Chain reaction - A self-sustaining nuclear reaction; a series of nuclear fissions in which neutrons released by splitting one atom leads to the splitting of others.
Clean Weapon - One in which measures have been taken to reduce the amount of residual radioactivity relative to a "normal" weapon of the same energy yield.
Cloud Chamber Effect - See Condensation cloud.
Cloud Column - The visible column of weapon debris (and possibly dust and water droplets) extending upward from the point of burst of a nuclear (or atomic) weapon. See Radioactive cloud.
Cloud Phenomena - See Base surge, Cloud column, Fireball, Radioactive cloud.
Column (or Plume) - A hollow cylinder of water and spray thrown up from an underwater burst or a nuclear (or atomic) weapon, through which the hot, high pressure gases formed in the explosion are vented to the atmosphere. A somewhat similar column of dirt is formed in an underground explosion.
Compton Current - Electron current generated as a result of Compton processes. See Compton effect, Compton electron.
Compton Effect - The scattering of photons (of gamma or X rays) by the orbital electrons of atoms. In a collision between a (primary) photon and an electron , some of the energy of the photon is transferred to the electron which is generally ejected from the atom. Another (secondary) photon, with less energy, then moves off in a new direction at an angle to the direction of motion of the primary photon. See Scattering.
Compton Electron - An electron of increased energy ejected from an atom as a result of a Compton interaction with a photon. See Compton effect.
Condensation Cloud - A mist or fog of minute water droplets which temporarily surround the fireball following a nuclear (or atomic) detonation in a comparatively humid atmosphere. The expansion of the air in the negative phase of the blast wave from the explosion results in a lowering of the temperature, so that condensation of water vapor present in the air occurs and a cloud forms. The cloud is soon dispelled when the pressure returns to normal and the air warms up again. The phenomenon is similar to that used by physicists in the Wilson cloud chamber and is sometimes called the cloud chamber effect.
Conflagration - A massive fire that occurs after a nuclear explosion, in which the flames spread along a common front. See also firestorm.
Contact Surface Burst - See Surface burst.
Contained Underground Burst - An underground detonation at such a depth that none of the radioactive residues escapes through the surface of the ground.
Contamination - The deposit and/or absorption of radioactive material, biological, or chemical agents or hazardous materials on, and by, structures, areas, people, or objects. In the case of a nuclear (or atomic) explosion, the material generally consists of fallout in which fission products and other weapon debris have become incorporated with particles of dirt, etc. Contamination can also arise from the radioactivity induced in certain substances by the action of neutrons from a nuclear explosion. See Decontamination, Fallout, Induced radioactivity, Weapon debris.
Crack - The light-colored region which follows closely behind the dark slick in an underwater burst. It is probably caused by the reflection of the water shock wave at the surface. See Slick.
Crater. The pit, depression or cavity formed in the surface of the earth by a surface or underground explosion. Crater formation can occur by vaporization of the surface material, by the scouring effect of air blast, by throwout of disturbed material, or by subsidence. In general, the major mechanism changes from one to the next with increasing dept of burst. The apparent crater is the depression which is seen after the burst; it is smaller than the true crater (i.e., the cavity actually formed by the explosion), because it is covered with a layer of loose earth, rock, etc. See also surface burst.
Critical Mass- The minimum mass of a fissionable material that will just maintain a fission chain reaction under precisely specified condition, such as the nature of the material and its purity, the nature and thickness of the tamper (or neutron reflector), the density (or compression), and the physical shape or geometry. For an explosion to occur, the system must be supercritical (i.e., the mass of the material must exceed the critical mass under the existing conditions). See supercritical.
Cube Root Law - A scaling law applicable to many blast phenomena. It relates the time and distance at which a given blast effect is observed to the cube root of the energy yield of the explosion.
Cumulative Dose (Radiation) - The total dose resulting from repeated exposure to radiation in the same region, or of the whole body.
Curie - A unit of radioactivity; it is the activity of a quantity of any radioactive species in which 3.700 x 1010 nuclear disintegrations occur per second. The gamma curie is sometimes defined correspondingly as the activity of material in which this number of gamma-ray photons are emitted per second.
Damage Criteria - Standards or measures used in estimating specific levels of damage.
Debris - See Weapon debris.
Decay (or Radioactive decay) - The decrease in activity of any radioactive material with the passage of time due to the spontaneous emission from the atomic nuclei of either alpha or beta particles, sometimes accompanied by gamma radiation. See Half-life, Radioactivity.
Decay Curve - The representation by means of a graph of the decrease of radioactivity with respect to time.
Decontamination - The reduction of removal of contaminating radioactive material from a structure, area, object or person. Decontamination may be accomplished by (1) treating the surface so as to remove or decrease the contamination; (2) letting the material stand so that the radioactivity is decreased as a result of natural decay; and (3) covering the contamination so as to attenuate the radiation emitted. Radioactive material removed in process (1) must be disposed of by burial in land or sea, or in other suitable way.
Delayed Fallout - See Fallout.
Deuterium - An isotope of hydrogen of mass 2 units; it is sometimes referred to as heavy hydrogen. It can be used in thermonuclear fusion reactions for the release of energy. Deuterium is extracted from water which always contains 1 atom of deuterium to about 6,500 atoms of ordinary (light) hydrogen. See Fusion, Isotope, Thermonuclear.
