118 RADIOACTIVITY; BIOLOGICAL TRACERS 



In review, the nature and properties of the four main types of emanation 

 have been considered. Positive ions and electrons lose kinetic energy by 

 charge interaction with the electron cloud of atoms in the path: the greater 

 the electron density the greater the absorption. Gamma rays lose energy 

 to the electron cloud principally by pair production or Compton scattering. 

 A neutron must hit a nucleus to lose energy. When it does, either the 

 nucleus (charged) recoils through the medium and ionizes as a positive ion, 

 or the neutron is absorbed by the nucleus, usually to form an unstable iso- 

 tope which decays with the expulsion of beta or gamma, proton or neutrons. 



The data of Table 5-3 illustrate these important principles. Note par- 

 ticularly the variation of the range in tissue for radiations of different type 

 and energy. Protons are the ionizing particles in tissue which is under fast- 

 neutron irradiation because hydrogen of water is the most plentiful target in 

 the tissue. . . . This table should be thoroughly studied and understood. 



USES AS BIOLOGICAL TRACERS 



One of the simplest, and yet one of the most intriguing applications of the 

 properties of radioactive substances has been in their use as tracers. The age 

 of the earth, the authenticity of oil paintings, the courses of water and wind 

 currents, have been probed simply by analyzing for the pertinent radioactive 

 isotope in the proper place in the proper manner. 



Three uses as tracers concern us here: (1) as an aid in determining the 

 steps and paths by which molecular reactions occur, whether simple hydra- 

 tions of ions, or the more complex syntheses and degradations of large bio- 

 chemicals; (2) in plotting the course of fluid flow, through the blood capil- 

 laries, across cell walls, etc.; (3) in plotting the time and space distribution 

 of biologically active chemicals. Examples of each are now given to illustrate 

 the principles. The book by Kamen, s now a classic in the subject of tracers, 

 is highly recommended for further study. 



Tracers of Molecular Reactions 



The first use of isotopic tracers on a biological problem was reported by 

 Hevesy in 1923; this was a study of lead metabolism in plants. When heavy 

 water (D^O) became available in Urey's laboratory after the discovery of 

 deuterium there in 1932, many biochemical problems were attacked: hydro- 

 genations and dehydrogenations, cholesterol synthesis from smaller frag- 

 ments, conversion of phenylalanine to tyrosine, etc. Then, by 1942, am- 

 monium sulfate containing N 7 15 , instead of the more common N 7 14 , became 

 available, and compounded the possibilities for biochemical investigations. 

 Thanks to the nitrogen tracer, the fate of amino acids in protein synthesis 

 could be followed. Probably the most important of all these investigations, 

 from the point of view of biology, was the demonstration that protein mole- 



