THE USE OF ISOTOPES AS TRACERS 



By A. H. W. Aten, Jr. 



and 



F. A. Heyn 



[With 1 plate] 

 INTRODUCTION 



When the periodic system of chemical elements was set up in the 

 course of the previous century, it was thought that each element con- 

 sisted of only one definite kind of atom. Later this was found to be 

 incorrect: an element may consist of different kinds of atoms which 

 have practically identical chemical properties — the criterion for denot- 

 ing the atoms by the name of the respective element — ^but differ in 

 atomic weight by one or more units. Such isotopic atoms, so called 

 because they have to be given the same position in the periodic system, 

 may be stable or unstable. In the latter case they undergo a gradual 

 change, accompanied by a radiation, into another kind of atom ; they 

 are then radioactive. 



Almost all the kinds of atoms occurring in nature are stable. There 

 are only a few unstable ones, namely the well-known substances with 

 natural radioactivity, such as radium, thorium, uranium, etc. In addi- 

 tion to these, howpver, it is nowadays possible to turn each element 

 into one or more new isotopes which do not occur in nature, all of 

 which are unstable (artificial radioactive substances). 



A single example will serve to illustrate the above. The element 

 calcium occurring in nature consists of six stable isotopic kinds of 

 atoms, namely, for 96.96 percent Ca*", i. e., calcium atoms with an 

 atomic weight of 40 (in round numbers), and further 0.64 percent 

 Ca«, 0.15 percent Ca*% 2.06 percent Ca", 0.0033 percent Ca*« and 0.19 

 percent Ca**. Furthermore, up to the beginning of the year 1944 it 

 had been found possible to make artificially six more radioactive cal- 

 cium isotopes with atomic weights 39, 39, 41, 45, 49, 49. The various 

 unstable isotopes can further be distinguished from each other by the 

 character of their radioactivity. In the case of a radioactive atom a 



* Reprinted by permission from Philips Technical Review, vol. 8, No. 10, October 1946. 



217 



