610 ADOLF F. VOIGT 



tion method does not put a limit on their specific activity. See 

 Table 1 for information on the biologically important radioisotopes. 



There are a number of important isotopes that cannot be produced 

 by neutron bombardment, but must be made in a cyclotron {3, p. 

 52). Some of these are included in Table I. Those producible by 

 cyclotron have not been applied extensively to tracer research, but 

 they doubtless would be if they were readily available. Many of 

 them are not limited in specific activity, if chemical separation can be 

 devised. For further detail consult the well known table of isotopes 

 by Seaborg {21,22). 



D. DETECTION AND MEASUREIVIENT OF RADIOACTIVITY 



The use of radioactive tracers is dependent upon the measurement 

 of their radioactivity, and the accuracy obtainable by using them is 

 only as good as the accuracy of this measurement. Fortunately 

 this is high in comparison to the other variables in biological research 

 and the method is, in general, capable of results as accurate as the 

 research requires {2, pp. 105, 141). 



Radioactivity is measured usually by the ionization produced by 

 the radiation. A large variety of instruments has been applied to the 

 problem and is discussed fairly extensively in the literature. Only a 

 few of these will be considered here, first as applied to the general 

 problem of radiation measurement and, second, to the special prob- 

 lem presented by isotopes whose radiation is of verj^ low energy, par- 

 ticularly C^"*. Three general principles of measurement are used: 

 photography, ionization chambers, and counters. The photographic 

 method does not have high sensitivity, but often it does give the 

 answer when no other method will. Its present use is largely confined 

 to two fields: radiation monitoring and radioautographs. Both of 

 these are considered in later sections. 



1. Ionization Chambers 



An ionization chamber is a gas-filled space in which two oppositely 

 charged electrodes collect the ions produced in the gas by radiation. 

 This collection of ions by what is essentially a charged condenser con- 

 stitutes a small electric current measured either as a current or as the 

 rate at which the electrodes are discharged. A number of conven- 

 tional circuits using various types of electrometers have been widely 

 used. With the development of electronics it has become possible to 



