Linear Attenuation Coefficient - See Attenuation.
Linear Energy Absorption Coefficient - See Absorption.
Lip Height - The height above the original surface to which earth is piled around the crater formed by an explosion. See Crater.
Loading - The force of an abject or structure or element of a structure. The loading due to blast is equal to the net pressure in excess of the ambient value multiplied by the area of the loaded object, etc, See Diffraction, Drag loading.
Mach Front - See Mach stem.
Mach Region - The region on the surface at which the Mach stem has formed as the result of a particular explosion in the air.
Mach Stem - The shock front formed by the merging of the incident and reflected shock fronts from an explosion. The term is generally used with reference to a blast wave, propagated in the air, reflected at the surface of the earth. The Mach stem is nearly perpendicular to the reflecting surface and and presents a slightly convex (forward) front. The Mach stem is also called the Mach front.See Shock front, Shock wave.
Mass Attenuation Coefficient - See Attenuation.
Mass Energy Absorption Coefficient - See Absorption.
Mass Number - See Nucleus.
Mean Free Path - The average part distance a particle (neutron or photon) travels before undergoing a specified reaction (with a nucleus or electron) in matter.
Megacurie - One million curies. See Curie.
Megaton Energy - Defined strictly as 10¹⁵ calories (or 4.2x ergs). This is approximately the amount of energy that would be released by the explosion of 1,000 kilotons (1,000,000 tons) of TNT. See TNT equivalent.
MEV (or Million Electron Volt) - A unit of energy commonly used in nuclear physics. It is equivalent to 1.6x10^-6 erg. Approximately 200 MeV of energy are produced for every nucleus that undergoes fission. See Electron volt.
Microcurie - A one-millionth of a curie. See Curie.
Micrometer - See Micron.
Micron - a one-millionth part of a meter (i.e.10^-6 meter or10^-4 centimeter), it is roughly four one-hundred-thousandths (4x10^-5) of an inch.
Microsecond - A one-millionth part of a second.
Million Electron Volt - See MeV.
Millirad - A one-thousandth part of a rad. See Rad.
Millrem - A one thousandth part of a rem. See Rem.
Milliroentgen - A one-thousandth part of a roentgen. See Roentgen.
Millisecond - A one-thousandth part of a second.
Mirror point - A point at which a charged particle, moving (in a spiral path) along the lines of a magnetic field, is reflected back as it enters a stronger magnetic field region. The actual location of the mirror point depends on the direction and energy of motion of the charged particle and the ratio of the magnetic field strengths. As a result only those particles satisfying the requirement of the existing situation are reflected.
Monitoring - The procedure or operation of locating and measuring radioactive contamination by means of survey instruments which can detect and measure (as dose rates) ionizing radiations. The individual performing the operation is called a monitor.
Negative Phase - See Shockwave.
Neutron - A neutral particle (i.e., with no electrical charge) of approximately unit mass, present in all atomic nuclei, except those of ordinary (light) hydrogen. Neutrons are required to initiate the fission process, and large numbers of neutrons are produced by both fission and fusion reactions in nuclear (or atomic) explosions.
Neutron Flux - See Flux.
Nominal Atomic Bomb - a term, now becoming obsolete, used to describe an atomic weapon with an energy release equivalent to 20 kilotons (i.e., 2,000 tons) of TNT. This is very approximately the energy yield of the bombs exploded over Japan and in the Bikini test of 1946.
Nuclear Cloud - See radioactive cloud.
Nuclear Contribution - Explosive energy released by nuclear fission or fusion reactions, as part of the total energy released by the accidental explosion of a nuclear weapon. Any nuclear contribution equivalent to four or more pounds of TNT is considered significant, and would add beta and gamma radiation hazards to other radiological and toxic hazards present at a nuclear weapon accident site.
Nuclear Detonation - A nuclear explosion resulting from fission or fusion reactions in nuclear material, such as a nuclear weapon.
Nuclear Radiation - Particulate and electromagnetic radiation emitted from atomic nuclei in various nuclear processes. The important nuclear radiation, from the weapons standpoint, are alpha and beta particles, gamma rays, and neutrons. All nuclear radiations are ionizing radiations, but the reverse in not true; X rays, for example, are included among ionizing radiations, but they are not nuclear radiations since they do not originate from atomic nuclei. See Ionizing radiation, X-rays.
Nuclear Reaction - a process that alters the energy or structure or composition of atomic nuclei. The particles and electromagnetic radiation emitted from atomic nuclei in various nuclear processes. All nuclear radiations, which include alpha particles, beta particles, gamma rays and neutrons, are ionizing radiation that can damage body tissues. See also alpha particle, beta particle, gamma rays and neutron.
Nuclear (or Atomic) Tests - Tests carried out to supply information required for the design and improvement of nuclear (of atomic) weapons and to study the phenomena and effects associated with nuclear (or atomic) explosions.
