XVII. RADIOACTIVE TRACERS 605 



Measured half-lives of radioisotopes range from 10~^ second to 10'^ 

 years. It is obvious that many isotopes will have decay periods 

 much too short to allow their use as tracers, while for others the peri- 

 ods are so long that the radioactivity will be of too low intensity to 

 measure. The useful range of half-lives is nearly spanned by the 

 carbon isotopes, C^', with half-life of 20 minutes, and C^"*, 5000 years. 



3. Units 



The most commonl}^ used unit for comparing quantity of radio- 

 activity is the curie (17). The unit was originally set up so that 

 other radioactive elements, including the daughter products of 

 radium, could be compared to radium, for many years the reference 

 substance. It is now widely used to mean the amount of radioactive 

 substance giving the same number of disintegrations as one gram of 

 radium, namely, 3.7 X 10^" per second. It is hoped that, although 

 the experimental value of the decay rate of radium may change, this 

 numerical definition of the curie will be retained. For most work the 

 fractional units, the millicurie (mc. = 3.7 X 10^ disintegrations per 

 second, 2.2 X 10^ disintegrations per minute) and microcurie (1 ^uc. = 

 2.2 X 10^ disintegrations per minute) are closer to the range of activity 

 being handled. 



In some instances the term "milligrams radium equivalent" is 

 used. One should check each author's definition of this term, since 

 some use it as a synonym for millicurie while others make the com- 

 parison in this case on the basis of 7-ray activity as measured by ioni- 

 zation chamber (see below). The two methods may give completely 

 different results. 



To avoid the conflicting definitions of the curie, a new unit, the 

 rutherford (rd.), has been proposed (18). It is defined on a purely 

 numerical basis as that quantity of any radioactive material decaying 

 at the rate of 10^ disintegrations per second. The new unit is thus 

 1/37 mc. according to the usual definition of the latter. It has not 

 had wide acceptance and seems to be considered superfluous. 



The differential equation for decay { — dN/dt — \N) relates the 

 activity, or disintegrations per unit time, to the weight of radioactive 

 material, since the number of atoms, .V, is equal to the weight of ma- 

 terial divided by' the molecular weight and multiplied by Avogadro's 

 number. From these various relations such problems as the weight 

 of isotope representing a millicurie of substance of any given half-life 

 can easily be solved. 



