decay: Spontaneous process in which a radioactive nucleus emits an alpha, beta, or gamma ray.