Diffraction - The bending of waves around the edges of objects. In connection with a blast wave impinging on a structure, diffraction refers to the passage around and envelopment of the structure by the blast wave. Diffraction loading is the force (or loading) on the structure during the envelopment process.
Direct Radiation - The nuclear radiation, composed mainly of gamma rays and neutrons, that occurs immediately at the time of detonation. It is usually very intense, but its range is limited to a few miles around the center of the explosion. Also known as prompt radiation.
Disaster Control - Measures taken before, during, or after hostile action, natural or man-made disasters, to reduce the probability of damage, minimize its effects, and initiate recovery.
Dome - The mound of water spray thrown up into the air when the shock wave from an underwater detonation of a nuclear (or atomic) weapon reaches the surface.
Dosage - see Dose.
Dose - A total (or accumulated) quantity of ionizing (or nuclear) radiation. The absorbed dose in rads represents the amount of energy absorbed from the radiation per gram of specified absorbing material. In soft body tissue the absorbed dose in rads is essentially equal to the exposure in roentgens. The biological dose (also called the RBE dose) in rems is a measure of biological effectiveness of the absorbed radiation.See Exposure, Rad, RBE, Rem, Roentgen.
Dose Equivalent - In radiation protection associated with peacetime nuclear activities, the dose equivalent in rems is a measure of the biological effectiveness of absorbed ionized radiation. It is similar to the biological dose which is used in connection with the large radiation exposures that might accompany a nuclear explosion. See Dose, Rem.
Dose Rate - As a general rule, the amount of ionizing (or nuclear) radiation which an individual or material would receive per unit of time. It is usually expressed as rads (or rems) per hour or in multiples or submultiples of these units, such as millirads per hour. The dose rate is commonly used to indicate the level of radioactivity in a contaminated area. See Survey meter.
Dose Rate Contour Line - A line on a map, diagram, or overlay joining all points at which the radiation dose rate at a given time is the same.
Dosimeter - An instrument for measuring and registering the total accumulated dose of (or exposure to) ionizing radiations. Instruments worn or carried by individuals are calledPersonal dosimeters.
Dosimetry - The theory and application of the principles and techniques involved in the measurement and recording of radiation doses and dose rates. Its practical aspect is concerned with the use of various types of radiation instruments with which measurements are made. See Dosimeter, Survey meter.
Drag Loading - The force of an object or structure due to the transient winds accompanying the passage of a blast wave. The drag pressure is the product of the dynamic pressure and the drag coefficient which is dependent upon the shape (or geometry) of the structure or object. See Dynamic pressure.
Dynamic Pressure - The air pressure which results from the mass air flow (or wind) behind the shock front of a blast wave. It is equal to the product of half the density of air through which the blast wave passes and the square of the particle (or wind) velocity behind the shock front as it impinges on the object or structure. See also overpressure.
Early Fallout - See fallout.
Effective Half-life - See Half-life.
Elastic Range - The stress range in which a material will recover its original form when the force (or loading) is removed. Elastic deformation refers to dimensional changes occurring within the elastic range. See Plastic range.
Elastic Zone - The zone beyond the plastic zone in crater formation in which the ground is disturbed by the explosion but returns to its original condition.
Electromagnetic Pulse (EMP) - A sharp pulse of radio frequency (long wavelength) electromagnetic radiation produced when an explosion occurs in an asymmetrical environment, especially at or near the earth's surface or at high altitudes. The intense electric and magnetic fields can damage unprotected electrical and electronic equipment over a large area. See electromagnetic radiation, High-altitude burst.
Electromagnetic Radiation - A traveling wave motion resulting from oscillating magnetic and electric fields. Familiar electromagnetic radiations range from X rays (and gamma rays) of short wavelength (high frequency), through the ultraviolet, visible, and infrared regions, to radar and radio waves or relatively long wavelength (low frequency). All electromagnetic radiations travel in a vacuum with the velocity of light. See Photon.
Electron - A particle of very small mass, carrying a unit negative or positive charge. Negative electrons, surrounding the nucleus, (i.e., orbital electrons), are present in all atoms: their number is equal to the number of positive charges (or protons) in the particular nucleus. The term electron, where used alone, commonly refers to negative electrons. A positive electron is usually called a positron, and a negative electron is sometimes called a negatron. See Beta particle.
Electron Volt (EV) - The energy imparted to an electron when it is moved through a potential difference of 1 volt. It is equivalent to 1.6 x 1012 erg.
Element - One of the distinct, basic varieties of matter occurring in nature which, individually or in combination, compose substances of all kinds. Approximately ninety different elements are known to exist in nature and several others, including plutonium, have been obtained as a result of nuclear reactions with these elements.
EMP - See Electromagnetic pulse.
Energy absorption - See Absorption.
Energy Partition - The distribution of the total energy released by a nuclear explosion among the various phenomena (e.g., nuclear radiation, thermal radiation and blast). The exact distribution is a function of time, explosion yield, and the medium in which the explosion occurs.
Expedient Shelters - Fallout shelters ( also known as hasty shelters) offering reasonable radiation protection that can be fashioned quickly in the event of nuclear attack. See also fallout shelter and protection factor.
Exposure - A measure expressed in roentgens of the ionization produced by gamma (or X) rays in air. The exposure rate is the exposure per unit time (e.g,, roentgens per hour). See Dose, Dose Rate, Roentgen.

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