Fusion: The combining of light nuclei into heavier ones with a release of energy. For example, deuteron + triton → alpha + neutron. About 18 million electron-volts are released in this process. Gamma ray: Energetic, penetrating electro-magnetic radiation emitted by certain radioactive nuclei, frequently after a beta emission. Genes: Parts of the chromosomes. They are big molecules that determine heredity. Ground state: The state of least energy and greatest stability of atoms, molecules, and nuclei. Ground zero: The point on the surface of the earth directly above or below a nuclear explosion. Half-life: The time required for one half of a large number of identical radioactive nuclei to disintegrate. H-bomb: Same as hydrogen bomb. Heavy hydrogen: Same as deuterium. Heavy water: Water with heavy hydrogen substituted for ordinary hydrogen. Hydrogen bomb: A high-yield thermonuclear bomb. Iodine¹³¹: A radioactive fission product with a half-life of 8 days. It emits an electron of average energy 0.2 million electron-volts and a gamma ray of energy 0.4 million electron-volts. Ion: A charged atom or molecule. Ions are produced in abundance when energetic charged particles pass through matter. Ionization: The process of removing electrons from neutral atoms or molecules. Neutrons and gamma rays as well as energetic charged particles are very effective in producing ionization. Iridium¹⁹²: 75 day radioisotope. It emits an electron of average energy 0.2 million volts and a 0.3 million volt gamma ray. Isotopes: Atoms whose nuclei have the same number of protons but a different number of neutrons. Such atoms have the same chemical behavior. Kiloton: The amount of energy released by a thousand tons of TNT. Krypton⁸⁵: A radioactive fission product. It has a ten year half-life and emits an electron of average energy 0.2 million volts and a 0.5 million volt gamma ray. Leukemia: A usually fatal disease in which white blood cells are overproduced. Local fallout: Radioactive fallout in the neighborhood of a nuclear explosion. Megaton: The amount of energy released by a million tons of TNT. Meson: A particle intermediate in weight between the electron and the proton. Actually, there are two kinds of mesons, called pi and mu. The pi meson weighs 276 times as much as the electron and is connected with the forces that hold the nucleus together. The mu meson weighs 212 times as much as the electron and contributes appreciably to the cosmic radiation. Microsecond: One millionth of a second. It takes light 5 microseconds to go a mile. Million volt particle: Short for million electron-volt particle. Moderator: A material used in nuclear reactors to reduce the speed of neutrons. Molecule: A combination of atoms held together chemically.