Nuclear Weapon (or Bomb) - A general name given to any weapon in which the explosion results from the energy released by reactions involving atomic nuclei, either fission or fusion or both. Thus, the A- (or atomic) bomb and the H- (or hydrogen) bomb are both nuclear weapons. It would be equally true to call the atomic weapons, since it is the energy of atomic nuclei that is involved in each case. However, it has become more-or-less customary, although it is not strictly accurate, to refer to weapons in which all the energy results from fission as A-bombs or atomic bombs. In order to make a distinction, those weapons in which part, at least, of the energy results from thermonuclear (fusion) reactions of the isotopes of hydrogen have been called H-bombs or hydrogen bombs.
Nuclear Yield - The energy released in the detonation of a nuclear weapon, measured in terms of the kilotons or megatons of trinitrotoluene (TNT) required to produce an equivalent energy release.
Nucleus (or Atomic Nucleus) - The small, central, positively charged region of an atom which carries essentially all the mass. Except for the nucleus of ordinary (light) hydrogen, which is a single proton, all atomic nuclei contain both protons and neutrons. The number of protons determines the the total positive charge, or atomic number, this is the same for all the atomic nuclei of a given chemical element. The total number of neutrons and protons, called the mass number, is closely related to the mass (or weight) of the atom. The nuclei or isotopes of a given element contain the same number of protons, but different numbers of neutrons. They thus have the same atomic number, and so are the same element, but they have different mass numbers (and masses). The nuclear properties (e.g., radioactivity, fission, neutron capture, etc.) of an isotope of a given element are determined by both the number of neutrons and the number of protons. See Atom, Element, Isotope, Neutron, Proton.
Nuclide - An atomic species distinguished by the composition of it nucleus (i.e., by the number of protons and the number neutrons). In isomeric nuclides the nuclei have the same composition but are in different energy states. See Atom, Neutron, Nucleus, Proton.
Oralloy - Enriched uranium. One of the primary fissionable materials in nuclear weapons.
Overpressure - The transient pressure, usually expressed in pounds per square inch (i.e., psi), exceeding the ambient pressure, manifested in the shock (or blast) wave from an explosion . The variation of the overpressure with time depends on the energy yield of the explosion, the distance from the point of burst, and the medium in which the weapon is detonated. The peak overpressure is the maximum value of the overpressure at a given location and is generally experienced at the instant the shock (or blast) wave reaches that location. See Shock wave.
Ozone Layer - A region of the Earth's atmosphere about 20 to 30 miles above the surface, characterized by high levels of ozone (O₃). This layer absorbs most of the ultraviolet radiation present in sunlight in outer space, protecting living cells from its harmful effects
Pair Production - The process whereby a gamma ray (or X-ray) photon, with energy in excess of 1.2 MeV in passing near the nucleus of an atom is converted in a positive electron and a negative electron. As a result, the photon ceases to exist. See Photon.
Particulate Radiation - Radiation in the form of particles (for example, neutron, electrons, alpha and beta particles) as opposed to electromagnetic radiation.
Photoelectric effect - The process whereby a gamma-ray (or X-ray) with energy somewhat greater of that of the binding energy of an electron in an atom, transfers all its energy to the electron which is consequently removed from the atom. Since it has lost all its energy. the photon ceases to exist. See Photon.
Photon- a unit or "particle" of electromagnetic radiation, carrying a quantum of energy which is characteristic of the particular radiation. If v is the frequency of the radiation in cycles per second and λ is the wavelength in centimeters, the energy quantum of the photon in ergs is hv or hc/λ , where h is Plank's constant, 6.62x 10^-27 erg-second and c is the velocity of light (2.997 925x10¹⁵ meters/second = 1896.281 miles/second). For gamma rays, the photon energy is usually expressed in million electron volt (MeV) units (i.e., 1.24x10^-10/λ where λ is in centimeters or 1.24x10^-2/λ if λ is in angstroms). An elementary particle that is its own antiparticle.
Plastic Range - The stress range in which a material will not fail when subjected to the action of a force, but will not recover completely, so that a permanent deformation results when the force is removed. Plastic deformation refers to dimensional changes occurring within the plastic range. See Elastic range.
Plastic Zone - The region beyond the rupture zone associated with crater formation in which there is no visible rupture but in which the ground is permanently deformed and compressed to a higher density. See Crater, Elastic zone, Rupture zone.
Plume - See Column.
Plutonium (Pu) - An artificially produced fissile material. The Pu-239 isotope is primarily used in nuclear weapons.
Popcorn Effect - Refers to burns caused on the epidermis by small overheated particles.
Positive Phase - See shock wave.
Precursor - An air pressure wave which moves ahead of the main blast wave for some distant as a result of a nuclear (or atomic) explosion of appropriate yield and low burst height over a heat-absorbing or dusty surface. The pressure at the precursor front increases more gradually that in a true (or ideal) shock wave, so that the behavior in the precursor region is said to be nonideal. See Blast wave, Shock front, Shock wave.
Prompt Effects - Effects of nuclear explosions that occur immediately after detonation , in contrast to later effects, such as fallout, ozone layer depletion, etc.
