Originally published in the Les Prix Nobel En 1940—1944, p. 



iO. 



98. SOME APPLICATIONS OF ISOTOPIC INDICATORS 



G. de Hevesy 



Nobel Lecture, dehvered at Stockholm, December 12, 1944 



The method of isotopic indicators had its ultimate origin in the Institute 

 of Physics at the University of Manchester, which then was under the 

 inspiring leadership of that great physicist, the late Lord (then Professor) 

 Ernest Rutherford. 



The cradle of radium is the Czecho-Slovakian town Joachimstal ; it 

 w^as from Joachimstal pitchblende ore that Professor and Madame Curie 

 isolated that element. The Austrian Government, the owners of these 

 mines, generously supplied Professor Rutherford not only with radium, 

 but also with the by-products of radium production, equally important 

 for the worker in the field of radioactivity. One of the most significant 

 by-products is radium D, which has a half-life period of 20 years and is 

 found associated with the very substantial amounts of lead present in pitch- 

 blende. The Austrian Government presented to Professor Rutherford 

 several hundred kilograms of such "radiolead". In view of its association 

 with very large amounts of lead, wdrich absorb the radiation emitted 

 by radium D, this precious radioactive material nevertheless proved to 

 be almost useless. When I met Professor Rutherford one day in 1911 

 in the basement of the laboratory where the radio-lead w^as stored, he 

 addressed me in his friendly and informal way, saying : "My boy, if 

 you are worth your salt, you try to separate radium D from all that 

 lead." In those days, I was an enthusiastic young man and, on immedia- 

 tely starting to attack the problem suggested to me, I felt quite con- 

 vinced that I would succed. However, although I made numerous 

 attempts to separate radium D from lead and worked for almost two 

 years at this task, I failed completely. In order to make the best of 

 this depressing situation, I decided to use radium D as an indicator 

 of lead, thus profiting from the inseparability of radium D from lead. 

 Suppose that we dissolve 1 gm of lead in the form of nitrate in water, 

 add radium D of negligible weight showing a radioactivity of one million 

 relative units (an electroscope being used to measure the activity), and 

 proceed to carry out the most intricate operations with this "labelled" 

 lead. If we then ascertain the presence of one radioactive unit in a 



