RADIOLOGICAL GLOSSARY

absorbed dose – the energy imparted to matter by ionizing radiation per unit mass of irradiated material at the point of interest. The unit of absorbed dose is the rad.

air burst – the explosion of a nuclear weapon at such a height that the fireball does not touch the earth’s surface. Fallout from an air burst is negligible.

alpha particle – a positively charged nuclear particle identical with the nucleus of a helium atom that consists of two protons and two neutrons and is ejected at high speed from the nucleus of certain atoms in radioactive decay processes.

alpha radiation – rays of alpha particles.

alpha ray – an alpha particle moving at high speed, or a stream of such particles.

atom – the smallest particle of an element that still retains the characteristics of that element. Every atom consists of a positively charged central nucleus, which carries nearly all the mass of the atom, surrounded by a number of negatively charged electrons, so that the whole system is normally electrically neutral.

background radiation – nuclear radiations arising from within the body and from the surroundings to which individuals are always exposed. The main sources of the natural background radiation are potassium-40 in the body, potassium-40, thorium, uranium, and their decay products present in rocks and soil, and cosmic rays.

beta burn – damage to the skin caused by prolonged contact with particles that emit beta radiation.

beta particle – an electron (negatively charged particle) or a positron (positively charged particle) ejected at high speed from the nucleus of certain atoms in radioactive decay processes.

beta ray – a beta particle moving at high speed, or a stream of such particles.

beta radiation – rays of beta particles.

blast wave – a violent pulse of air in which the pressure increases sharply at the front, accompanied by winds, propagated from an explosion.

bone seeker – any compound or ion that migrates in the body preferentially into bone.

contamination – the deposit of radioactive materials on the surfaces of structures, areas, objects, or personnel.

cumulative dose – the total dose resulting from continued or repeated exposures to radiation.

curie (abbr., Ci ) – unit of radioactivity equal to 3.7 x 1010 disintegrations per second.

decontamination – the removal of radioactive material from a structure, area, object, or person, or the reduction of radiation from a surface or area by covering it.

dose – a general term indicating the quantity of radiation or energy absorbed.

dose equivalent (abbr., DE ) – a quantity that is related to the expected detriment resulting from exposure to any kind of nuclear radiation, defined as the product of the absorbed dose in rads and modifying factors; the unit of 0E is the rem.

dose rate – absorbed dose delivered per unit time.

dosimeter – an instrument for measuring accumulated exposure to nuclear radiation.

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.

electron – an elementary particle having a negative electric charge of 1.6 x 10-19 coulomb and a rest mass 1/1836 that of the proton. In atoms, electrons surround the positively charged nucleus.

element – one of the known chemical substances that cannot be divided into simpler substances by chemical means.

emergency services – elements of government that are responsible for the protection of life and property, such as fire, police, welfare, and rescue services.

EOC ( Emergency Operating Centre) – a well-protected headquarters at various levels of government, such as city, county, state, or region, with two-way radio and telephone communications with shelters, emergency services, other EOCs, and various government headquarters.

exposure – a quantitative measure of gamma or x-ray radiation at a certain place, based on its ability to produce ionization in air, measured in units of roentgens or similar unit of measurement.

fallout – the process of the settling to the earth’s surface of airborne particles containing radioactive material following a nuclear explosion; also refers to the particles themselves. Early fallout, also called local fallout, is that fallout which settles to the surface of the earth during the first 24 hours after a nuclear explosion. Delayed fallout, also called worldwide fallout, is that fallout which settles to the surface of the earth at some time later than the first 24 hours after a nuclear explosion. Most of the fallout from a surface burst will be deposited within 24 hours after a nuclear explosion and within 400 to 500 miles downwind from the explosion.

fallout half-value thickness – the thickness of a given material which will absorb half the gamma radiation incident upon it. This thickness depends on the nature of the material–it is roughly inversely proportional to its density–and also to the energy of the gamma rays. These factors are specially calculated for fallout radiation and include all processes of attenuation of radiation.

fallout protection factor (FRF ) – an indication of the degree of protection provided by a location against gamma radiation from fallout. The FPF for a location is defined as the ratio of the radiation exposure rate at 3 feet above a flat, smooth, large, open area to the radiation exposure rate at the location in question, when the same amount of fallout is deposited uniformly over both locations. If the FPF of a location is one, that location provides no protection against gamma radiation. This factor is also called the protection factor (PF). It is often called “fallout protection factor” because “protection factor” can mislead people into thinking that a location with a high “protection factor” will also protect against blast and thermal radiation.

fallout shelter – an enclosed area or place which can provide refuge and protection against fallout radiation by absorbing some or most of the radiation directed toward the shelter.

