R 



race — A very fast current flowing through a 

 relatively narrow channel. (50) 



radiance — In radiometery, a measure of the in- 

 trinsic radiant intensity emitted by a radiator 

 in a given direction. It is the irradiance 

 (radiant flux density) produced by radiation 

 from the source upon a unit surface area oriented 

 normal to the line between source and receiver, 

 divided by the solid angle subtended by the 

 source at the receiving surface. It is assumed 

 that the medium between the radiator and re- 

 ceiver is perfectly transparent; therefore, radi- 

 ance is independent of attenuation between 

 source and receiver. (5) 



radiance meter — An irradiance meter which 

 collects radiant energy from a set of directions 

 and which has its field of view limited to a cir- 

 cular solid angle of O magnitude (defined, for 

 example, by a cylindrical tube) whose axis is 

 fixed normal to the plane of the collecting area 

 of the meter. If E is the reading of the meter, 

 the associated radiance is L=E/Q,. (8) 



radiant emittance (at a point on a surface) — The 

 radiant flux emitted by an infinitesimal element 

 of surface containing the point under considera- 

 tion, divided by the area of that element. Unit 

 of measurement is watt per square meter 

 (W/m=). (8) 



radient energy — (also called radiation). 1. The 

 energy of any type of electromagnetic 

 radiation. 



2. Infrequently, any energy that may be 

 radiated, as, for example, sonic energy. 



radiant flux — The time rate of flow of radiant 

 energy. Unit of measurement is watts. (8) 



radiant intensity (of a source in a given direc- 

 tion) — The radiant flux emitted by a source, 

 or by an element of a source, in an infinitesimal 

 cone containing the given direction, divided by 

 the solid angle of that cone. 



Note: For a source which is not a point 

 source ; the quotient of the radiant flux received 

 on an elementary surface by the solid angle 

 which this surface subtends at any point of the 

 source, when this quotient is taken to the limit 

 as the distance between tlie surface and the 

 source is increased. 



Unit of measurement is watt pier steradian 

 (W/sr). (8) 



radiated noise — The underwater sound energy 

 emitted by shijDS, submarines, and torpedoes. 



radiation — 1. The emission and propagation of 

 energy through space or through a material 

 medium in the form of waves ; for instance, the 

 emission and propagation of electromagnetic 

 waves, or of sound and elastic waves. 



2. The energy propagated through space or 

 through a material medium as waves; for 

 example, energy in the form of electromagnetic 

 waves or of elastic waves. The term radiation 

 or radiant energy, when unqualified, usually 

 refers to electromagnetic radiation; such 

 radiation commonly is classified according to 

 frequency, as Hertzian, infrared, visible (light) , 

 ultra-violet. X-ray, and gamma ray. {See 

 photon) . 



3. By extension, corpuscular emissions, such 

 as alpha and beta radiation, or rays of mixed or 

 unknown type, as cosmic radiation. 



radiation absorbed dose — (abbreviated rad) . A 

 measure of the dose of any ionizing radiation to 

 body tissues in terms of the energy absorbed per 

 unit mass of the tissue. One rad is the dose cor- 

 responding to the absorbtion of 100 ergs per 

 gram of tissue. One millirad (mrad) equals 

 0.001 rad. (As defined by the Atomic Energy 

 C ommission.) 



radiation detector — Any device for converting 

 radiant energy to a form more suitable for 

 observation. (Reactor engineering) An instru- 

 ment used to determine the presence and some- 

 times the amount of radiation or neutron flux. 

 (70) 



radioactive age determination — The determina- 

 tion of the age of a rock or sediment by measur- 

 ing the proportion of the radioisotope Carbon" 

 in the organic material it contains. A method 

 (radiocarbon dating) is based upon the known 

 rate of conversion of carbon to its isotope and is 

 accurate to a maximum age of about 30,000 

 years. Other chemical elements can be used 

 similarly for age determination. 



radioactive decay — The disintegration of the 

 nucleus of an unstable nuclide by the sponta- 

 neous emission of charged particles and/or 

 photons. (70) 



radioactive equilibrium — A condition which may 

 occur in the course of the decay of a radioactive 

 parent having shorter-lived descendants, in 

 which the ratio of the activity of the parent to 

 that of a descendant is independent of time. 



130 



