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absorbed dose (D) - The energy imparted per unit mass by ionizing radiation to matter at a specific point. The SI unit of absorbed dose is joule per kilogram (J kg-1). The special name for this unit is gray (Gy). The previously used special unit of absorbed dose, the rad, was defined to be an energy absorption of 100 ergs/gram. Therefore, 1 Gy = 100 rad.

activity - Quantity of a radionuclide which describes the rate at which decays occur in an amount of a radionuclide. The SI unit of radioactivity is the becquerel (Bq), which replaced the old unit, the curie (Ci). One becquerel corresponds to 1 disintegration of a radionuclide per second.

alpha particle (a) - A positively charged particle emitted by radioactive material. It consists of two neutrons and two protons bound together; hence, it is identical with the nucleus of a helium atom. It is the least penetrating of the three common types of radiation -- alpha, beta, and gamma -- and it is stopped by a sheet of paper. It is not dangerous to plants, animals, or humans unless the alpha-emitting substance has entered the organisms. Alpha particles are ejected from a nucleus during the decay of some radioactive elements; for example, an alpha particle is emitted when either of the radon progeny polonium-218 or polonium-214 decays.

alpha ray - A stream of alpha particles. Used loosely as a synonym for alpha particles.

atom - A particle of matter indivisible by chemical means. It is the fundamental building block of the chemical elements, which differ from each other because they contain different kinds of atoms. According to present-day theory, an atom contains a dense inner core, the nucleus, and a much less dense outer domain consisting of electrons in motion around the nucleus. Atoms are electrically neutral. Compare with  ion, molecule. See also matter.



beam - A stream of particles or electromagnetic radiation moving in a single direction.

becquerel (Bq) - SI unit of radioactivity, corresponding to one radioactive disintegration per second. See activity.

beta particle (b) - An elementary particle emitted from a nucleus during radioactive decay, with a single electrical charge and a mass equal to 1/1837 of that of a proton. A negatively charged beta particle is identical to an electron. A positively charged beta particle is called a positron. Beta radiation may cause skin burns, and beta emitters are harmful if they enter the body. Beta particles, however, are easily stopped by a thin sheet of metal.

biologically effective dose (BED) - In fractionated radiotherapy, the total dose that would be required in very small dose fractions to produce a particular effect, as indicated by the linear-quadratic equation. Otherwise known as 'extrapolated total dose' (ETD). BED values calculated for different a/b ratios are not strictly comparable.

Bremsstrahlung x rays - Literally translated "braking radiation;" these are x rays resulting from interaction of a projectile electron with a target nucleus.



cell cycle - Sum of the phases of growth of an individual cell type; divided into G1 (gap 1), S (DNA synthesis), G2 (gap 2), and M (mitosis); the cycle of cellular events from one mitosis to the next.

centromere - The chromosome constriction to which the spindle fiber attaches. The position of the centromere determines whether chromosomes are metacentric (X-shaped; e.g., chromosomes 1, 3, 16, 19, 20) or acrocentric (inverted V-shaped; e.g., chromosomes 13-15, 21, 22, Y). During mitosis the identical chromatids of each chromosome are separated by shortening of the spindle fibers attached to opposite poles of the dividing cell.

charged particle - An ion; an elementary particle that carries a positive or negative electric charge.

chromosomal aberration - Any change resulting in the duplication, deletion, or rearrangement of chromosomal material.

chromosomal instability - An effect of irradiation in which chromosomal aberrations continue to appear through many cell generations.

clonogenic cells - Cells that have the capacity to produce an expanding family of descendants (usually at least 60). Also called 'colony-forming cells' or 'clonogens'.

Cobalt-60 - A radioactive substance used as a radiation source to treat cancer.

collective dose - Usually refers to 'collective effective dose' obtained by multiplying the average effective dose by the number of persons exposed to that given dose. Expressed in person-sievert. The old unit was the man-rem.

colony - A group of identical cells derived from a single ancestor cell.

Compton effect - Scattering of x rays resulting in ionization and loss of energy; the energy lost by the photon is given to the ejected electron as kinetic energy.

curie (Ci) - Old unit of radioactivity, corresponding to 3.7 x 1010 radioactive disintegrations per second. Now replaced by the becquerel.



deterministic effect - An effect for which the severity of the effect in affected individuals varies with the dose, and for which a threshold usually exists.

dicentric chromosome - A chromosome having two centromeres.

differentiation - The process of complex changes by which cells and tissues attain their adult structure and functional capacity.

direct action - Ionization or excitation of atoms within DNA leading to free radicals, as distinct from the reaction with DNA of free radicals formed in nearby water molecules.

