440 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



radiations, and the application of the principles of this science to the diagnosis and 

 treatment of disease. 



radionuclide : Any nuclide that is radioactive. 



radioresistance : Relative resistance of cells, tissues, organs, or organisms to the 

 mjurious action of radiation. The term may also be applied to chemical compounds 

 or to any substances. See also radiosensitivity. 



radiosensitivity: Relative susceptibility of cells, tissues, organs, organisms, or any 

 substances to the injurious action of radiation. Radioresistance and radiosusceptihility 

 are at present employed in a qualitative or comparative sense rather than in a quan- 

 titative or absolute one. 



range : The thickness of matter, expressed in length or in mass per unit area, required 

 to remove all detectable high-speed charged particles from a beam by slowing them 

 down below the threshold for measurement. Alpha particles show well-defined ranges 

 with some straggling; fission fragments show more straggling; and electrons show much 

 less well-defined ranges. The ranges depend on the initial maximum energy of the 

 particles in the beam. 



reaction, nuclear: An induced nuclear disintegration; that is, a process occurring 

 when a nucleus comes into contact with a photon, an elementary particle, or another 

 nucleus. In many cases the reaction can be represented by the symbolic equation: 

 X+a^ Y + h or, in abbreviated form, X{a,h)Y, in which X is the target nucleus, 

 a is the incident particle or photon, h is an emitted particle or photon, and Y is the 

 product nucleus. 



relative abundance of an isotope : The fraction or percentage of the atoms of an 

 element which are a given isotope. It usually refers to the natural isotopic mixture 

 of an element. 



relative biological effectiveness of radiation (RBE) : The inverse ratio of tissue doses 

 of two different types of radiation which produce a particular biologic effect under 

 otherwise identical conditions. In general, the RBE may vary with the kind and 

 degree of biological effect considered, the duration of the exposure, and other factors 



relative plateau slope (counter tubes) : The percentage of change in counting rate 

 per 100 volts increase of applied potential along the plateau. 



roentgen : The quantity of X or gamma radiation such that the associated corpus- 

 cular emission per 0.001293 g of air produces, in air, ions carrying one electrostatic unit 

 of quantity of electricity of either sign. 



roentgen rays : X rays. 



sarcoma : Malignant neoplasm composed of cells imitating the appearance of the 

 supportive and lymphatic tissues. 



saturation : The condition that obtains when an induced nuclear reaction has contin- 

 ued sufficiently long to produce the asymptotic activity of a given radionuclide. In 

 this condition the decay rate of the nuclide in question is equal to its rate of production. 



scaler, scaling circuit: A device that produces an output pulse whenever a pre- 

 scribed number of input pulses have been received. 



scaling factor : The number of input pulses per output pulse of a scaling circuit. 



scattered radiation (in radiology) : Radiation that, during its passage through a 

 substance, has been deviated in direction. It may also have been modified by an 

 increase in wavelength. 



scavenging : The use of an unspecific precipitate to remove from solution by adsorp- 

 tion or coprecipitation a large fraction of one or more undesirable radionuclides. 

 Voluminous gelatinous precipitates are ordinarily used as scavengers, e.g., Fe(0H)3. 



scintillation : A flash of light produced in a phosphor by an ionizing event. 



scintillation counter: The combination of phosphor, photomultiplier tube, and 

 associated circuits for counting scintillations. 



