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 nuclear species, 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 plants to animals to man. Thecollection 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 andtritium, 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 en- ergy 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. The half-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 time in which the quantity in the body will decrease to half as a result of both radioactive decay and biological 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, ete. 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: The process in which the body breaks down foods into usable materials that are taken into the cells and manufactured into the living tissues of the body. RN MICROCURIE: A one-millionth part of a curie, MILLIREM: A one-thousandth part of a rem. MILLIROENTGEN: A one-thousandth part of a roentgen. OVERPRESSURE: Thetransient pressure, usually expressed in pounds per square inch, exceeding the ambient pressure, manifested in the shock (or blast) wave from an explosion. PICOCURIE: Onemillionth of a millionth of a curie. RAD: A unit of absorbed dose of radiation; it represents the absorption of 100 ergs of nuclear (or ionizing) radiation per gram of the absorbing material or tissue. . RBE (OR RELATIVE BIOLOGICAL EFFECTIVENESS): The ratio of the number of rads of gamma (or X-) radiation of a certain energy which will produce a specified biological effect to the number of rads of another radiation required to produce the sameeffect is the RBE of this latter radiation. REM: A unit of biological dose of radiation; the nameis derived from theinitial letters of the term “roentgen equivalent man (or mammal).” The number of rems of radiation is equal to the number of rads absorbed multiplied by the RBE ofthe given radiation(for a specified effect). REP: A unit of absorbed dose of radiation now being replaced by the rad; the namerep is derived from the initial letters of the term “roentgen equivalent physical.” Basically, the rep was intended to express the amount of energy absorbed per gram of soft tissue as a result of exposure to 1 roentgen of gamma (or X-) radiation. RESIDENCE HALF-TIME: As applied to delayed fallout, it is the time required for the amount of weapon debris deposited in a particular part of the atmosphere,e.g., stratosphere or troposphere, to decrease to half of its initia) value. ROENTGEN: A unit of exposure dose of gamma (or X-) radiation. It is defined precisely as the quantity of gamma (or X-) radiation such that the associated corpuscular emission per 0.001293 gram of air produces, in air, ions carrying one electrostatic unit quantity of electricity of either sign. STRATOSPHERE: A relatively stable layer of the atmosphere between the tropopause and a height of about 30 miles in which the temperature changes very little (in polar and temperate zones) or increases (in the tropics) with increasing altitudes. In the stratosphere clouds of water never form and there is practically no convection. TNT EQUIVALENT: A measure of the energy released in the detonation of a nuclear (or atomic) weapon, or in the explosion of a given quantity offissionable material, expressed in terms of the weight of TNT which would release the same amount of energy when exploded. The TNT equivalent is usually stated in kilotons or megatons. TRITIUM: A radioactive isotope of hydrogen, having a mass of 3 units; it is produced in nuclear reactors by the action of neutrons on lithium nuclei. TROPOPAUSE: The imaginary boundary layer dividing the stratosphere from the lower part of the atmosphere, the troposphere. The tropopause normally occurs at an altitude of about 25,000 to 45,000 feet in polar and temperate zones, and at 55,000 feet in the tropics. TROPOSPHERE: The region of the atmosphere immediately above the earth’s surface and up to the tropopause in which the temperaturefalls fairly regularly with increasing altitude, clouds form, convection is active, and mixing is continuous and more or less complete. WEAPON DEBRIS: The highly radioactive material, consisting of fission products, various products of neutron capture, and uranium and plutonium that haveescaped fission, remaining after the explosion. 51