430 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



contamination, radioactive : (a) A condition in which an undesirable radioactive 

 substance is mixed with a desired substance, (b) A condition in which radioactive 

 material has spread to places where it may harm persons, spoil experiments, or make 

 products or equipment unsuitable or unsafe for some specific use. 



coprecipitation : (a) The carrying of trace amounts of (active) material on a ponder- 

 able precipitate of another composition. It may involve trace-ion replacement in the 

 crystal lattice of the carrier when the two substances are truly isomorphous; it may 

 involve the formation of anomalous mixed crystals; it may involve adsorption, at the 

 time of formation or later, of the trace ion on the highly charged surface of the carrier 

 precipitate; or it may involve the internal adsorption of the trace in aggregate spots 

 within the carrier precipitate, (b) The precipitation of a substance from concentra- 

 tions below its solubility when induced by the precipitation of another substance. 



cosmic rays : Radiation that has its ultimate origin outside the earth's atmosphere, 

 that is capable of producing ionizing events in passing through the air or other matter, 

 and that includes constituents capable of penetrating many feet of material such as 

 rock. 



count (radiation measurements) : The external indication of a device designed to 

 enumerate ionizing events. It may refer to a single detected event or to the total 

 registered in a given period of time. The term is loosely used to designate a disinte- 

 gration, ionizing event, or voltage pulse. 



counter : A device for counting ionizing events. The term may refer to a complete 

 instrument or, loosely, to the detector. 



counting loss : See dead-time correction. 



counting rate : The average rate of occurrence of ionizing events as observed by 

 means of a counting system. 



counting-rate meter: A device that gives a continuous indication of the average 

 rate of ionizing events. 



cross section : A measure o- of the probability of occurrence of a given reaction. For 

 a particular nuclear reaction, <r may be greater or smaller than the geometric cross 

 section ttR^. If the reaction cannot take place, the cross section is zero. For any 

 collision reaction between nuclear or atomic particles or systems, o- is an area such that 

 the number of reactions taking place is equal to the product of the number of incident 

 particles that would pass through this area at normal incidence and the number of 

 target particles or systems. 



curie: (a) A unit of radioactivity, symbol c, equal to 3.7 X lO'** dis/sec. It is 

 approximately the activity of 1 g radium. (6) A quantity of a nuclide having an 

 activity of 1 c. 



cyclotron : A device for accelerating charged particles to high energies by means of 

 an alternating electric field between electrodes placed in a constant magnetic field. 



daughter : A synonym for decay product. 



dead time (counters) : The time interval, after a covuit, during which a radiation 

 detector and /or its circuit are insensitive to ionizing events. 



dead-time correction : Correction to the observed counting rate to allow for the 

 probability of the occurrence of events within the dead time. 



decade scaler: A scaler whose scaling factor is 10. 



decay, radioactive : The decrease with time of the number of radioactive atoms in a 

 sample, as a result of their spontaneous transformation. 



decay constant : See disintegration constant. 



decay curve : A curve showing the relative amount of radioactive substance remain- 

 ing after any time interval. 



decay product: A nuclide resulting from the radioactive disintegration of a radio- 