fireball – the luminous sphere of hot gases which forms a few millionths of a second after a nuclear explosion as a result of the absorption by the surrounding air of the radiation emitted by the extremely hot weapon residues. The exterior of the fireball is initially sharply defined by the luminous shock front and later by the hot gases themselves and may be visible for several minutes.

fission fraction – the fraction (or percentage) of the total yield of a nuclear weapon which is due to fission, the remaining fraction of the yield being due to fusion. For thermonuclear weapons the average value of the fission fraction is about 50 percent.

fission, nuclear – a nuclear transformation characterized by the splitting of a high-mass nucleus into at least two other nuclei of lower mass and the conversion of some of the initial mass into a relatively large amount of energy.

fission products – a general term for the complex mixture of substances produced as a result of nuclear fission. About 80 different fission fragments result from approximately 40 different modes of fission. 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.

FPF – see fallout protection factor.

fusion, nuclear – a nuclear transformation characterized by the uniting together of two or more low-mass nuclei into a nucleus of higher mass and the conversion of some of the initial mass into a relatively large amount of energy.

gamma radiation – rays of high-energy photons from radioactive material.

gamma ray – a photon of high energy, or a stream of such photons, emitted by the nuclei of certain atoms in radioactive decay processes.

ground burst – a nuclear detonation at the surface of the earth, or at such a height above the earth that the fireball makes contact with the surface.

ground zero – the point on the surface of the earth vertically below, at, or above the point at which a nuclear explosion is initiated.

group dosimetry – a method for estimating radiation exposures of individual members of a group when there aren’t enough dosimeters for each member to have one.

half-life (radioactive half-life ) – the time in which half the atoms of a particular substance undergo radioactive decay.

high-risk areas – geographical areas in the United States estimated to be subject to a 50 percent or greater probability of receiving blast overpressures of 2 psi or more in a nuclear war, or to a 50 percent or greater probability of receiving a radiation exposure of 10,000 R or more.

hot spot – a localized surface area of higher than average radiation.

initial nuclear radiation – nuclear radiation (essentially neutrons and gammas) emitted from the fireball and the cloud column during the first minute after a nuclear explosion. The time limit of one minute is set somewhat arbitrarily as that required for the source of the nuclear radiations to attain such a height that only insignificant amounts of radiation reach the earth’s surface.

ion – an atom or molecule that has lost or gained one or more electrons to become electrically charged.

ionization – the process of adding electrons to or removing electrons from atoms or molecules.

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 also differing in their nuclear properties, such as half-life, energy, and type of nuclear radiation emitted.

kiloton energy – approximately the amount of energy that would be released by the explosion of 1,000 tons of TNT, defined precisely as 1012 calories, or 4.19 x 1019 ergs.

latency or latent period – the period of time between exposure to radiation and the detection of a specified effect of that exposure; or, for acute radiation sickness, the time during which no symptoms appear between the first reaction to radiation exposure and the later radiation sickness.

lethal radiation dose – the total -body radiation exposure required to cause death in 100 percent of a large group of people within a specified time period. For example, LD 100/60 indicates a dose which is lethal to 100 percent of the people exposed within 60 days after the exposure.

megaton energy – approximately the amount of energy that would be released by the explosion of one million tons of TNT, defined precisely as 1015 calories, or 4.19 x 1022 ergs.

midlethal or median lethal radiation dose – the short-term, total-body radiation exposure to cause death in 50 percent of a large group of people within a specified time period. For example, LD 50/60 indicates a dose which is lethal to 50 percent of the people exposed within 60 days after the exposure.

milliroentgen (mR) – 1/1000 of a roentgen. 1000 milliroentgens equal one roentgen.

neutron – an elementary particle having no electric charge and a rest mass of 1.675 x 10-27 kilogram. The neutron is a constituent of the nucleus of every atom heavier than ordinary hydrogen.

nuclear radiation – particulate and electromagnetic radiation emitted from atomic nuclei in various nuclear processes. The important nuclear radiations, from the weapons standpoint, are alpha and beta particles, gamma rays, and neutrons. All nuclear radiations are ionizing radiations, but the reverse is not true; X rays and nearly all ultraviolet radiation, for example, are included among ionizing radiations, but they are not nuclear radiations since they do not originate from atomic nuclei.

nuclear weapon – any weapon which attains its energy release from the fission or fusion of atomic nuclei.

nucleus – the positively charged central portion of an atom, composed of protons and neutrons and containing almost all of the mass of an atom but only a tiny part of its volume.

overpressure – the transient pressure, usually expressed in pounds per square inch, exceeding the ambient pressure, in the shock (or blast) wave from an explosion. The variation of the overpressure with time depends on the yield of the explosion, the distance from the point of burst, and the medium, whether air, water, or soil, 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.

