84 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



if the procedures must be prohibitively expensive or cumbersome, then 

 full benefits will not be realized. Experience in operations of the U.S. 

 Atomic Energy Commission and in private laboratories has shown that 

 almost any level of radioactive material can be handled without damage 

 to personnel provided adequate precautions are taken. The extent of 

 these precautions primarily depends upon the amount of radioactivity 

 used. This discussion will be limited primarily to the lower levels of 

 operation most customary for biological experiments. Only such consid- 

 erations will be treated as are necessary for the intelligent application of 

 measures for radiation protection. . 



The ultimate basis for protective measures will undoubtedly result 

 from fundamental studies of radiation biology. However, at the present 

 time and probably for some time to come it will be necessary to rely upon 

 empirical considerations. The problems of the action of radiation on the 

 living cell are so complex as to fall beyond the scope of this text. The 

 interested reader may consult references (12 to 23a) for a general treat- 

 ment of this subject. 



Nomenclature and Units. In order to discuss problems of radiation 

 protection, it is first necessary to establish the meaning of certain units of 

 measurement and to outline some aspects of the interaction of radiation 

 with matter and with biological systems. We are not particularly inter- 

 ested in formal definitions or in the precise values of various constants. 

 Rather it is important to understand the interrelationships between the 

 units, the characteristics of the radiation, and the biological response. 



Specific Ionization. The radiation from radioactive materials, as well 

 as X rays, is characterized by its ability to produce ionization, that is to 

 say, the removal of electrons from certain atoms and their attachment to 

 other atoms, thus forming pairs of positive and negative ions. Such 

 radiation is therefore called ionizing radiation. It is generally accepted 

 that the biologic effects of radiation are primarily due to the ionization 

 produced in the tissues. The number of ions formed per unit length of 

 path is called the specific ionization and is a characteristic of the radiation. 

 In higher plants and animals, at least, the amount of injury generally 

 increases with the specific ionization, because the more densely ionizing 

 radiation releases more of its energy in localized regions. 



Energy of Radiation. It is necessary to have a unit of energy to 

 describe in part the radiation emitted by radioisotopes. The Mev {mil- 

 lion electron volts) has been found convenient to use. It is equal to the 

 amount of work done when an electron is accelerated by a potential of a 

 million volts. Other fractions and multiples of this unit are often 

 employed, such as the ev {electron volt), kev {kilo-electron-volt) , and Bev {bil- 

 lion electron volts). The energies involved in ordinary chemical reactions 

 range up to 10 ev, whereas those encountered in nuclear interactions are 



