Fallout- The process or phenomenon of the descent to the earth's surface of particles contaminated with radioactive material from the radioactive cloud. The term is also applied in a collective sense to the contaminated particular matter itself. The early (or local) fallout is defined, somewhat arbitrarily, as those particles which reach the earth within 24 hours after a nuclear explosion. The delayed (or worldwide) fallout consists of the smaller particles which ascend into the upper troposphere and into the stratosphere and are carried by winds to all parts of the earth. The delayed fallout is brought to earth, mainly by rain and snow, over extended periods ranging from months to years.
Fallout Shelter - A structure designed to protect its inhabitants against the effects of radioactive fallout. Generally, it will have thick walls and roof to attenuate nuclear radiation, plus provisions for air filtration, food, water, etc. See also fallout and protection factor.
Fireball - The luminous sphere of hot gases which forms a few millionths of a second after a nuclear (or atomic) explosion as the result of the absorption by the surrounding medium of the thermal X rays emitted by the extremely hot (several tens of million degrees) weapon residues. The exterior of the fireball in air is initially sharply defined by the luminous shock front and later by the limits of the hot gases themselves (radiation front). See Breakaway, Thermal X. rays.
Firestorm - Stationary mass fire, general in builtup urban areas, causing strong, inrushing winds from all sides; the winds keep the fire from spreading while adding fresh oxygen to increase their intensity.
Fission - The process whereby the nucleus of a particular heavy element splits into (generally) two nuclei of lighter elements, with the release of substantial amounts of energy. The most important fissionable materials are uranium-235 and plutonium-239; fission is caused by the absorption of neutrons. See also fusion.
Fission Fraction - The fraction (or percentage) of the total yield of a nuclear weapon which is due to fission. For thermonuclear weapons the average value of the fission fraction is about 50 percent.
Fission Products - A general term for the complex mixture of substances produced as a result of nuclear fission. A distinction should be made between these and the direct fission products or fusion fragments which are formed by the actual splitting of the heavy-element nuclei. Something like 80 different fission fragments result from roughly 40 different modes of fission of a given nuclear species (e.g., uranium-235 or plutonium-239). The fission fragments, being radioactive, immediately begin to decay, forming additional (daughter) products, with the result that the complex mixture of fission products so formed contains over 300 different isotopes of 36 elements.
Flash Burn : A burn caused by excessive exposure (of bare skin) to thermal radiation. See Thermal radiation.
Flashblindness - A temporary blindness, lasting up to several minutes, that results in people who are looking in the direction of a nuclear explosion at the instant of detonation.
Fluence (or Integrated Flux) - The product (or integral) of particle (neutron or photon) flux and time, expressed in units of particles per square centimeter. The absorbed dose of radiation (in rads) is related to fluence. See Flux.
Flux (or Flux Density) - The product of the particle (neutron or photon) density (i.e., number per cubic centimeter) and the particle velocity. The flux is expressed as particles per square centimeter per second and is related to the absorbed dose rate. It is numerically equal to the total number of particles passing in all directions through a sphere of 1 square centimeter cross sectional area per second.
Fractionation - Any one of several processes, apart from radioactive decay, which results in change in the composition of the radioactive weapon debris. As a result of fractionation, the delayed fallout generally contains relatively more of strontium-90 and cesium-137, which have gaseous precursors, that does early fallout from a surface burst.
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. See Overpressure.
Fuel Loading - The density of flammable materials (usually measured in lb/ft2 or kg/m2) occurring in the are subject to a nuclear blast. The ignition of a firestorm is thought to require a food loading of 8 lb/ft2. See also conflagration and firestorm.
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. See Thermonuclear.
Gamma Rays (or Radiation) - Electromagnetic radiations of high photon 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 X rays do not originate from atomic nuclei, but are produced in other ways (e.g., by slowing down (fast) electrons of high energy). See Electromagnetic radiation, Photon, X rays
Genetic Effect - The effect of various agents (including nuclear radiation) in producing changes (mutations) in the hereditary components (genes) of germ cells present in the reproductive organs (gonads). A mutant gene causes changes in the next generation which may or may not be apparent.
Ground Zero - The point on the surface of land vertically below or above the center of burst of a nuclear (or atomic) weapon; frequently abbreviated to GZ. For a burst over or under water the corresponding term is surface zero (SZ). Surface zero is also commonly used for ground surface and underground bursts.
Gun Type Weapon - A device in which two or more pieces of fissionable material, each less that a critical mass, are brought together very rapidly so as to form a supercritical mass which can explode as the result of a rapidly expanding fission chain. See Critical mass, Supercritical.
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 (or an organ) will decrease to half as a result of both radioactive decay and biological elimination. See Biological half-life.
Half-Residence 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 initial value.
Half-Value Thickness - The thickness of a given material which will absorb half the gamma radiation incident on it. The thickness depends on the nature of the material - it is roughly inversely proportional to its density - and also on the energy of the gamma rays.
H-Bomb - an abbreviation for hydrogen bomb. See Hydrogen bomb.
Height of Burst (HOB) - The height above the earth's surface at which a bomb is detonated in the air. The optimum height of burst for a particular target (or area) is that at which it it is estimated a weapon will produce a certain desired effect over the maximum possible area.