Protection Factor (PF) - The reduction in levels of nuclear radiation, from outdoor levels to those inside a fallout shelter, that can be achieved by a particular selection and thickness of shielding materials. For example, a protection factor of 10 will reduce the inside level of radiation to one-tenth of that experienced just outside.
Proton - a particle of mass (approximately) unity carrying a unit positive charge; it is identical physically with the nucleus of the ordinary (light) hydrogen atom. All atomic nuclei contain protons. See Nucleus.
Quantum - The smallest discrete quantity of some physical property that a system can possess (according to quantum theory). See Photon.
RAD - A unit of the dose of radiation absorbed that represents the absorption of 100 ergs of nuclear (or ionizing) radiation per gram of absorbing material, such as body tissue. One rad represents the absorption of 0.01 joule of nuclear (or ionizing) radiation energy per kilogram of the absorbing material or tissue. See also rem and röntgens.
RADIAC - A term designating various types of radiological measuring instruments or equipment. (this term is derived from the words "radioactivity detection, indication and computation," and is normally an adjective).
Radiant Exposure - The total amount of thermal radiation energy received per unit area of exposed surface; it is usually expressed in calories per square centimeter.
Radiation - See Ionizing radiation, Nuclear radiation, Thermal radiation.
Radiation Injury (or Syndrome) - See Syndrome (Radiation).
Radioactive (or Nuclear) Cloud - An all-inclusive term for the cloud of hot gases, smoke, dust and other particulate matter from the weapon itself and from the environment, which is carried aloft in conjunction with the rising fireball produced by the detonation of a nuclear (or atomic) weapon.
Radioactivity - The spontaneous emission of radiation, generally alpha or beta particles, often accompanied by gamma rays for nuclei of an unstable isotope. As a result of this emission the radioactive isotope is converted (or decays) into the isotope of a different (daughter) element which may (or may not) also be radioactive. Ultimately, as a result of one of more stages of radioactive decay, a stable (nonradioactive) end product is formed. See Isotope.
Radio Blackout - The complete disruption of radio (or radar) signals over large areas caused by the ionization accompanying a high altitude nuclear explosion, especially above 40 miles.
Radioisotope - A radioactive isotope. See Isotope, Radioactivity.
Radiological Survey - The directed effort to determine the distribution of radiological material and dose rates in an area.
Radionuclide - A radioactive nuclide (or radioactive atomic species). See Nuclide.
Rainout - The removal of radioactive particles from a nuclear cloud by precipitation when this cloud is within a rain cloud. See Washout.
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 same effect is the RBE of the latter radiation.
Reflected Pressure - The total pressure which results instantaneously at the surface when a shock (or blast) wave traveling in one medium strikes another medium (i.e., at the instant when the front of a blast wave in air strikes the ground or a structure). If the medium struck (e.g., the ground of a structure) is more dense than that in which the shock wave is traveling (e.g., air), the reflected pressure is positive (compression). If the reverse is true (i.e., when a shock wave in the ground or water strikes the air surface) the reflected pressure in negative (rarefaction or tension).
Reflection Factor - The ratio of the total (reflected) pressure to the incident pressure when a shock (or blast) wave traveling in one medium strikes another.
REM (Roentgen Equivalent Man/Mammal) - A unit of biological dose of radiation. The dosage of ionizing radiation that will cause the same amount of injury to human tissue as 1 roentgen of X-rays. The number of rems of radiation is equal to the number of rads absorbed multiplied by the relative biological effectiveness of the particular type of radiation (e.g., gamma rays, alpha particles, neutrons). The rem is also the unit of dosed equivalent, which is equal to the product of the number of rads absorbed and the "quality factor" of the radiation. The SI unit replaced the rem. See Dose, Dose equivalent, Rad, RBE,Röntgens.
Residual Contamination - Contamination which remains after steps have bee taken to remove it. These steps may consist of nothing more than allowing the contamination to decay naturally.
Residual Nuclear Radiation - Nuclear radiation, chiefly beta particles and gamma rays, which persists for some time following a nuclear (or atomic) explosion. The radiation is emitted mainly by the fission products and other bomb residues in the fallout, and to some extents by earth and water constituents, and other materials, in which radioactivity has been induced by the capture on neutrons. See Fallout, Induced radioactivity, Initial nuclear radiation.
Röntgens (or Roentgen) - A unit of exposure to gamma (or X) radiation. It it defined precisely as the quantity of gamma (or X) rays that will produce electrons (in ion pairs) with a total charge of 2.58x10^-4 coulomb in 1 kilogram of dry air. An explosion of 1 roentgen results in the deposition of about 94 ergs of energy in 1 gram of soft body tissue. Hence, an exposure of 1 roentgen is approximately equal to an absorbed dose of 1 rad in soft tissue. See Dose, Rad.
Rupture Zone - The region immediately adjacent to the crater boundary in which the stresses produced by the explosion have exceeded the ultimate strength of the ground medium. It is characterized by the appearance of numerous radial (and other) cracks of various sizes. See Crater, Plastic zone.

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