by the actual splitting of the heavy-element nuclei. Something like 80 different fission fragments result from roughly 40 different modesof fission of a given nuclearspecies, i.e., uranium 235 or plutonium 239. Thefission fragments, being radioactive, immediately begin to decay, forming additional (daughter) products, with the result that the complex mixtureof fission products so formed contains about 200 different isotopes of 36 elements. FOOD CHAIN: The sequence of events in which nutrients are transferred from the soil to piants to animals to man. The collection of these various stages is referred to generally as the biosphere. FREE AIR OVERPRESSURE (OR FREE FIELD OVERPRESSURE): The unreflected pressure, in excess of the ambient atmospheric pressure, created in the air by the blast wave from an explosion. FUSION: The process whereby the nuclei of light elements, especially those of the isotopes of hydrogen, namely, deuterium and tritium, combine to form the nucleus of a heavier element with the release of substantial amounts of energy. GAMMA RAYS (OR RADIATIONS): Electromagnetic radiations of high energy originating in atomic nuclei and accompanying many nuclear reactions, e.g., fission, radioactivity, and neutron capture. Physically, gamma rays are identical with X-rays of high energy, the only essential difference being that the X-rays do not originate from atomic nuclei, but are produced in other ways, e.g., by slowing down (fast) electrons of high energy. GROUND ZERO: The point on the surface of land or water vertically below or above the center of a burst of a nuclear (or atomic) weapon; frequently abbreviated to GZ. HALF-LIFE: The time required for the activity of a given radioactive species to decrease to half of its initial value due to radioactive decay. Thehalf-life is a characteristic property of each radioactive species and is independent of its amount or condition. The effective half-life of a given isotope is the er time in which the quantity in the body will decrease to half as a result of both radioactive decay andbiological elimination. INDUCED RADIOACTIVITY: Radioactivity produced in certain materials as a result of nuclear reactions. particularly the capture of neutrons, which are accompanied by the formation of unstable (radioactive) nuclei. The activity induced by neutrons from a nuclear (or atomic) explosion in materials containing the elements sodium, manganese, silicon, or aluminum may be significant. INVERSE SQUARE LAW: The law which states that when radiation (ther- mal or nuclear) from a point source is emitted uniformly in all directions. the amount received per unit area at any given distance from the source, assuming no absorption, is inversely proportional to the square of that distance. ISOTOPES: Forms of the same element having identical chemical properties but differing in their atomic masses (due to different numbers of neutrons in their respective nuclei) and in their nuclear properties, e.g.,radioactivity, fission, etc. KILOTON ENERGY: The energy of a nuclear (or atomic) explosion which is equivalent to that produced by the explosion of 1 kiloton(i.e. 1,000 tons) of TNT. MEGATON ENERGY: The energy of a nuclear (or atomic) explosion which is equivalent to 1 million tons (or 1,000 kilotons) of TNT. METABOLISM: Theprocess in which the body breaks down foodsinto usable materials that are taken into the cells and manufactured into the livingtissues of the body. 50