RATE OF DECAY 



This condition can exist only when no activity 

 longer-lived than that of the parent is interposed 

 in the decay chain. (41) 



radioactive half-life — The time required for a 

 radioactive substance to lose 50 percent of its 

 activity by decay. Each radionuclide has an 

 unique half-life. (70) 



radioactive series — A succession of nuclides, each 

 of which transforms by radioactive disintegra- 

 tion into the next until a stable nuclide results. 

 The fii-st member is called the parent, the inter- 

 mediate members are called daughters, and the 

 final stable member is called the end product. 

 Three such series are encountered in natural 

 radioactivity, and many others are encountered 

 in induced radioactivity, particularly among 

 the heavy elements and fission products. (41) 



radioactive tracer — A radionuclide used as an 

 oceanographic tracer having radioactivity as 

 its disting-uishing property. Kadioactive 

 oceanographic tracers include Tritium (H^), 

 Carbon", and fallout radioisotopes (fission 

 products) . 



radioactivity — 1. The process whereby certain nu- 

 clides undergo spontaneous disintegration in 

 which energy is liberated, generally resulting in 

 the formation of new nuclides. The process is 

 accompained by the emission of one or more 

 types of radiation, such as alpha particles, and 

 gamma photons. 



2. A particular radiation component from a 

 radioactive source, such as gamma radioactivity. 



3. A radionuclide, such as a radioactivity 

 produced in a bombardment. 



(70) 



radiobiology — That branch of biology which deals 

 with the effects of radiation on biological sys- 

 tems. (41) 



radiocarbon age — This age is calculated from the 

 specific activity, due to Carbon", of the carbon 

 in a once-living object; such radiocarbon dating 

 is possible because Carbon^* is produced in the 

 atmosphere by cosmic rays and is incorporated 

 into all living objects; after death the Carbon" 

 activity decays exponentially with a half -life of 

 5,568 years. (41) 



radiocarbon dating — See radioactive age 

 determination. 



radiochemistry — The aspects of chemistry con- 

 nected with radionuclides and their properties, 

 with the behavior of minute quantities of radio- 

 active materials by means of their radioactivity 

 and with the use of radionuclides in the study 

 of chemical problems. (70) 



radioisotope — 1. Any radioactive isotope of an 

 element. 



2. A word loosely used as a synonym for 

 radionuclide. 

 (41) 



radioisotopic oceanography — That oceano- 

 graphic disclipline dealing with the measure- 



ment and distributional analysis of ocean-borne 

 radioisotopes. 

 Radiolaria — See radiolarian. 



radiolarian — One of an order (Radiolaria) of 

 single-celled plankton ic protozoa possessing a 

 skeleton of siliceous spicules and radiatmg 

 threadlike pseudopodia. Most members are 

 pelagic, and many are luminescent. 



radiolarian ooze — Deposits of siliceous sediments 

 distinguished by large proportions of minute 

 opaline silica shells (tests) of radiolarians. 

 Water depths between about 13,000 and 25,000 

 feet are most favorable for the preservation of 

 radiolarian tests. See ooze. 



radionuclide — A synonym for radioactive nu- 

 clide. (41) See radioisotope. 



rafted ice — (also called telescoped ice). Pres- 

 sure ice in which one ice floe overrides 

 another. (74) 



rafting — 1. The process by which rafted ice is 

 formed. (65) 



2. The transporting of sediment, rocks, silt, 

 and other matter of land origin out to sea by 

 ice, logs, etc., with subsequent deposition of the 

 rafted matter when the carrying agent 

 disintegrates. (68) 



ram — 1. (also called is/>w or ap'o?i). An under- 

 water ice projection from an iceberg or a 

 hummocked ice floe. Its formation is usually 

 due to a more intensive melting of the unsub- 

 merged part of the floe. (47) 



2. In ice navigation, to charge obstructing 

 ice with a ship. (59) 



ramp — (also called drift ice foot) . An accumula- 

 tion of snow that forms an inclined plane be- 

 tween land or land ice elements and sea ice or 

 shelf ice. See bridge. (65) 



random noise — An oscillation whose instanta- 

 neous magnitude is not specified for any given 

 instant of time. The instantaneous magnitudes 

 of a random noise are specified only by proba- 

 bility distribution functions giving the fraction 

 of the total time that the magnitude, or some 

 sequence of magnitudes, lie within a specified 

 range. (6) 



range — 1. The difference between the maximum 

 and minimum of a given set of numbers ; in a 

 periodic process it is twice the amplitude, that 

 is, the wave height. 



2. The distance between two objects, usually 

 an observation point and an object under 

 observation. 



3. A maximum distance attributable to some 

 process, as in visual range or the range of an 

 aircraft. 



(5) 



rapture of the deep — See nitrogen narcosis. 



rate of decay — The time rate at which the somid 

 pressure level, or any other stated characteristic, 

 decreases at a given pomt in a given time. A 

 commonly used unit to express the rate of decay 



131 