DNA (deoxyribonucleic acid) - An organic acid composed of four nitrogenous bases (adenine, thymine, cytosine, and guanine) linked via sugar and phosphate units. DNA is the genetic material of most organisms and usually exists as a double-stranded molecule in which two antiparallel strands are held together by hydrogen bonds between adenine-thymine and cytosine-guanine base pairs.

dose - General term for the quantity of radiation. See absorbed dose, equivalent dose, effective dose, collective effective dose.

dose rate - The radiation dose delivered per unit time and measured, for instance, in Gy per hour.

double helix - The model for DNA structure proposed by James Watson and Francis Crick, involving two antiparallel, hydrogen-bonded polynucleotide chains wound into a right-handed helical configuration, with 10 base pairs per full turn of the double helix. Often called B-DNA.

doubling dose - the radiation dose required to double the spontaneous mutation rate.



early responses - Radiation-induced normal-tissue damage that is expressed in weeks to a few months after exposure. Generally due to damage to parenchymal cells. a/b ratio tends to be large.

effective dose - The radiation dose allowing for the fact that 1) some types of radiation are more damaging than others, and 2) some parts of the body are more sensitive to radiation than others. It is defined as the sum over specified tissues of the products of the equivalent dose in a tissue (T) and the weighting factor for that tissue (WT).

electromagnetic radiation - Radiation consisting of associated and interacting electric and magnetic waves that travel at the speed of light; in order of increasing energy these include radio waves, visible light, x rays, and gamma rays. All electromagnetic radiation can be transmitted through a vacuum.

electron (e) - An elementary particle with a unit negative electrical charge and a mass 1/1837 that of the proton. Electrons surround the positively charged nucleus and determine the chemical properties of the atom. Positive electrons, or positrons, also exist.

electron volt (eV) - The amount of energy gained by a particle of charge e (-1.6 x 10-19 C) when accelerated by a potential difference of one volt. 1 eV ~ 1.6 x 10-19  J.

equivalent dose (HT) - A quantity used for radiation-protection purposes that takes into account the different probability of effects which occur with the same absorbed dose delivered by radiations with different WR values. It is defined as the product of the averaged absorbed dose in a specified organ or tissue (DT) and the radiation weighting factor (WR) values. If dose is in gray (Gy), equivalent dose is in sieverts (Sv).



fast neutrons - Neutrons with energy greater than approximately 100,000 eV (electron volts). Compare intermediate neutrons, prompt neutrons, thermal neutrons.

FISH (fluorescence in situ hybridization) - Fluorescent dyes are attached to specific region of the genome, thus aiding the identification of chromosomal damage.

free radical - A fragment of an atom or molecule containing an unpaired electron, therefore very reactive.



gamma rays (g) - High-energy, short-wavelength electromagnetic radiation. Gamma radiation frequently accompanies alpha and beta emissions and always accompanies fission. Gamma rays are very penetrating and are stopped best or shielded against by dense materials, such as lead or depleted uranium. Gamma rays are indistinguishable from x rays except for their source. Gamma rays originate inside the nucleus; x rays from outside.

genetic counseling - Analysis of risk for genetic defects in a family and the presentation of options available to avoid or ameliorate possible risks.

genetic effects - Changes in reproductive cells that may result in abnormal offspring of persons or animals. Now called heritable effects.

genetic effects of radiation - Radiation effects that can be transferred from parent to offspring. Any radiation-caused changes in the genetic material of sex cells. Now called hereditary effects.

genetically significant dose (GSD) - The dose to the gonads weighted for the age and sex distribution in those members of the population expected to have offspring. The GSD is measured in sieverts (rem).

GeV - One billion electron volts. Also written BeV.

gray (Gy) - The special name for the SI unit of absorbed dose, kerma, and specific energy imparted equal to one joule per kilogram. One gray equals one joule per kilogram. The previous unit of absorbed dose, rad, has been replaced by the gray. 1 Gy = 100 rad.



hereditary effects - Radiation effects that can be transferred from parent to offspring. Any radiation-caused changes in the genetic material of sex cells.

heredity - Transmission of traits from one generation to another.

heritability - A measure of the degree to which observed phenotypic differences for a trait are genetic.

heritable effects - Changes in reproductive cells that may result in abnormal offspring of persons or animals.



indirect action - Damage to DNA by free radicals formed through the ionization of nearby water molecules.

ion - An electrically charged atom or group of atoms.

ion pair - A closely associated positive ion and negative ion (usually an electron) having charges of the same magnitude and formed from a neutral atom or molecule by radiation.

ionization - The process of adding one or more electrons to, or removing one or more electrons from, atoms or molecules, thereby creating ions. High temperatures, electrical discharges, or nuclear radiations can cause ionization.

ionization chamber - A device for detection of ionizing radiation or for measurement of radiation dose and dose rate.

ionizing radiation - Any radiation displacing electrons from atoms or molecules, thereby producing ions. Examples of ionizing radiation are alpha-, beta-, and gamma-radiation and short-wave ultraviolet light. Ionizing radiation may produce severe skin or tissue damage. See radiation burn, radiation illness.

isotopes - Forms of a chemical element that have the same number of protons and electrons, but differ in the number of neutrons contained in the atomic nucleus. Unstable isotopes undergo a transition to a more stable form with the release of radioactivity.



kilovolt (kV) - A unit of electrical potential difference equal to 1,000 volts.



late responses - Radiation-induced normal-tissue damage that in humans is expressed months to years after exposure. Generally due to damage to connective tissue cells. a/b ratio tends to be small (< 5 Gy).