High-Altitude Burst - This is defined, somewhat arbitrarily, as a detonation at an altitude over 100,000 feet. Above this level the distribution of the energy of the explosion between blast and thermal radiation changes appreciably with increasing altitude due to changes in fireball phenomena.
Hot Spot - Region in a contaminated area in which the level of radioactive contamination is somewhat greater than in neighboring regions in the area. See Contamination.
Hydrogen Bomb (or Weapon) - A term sometimes applied to nuclear weapons in which part of the explosive energy is obtained from nuclear fusion (or thermonuclear) reactions. See Fusion, Nuclear weapon, Thermonuclear.
Hypocenter - A term sometimes used for ground zero. See Ground zero.
Implosion Weapon - A device in which a quantity of fissionable material, less than a critical mass, has its volume suddenly decreased by compression, so that it becomes supercritical and an explosion can take place. The compression is achieved by means of a spherical arrangement of specially fabricated shapes of ordinary high explosive which produce an inwardly-directed implosion wave, the fissionable material being at the center of the sphere. See Critical mass, Supercritical.
Impulse (per Unit Area) - The product of the overpressure (or dynamic pressure) from the blast wave of an explosion at the time during which it acts at a given point. More specifically, it is the integral, with respect to time of overpressure (or dynamic pressure), the integration being within the time of arrival of the blast wave and that at which the overpressure (or dynamic pressure) returns to zero at the given point.
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. In a nuclear explosion, neutrons can induce radioactivity in the weapon materials, as well as in the surroundings (e.g., by interaction with nitrogen in the air and with sodium, manganese, aluminum and silicon in soil and sea water).
Infrared - Electromagnetic radiations of wavelength between the longest visible red (7,000 Angstroms or 7x104 millimeter) and about 1 millimeter. See Electromagnetic radiation.
Infrared Radiation - Invisible electromagnetic radiation much like visible light but with a longer wavelength and lower energy. Almost half the energy in sunlight comes as infrared radiation of a slighter lower wavelength that visible light. A large portion of the "heat radiation" from a nuclear blast is infrared radiation.
Ingestion Pathway - The means by which a person is exposed to radiation through the food chain.
Inhalation Pathway - The means by which a person in a contaminated area or downwind is subject to respiratory radiation exposure.
Initial Nuclear Radiation - Nuclear radiation (essentially neutrons and gamma rays) emitted from the fireball and the cloud column during the first minute after a nuclear (or atomic) explosion. The time limit of one minute is set, somewhat arbitrarily, as that required for the source of part of the radiation (fission products, etc., in the radioactive cloud) to attain such a height that only insignificant amounts or radiation reach the earth's surface. See Residual nuclear radiation.
Integrated Neutron Flux - See Fluence.
Intensity - The amount of energy of any radiation incident upon (or flowing through) unit area, perpendicular to the radiation beam, in unit time. The intensity of thermal radiation is generally expressed in calories per square centimeter per second falling falling on a given surface at any specified instant. As applied to nuclear radiation, the term intensity is sometimes used, rather loosely, to express the exposure (or dose) rate at any given location.
Internal Radiation - Nuclear radiation (alpha and beta particles and gamma radiation) resulting from radioactive substances in the body. Important sources are iodine-131 in the thyroid gland, and strontium-90 and plutonium-239 in bone.
Inverse Square Law - The law which states that when radiation (thermal 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.
Ionization - The separation of a normally electrical neutral atom or molecule into electrically changed components. The term is also employed to describe the degree or extent to which this separation occurs. In the sense used in this book, ionization refers especially to the removal of an electron (negative charge) from the atom or molecule, either directly or indirectly, leaving a positively charged atom. The separated electron and ion are referred to as an ion pair. See Ionizing radiation.
Ionizing Radiation - Electromagnetic radiation (gamma rays or X rays) or particulate radiation (alpha particles, beta particles, neutrons, etc.) capable of producing ions, i.e., electrically charged particles, directly or indirectly, in it passage through matter.See nuclear radiation.
Ionosphere - The region of the atmosphere, extending from roughly 40 to 250 miles of altitude, in which there is appreciable ionization. The presence of charged particles in this region profoundly affects the propagation of long-wavelength electromagnetic radiations (radio and radar waves).
Ion Pair - See Ionization.
Isomer (or Isomeric Nuclide) - See Nuclide.
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.). For example, hydrogen has three isotopes, with masses of 1(hydrogen), 2 (deuterium), and 3 (tritium) units, respectively. The first two of these are stable (non radioactive), but the third (tritium) is a radioactive isotope. Both of the common isotopes un uranium, with masses of 235 and 238 units, respectively, are radioactive, emitting alpha particles, but their half-lives are different. Furthermore uranium-235 is fissionable by neutrons of all energies, but uranium-238 will undergo fission only with neutrons of high energy. See Nucleus.
Joule - A unit of electrical energy equal to the work done when a current of one ampere passes through a resistance of one ohm for one second.
Kilo-Electron Volt (or KEV) - an amount of energy equal to 1,000 electron volts. See Electron Volt.
Kiloton Energy - Defined strictly as 1012 calories (or 4.2 x 1019 ergs). This is approximately the amount of energy that would be released by the explosion of 1 kiloton (1,000 tons) of TNT. See TNT equivalent.
Kiloton (kt) - An equivalent amount of energy released in a nuclear explosion that approximately equal to that released in the explosion of 1.000 tons of TNT.

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