162 ISOTOPIC TRACERS 



mass. Generally the symbol of the element gives adequate information, 

 so the subscript atomic numbers are omitted. H" is frequently called 

 deuterium and is sometimes given the symbol D, whereas H^ is called 

 tritium (T). 



Radioactivity and radiation 



The nuclear combination of one proton and one neutron is stable; 

 that is, there is no tendency for this nucleus to decompose. The tritium 

 nucleus, however, is unstable and undergoes spontaneous degradation to 

 a more stable form. The excess energy of the unstable form is given off 

 as a radioactive emission, in this case y3~ particles or electrons, as one of 

 the neutrons changes to a proton and an electron. The atom becomes 

 2He^, and the electron or /3~ particle is accelerated through space. Other 

 radioactive materials disintegrate in this manner or in other patterns 

 which yield a. particles (2 protons + 2 neutrons), P~ (electrons) or ^8+ 

 (positrons), y quanta (electromagnetic radiation), or some combination 

 of these. Any isofope disintegrates at a characteristic rate in a character- 

 istic way. 



The oi, p ,1^ y emanations possess great energy. As they move 

 through air or other materials they produce pairs of ions. They share 

 this quality with X rays, which are produced by a different principle. 

 Quanta of visible light and ultraviolet are basically similar to y rays and 

 X rays, but the quanta possess insufficient energy to produce ionization. 

 For this reason, only X rays and y rays are classified as ionizing radia- 

 tion. 



Amounts of radioactive materials are measured in curies, one curie 

 being the amount of a radioactive material such that 3.7 X 10^^ atoms dis- 

 integrate per second. A curie is a large amount of radioactive material, 

 and biologists are more likely to work with millicuries (mc) or micro- 

 curies (/w-c). The basic unit of ionizing radiation is the roentgen (r). The 

 roentgen is an amount of X or y radiation sufficient to produce about 

 2X10® ion pairs in 1 cm'^ of dry air. Several other units of radiation have 

 been developed for use in studies on the biological effects of radiation. 

 These include rep (radiation equivalent, physical), used in measuring 

 radiation absorbed by soft tissue; the rad (radiation, absorbed dose), the 

 amount absorbed in any medium; and the rem (radiation equivalent, 

 man), which applies sp>ecific corrections for man. 