lethal dose - A dose of ionizing radiation sufficient to cause death. Median lethal dose (MLD or LD50) is the dose required to kill, within a specified period of time, half the individuals in a large group of organisms similarly exposed. The LD50/60 for humans is about 4 Gy (400 rad).

linear energy transfer (LET) - The rate of energy loss along the track of an ionizing particle. Usually expressed in keV/mm.



metaphase - The stage of cell division in which the condensed chromosomes lie in a central plane between the two poles of the cell, and in which the chromosomes become attached to the spindle fibers.

mitosis - Process of nuclear division in which chromosomes move along a spindle resulting in two new nuclei with the same number of chromosomes as the original nucleus.

mutation - A relatively stable change in the DNA of the cell nucleus. Mutations in the germ cells of the body (ova and sperm) may lead to inherited effects in the offspring. Mutations in the somatic cells of the body may lead to effects in the individual, e.g., cancer.

mutation rate - The frequency with which mutations take place at a given locus or in a population.



neutron (n) - An uncharged elementary particle with a mass slightly greater than that of the proton and found in the nucleus of every atom heavier than hydrogen. A free neutron is unstable and decays with a half-life of about 13 minutes into an electron, a proton, and a neutrino. Neutrons sustain the fission chain reaction in a nuclear reactor.



particle - A minute constituent of matter, generally one with a measurable mass. The primary particles involved in radioactivity are alpha-particles, beta-particles, neutrons, and protons.

photoelectric effect - Absorption of an x ray by ionization.

photon - The carrier of a quantum of electromagnetic energy. Photons have an effective momentum but no mass or electric charge.

positron (b+) - An elementary particle with the mass of an electron but charged positively. It is the "anti-electron." It is emitted in some radioactive disintegrations and is formed by the interaction of high-energy gamma-rays with matter.

proton - An elementary particle that is a component of all nuclei with a single positive electrical charge and a mass approximately 1837 times that of the electron. The nucleus of an ordinary or light hydrogen atom. The atomic number of an atom is equal to the number of protons in its nucleus.



rad - The old unit of absorbed dose, equivalent to an energy absorption of 10-2 J/kg. Superseded by the gray. See absorbed dose.

radiation - The emission and propagation of energy through matter or space by means of electromagnetic disturbances that display both wave-like and particle-like behavior; in this context the "particles" are known as photons. Also, the energy so propagated. The term has been extended to include streams of fast-moving particles, such as alpha- and beta-particles, free neutrons, and cosmic radiation. Nuclear radiation is that emitted from atomic nuclei in various nuclear reactions, including alpha-, beta-, and gamma-radiation and neutrons.

radiation (ionizing) - Any electromagnetic or particulate radiation capable of producing ions, directly or indirectly, by interaction with matter. Examples are x rays, photons, charged atomic particles and other ions, and neutrons.

radiation absorbed dose - See rad.

radiation weighting factor - Approximate values of the relative biological effectiveness, applicable to low doses and relevant to carcinogenesis and hereditary effects. Radiation weighting factors are chosen by the ICRP based on experimental relative biological effectiveness values with a large judgment factor.

radioresistance - A relative resistance of cells, tissues, organs or organisms to the harmful action of radiation.

radiosensitivity - (1) A relative susceptibility of cells, tissues, organs, or organisms to the effects of radiation. (2) The radiation dose required to produce a defined level of cell inactivation. Usually indicated by the surviving fraction at 2 Gy (i.e., SF2) or by the parameters of the linear-quadratic or multitarget equations.

relative biological effectiveness (RBE) - A factor used to compare the biological effectiveness of different types of ionizing radiation. It is the inverse ratio of the amount of absorbed radiation, required to produce a given effect, to a standard (or reference) radiation required to produce the same effect.

rem - Old unit of equivalent or effective dose. It is the product of absorbed dose (in rad) and the radiation weighting factor. 1 rem = .01 Sv.

repair - Restoration of the integrity of damaged macromolecules.

reproductive integrity - Ability of cells to divide many times and thus be "clonogenic."



scattered radiation - Radiation that, during passage through matter, is changed in direction (the change is usually accompanied by a decrease in energy).

sievert (Sv) - SI unit of equivalent dose or effective dose. 1 Sv = 100 rem.

sublethal damage - Non-lethal cellular injury that can be repaired or accumulated with further dose to become lethal.

stochastic effect - Effects, the probability of which, rather than their severity, is a function of radiation dose without threshold. (More generally, stochastic means random in nature).



thermal neutrons - Neutrons in thermal equilibrium with their surrounding medium. Thermal neutrons are those that have been slowed down by a moderator to an average speed of about 2200 m/s (meters per second) at room temperature from the much high initial speeds that they had when expelled by fission.

thymine dimer - A pair of adjacent thymine bases in a single polynucleotide strand between which chemical bonds have formed. This lesion, usually the result of damage caused by exposure to ultraviolet light, inhibits DNA replication unless repaired by the appropriate enzyme system.



x ray - A penetrating form of electromagnetic radiation emitted either when the inner orbital electrons of an excited atom return to their normal state (these are characteristic x rays) or when a metal target is bombarded with high-speed electrons. X rays are always non-nuclear in origin.

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This page was last modified on October 23, 2011