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.
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.