Radioactivity 



The spontaneous breakdown of certain atomic nuclei 

 usually resulting in the emission of radiant energy in the 

 form of Alpha or Beta particles, or Gamma rays. 



Radioautograph 



A representation of an object containing radioactive 

 Isotopes (q. v.) such as a leaf on a photographic negative to 

 show the distribution of the radioactive material. 



Radiocarbon Dating 



Determination of the age of organic remains such as 

 long-buried wood by measuring its radioactivity caused by 

 C 14 , which has a half-life of 5568 years and begins to break 

 down upon the death of organisms. 



Radioisotope 



An Isotope (q. v.) that is unstable, disintegrates, and 

 emits radiations, e.g., uranium-235 which emits alpha and 

 gamma rays. 



Radiometer 



An instrument that measures the intensity of solar Radia- 

 tion, cf. Pyrheliometer. 



Radiosensitivity 



The sensitivity, or lack of tolerance of organisms to 

 endure Ionizing radiation (q. v.). 



Radiosonde 



A free balloon with attached instruments and radio trans- 

 mitter for securing measurement of temperature and atmos- 

 pheric pressure. 



Rainfall 



The total amount of Precipitation including rain, snow, 

 hail, and other forms. 



Rain Forest 



A type of vegetation consisting of tall, evergreen trees, 



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