PF – see protection factor or fallout protection factor.

photon – a packet of electromagnetic energy having zero mass and no electric charge. Visible light is made up of low-energy photons, and gamma rays are high-energy photons.

protection factor (PF ) – also called the “fallout protection factor”
An indication of the degree of protection provided by a location against gamma radiation from fallout. The PF for a location is defined as the ratio of the radiation exposure rate at 3 feet above a flat, smooth, large, open area to the radiation exposure rate at the location in question, when the same amount of fallout is deposited uniformly over both locations. If the PF of a location is one, that location provides no protection against gamma radiation. The term “protection factor” can mislead people into thinking that a shelter with a high protection factor will provide protection against blast, so it is sometimes called “fallout protection factor” to clarify what is meant.

proton – an elementary particle having a positive electric charge numerically equal to that of the electron and a mass of 1.672 x 10-27 kilogram. The proton constitutes the nucleus of the hydrogen atom and is a part of the nucleus of every atom.

rad – a special unit of absorbed dose equal to 100 ergs of energy imparted by ionizing radiation per gram of absorbing material, such as body tissue. The exposure rate measured at a point in roentgens /hr may be taken to be numerically equal to the absorbed dose rate in rad/hr at that point for external sources of gamma radiation.

radioactive decay – a spontaneous nuclear transformation in which a nucleus emits alpha or beta particles, often accompanied by gamma radiation, resulting in a progressive decrease in the number of radioactive atoms in a substance.

radioactivity – the spontaneous emission of radiation, generally alpha or beta particles, often accompanied by gamma rays, from the 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 or more stages of radioactive decay, a stable, nonradioactive end product is formed.

rainout – the process of removal of particles of fallout from the air either by the formation of water droplets around the particles which then fall as rain, or by rain falling into the fallout cloud and “washing” the particles down to earth. Rainout does not affect fallout particles that are higher than about 10 km (33,000 ft).

rem – a unit of dose equivalent, numerically equal to the dose in rads multiplied by factors such as the quality factor, which takes into account the higher risk of late biological effects by certain radiations such as heavy ionizing particles (alphas, neutrons, protons) along their paths through cells of the body.

RM (radiological monitor) – the person who uses radiological instruments to (1) measure nuclear radiation intensities, (2) estimate the radiation exposure of shelter occupants, (3) find the places with the lowest nuclear radiation levels in a shelter, (4) advise on , the improvement of radiation protection in a shelter, (5) advise when (and for how long) someone can go outside the shelter on short emergency trips, and (6) advise when to leave for longer trips, and when to leave permanently.

roentgen (R) – A unit of radiation exposure determined by the amount of ionization produced in air. Specifically, it has been defined as the quantity of radiation that will ionize dry air at zero degrees centigrade and standard atmospheric pressure to produce one electrostatic unit of electric charge of each sign, both positive and negative, in one cubic centimetre.

shielding – any material or obstruction which absorbs or attenuates radiation and thus protects personnel or materials from the radiation effects of a nuclear explosion. A moderately thick layer of any opaque material will provide satisfactory shielding from thermal radiation, but a considerable thickness of material of high density may be needed to protect adequately from nuclear radiation.

skyshine – radiation, particularly gamma rays from a nuclear explosion or from fallout, reaching a target from many directions, mostly from above, as a result of scattering by air.

surface burst – same as ground burst.

survey meter – an instrument used to measure the exposure rate in roentgens per hour, or a similar unit of measurement, at the location being metered.

tenth-value thickness – the thickness of a given material which will decrease the intensity of gamma radiation to one-tenth of the amount incident upon it. Two tenth-value thicknesses will reduce the intensity received by a factor of 10 x 10, or 100, and so on. The tenth-value thickness of a given material depends on the gamma-ray energy, but for radiation of a particular energy it is roughly inversely proportional to the density of the material.

thermonuclear – an adjective referring to the process in which very high temperatures are used to bring about the fusion of light nuclei, such as those of the hydrogen isotopes deuterium and tritium, with the accompanying liberation of energy. A thermonuclear bomb is a weapon in which part of the explosion energy results from thermonuclear fusion reactions. The high temperatures required are obtained in this case by means of a fission explosion.

x ray – a photon of high energy, or a stream of such photons, resulting from processes other than nuclear transformations.

yield – the total effective energy released in a nuclear explosion. It is usually expressed in terms of the equivalent tonnage of TNT that would be required to produce the same energy release in an explosion.