24 ADVEXTURES IN RADIOISOTOPE RESEARCH 



labelled lead from such seedlings. This was the first application of radio- 

 active tracers in biological studies. The following year we extended 

 these studies with my friends Christiansen and Lomholt to the distri- 

 bution of lead and bismuth in the animal organism. 



Potassium is one of the few radioactive elements found in nature 

 outside the members of the disintegration series. We were interested 

 to find out which of the potassium isotopes is radioactive. For this 

 purpose we carried out a partial separation of the potassium isotopes, 

 applying the same method used when separating the isotopes of mercury. 

 A few kilograms of metallic potassium were distilled and a heavy and a 

 light potassium fraction obtained. From the difference in the activity 

 of these samples and the difference in their atomic weight, the mass 

 number of the active isotope could be calculated. Among other instru- 

 ments that were used to measure the activities of our samples was the 

 first counter built by Geiger in his institute at Kiel. The atomic weight 

 of our sample was determined by Honigschmid in Munich. From these 

 data it was concluded that the mass number of radioactive potassium 

 is 41. 



The first one to draw my attention to the fact that this result was 

 probably wrong was Baxter, when I visited him at Harvard. He had 

 found that, in constrast to all other atomic weight figures determined 

 by Honigschmid, that of potassium was wrong. Baxter proposed to 

 determine the atomic weight of our potassium samples. From his results 

 it followed that the mass number of radioactive potassium is 40. The 

 two greatest authorities in the field of atomic w^eight determination thus 

 arrived at different results as to the atomic weight of our potassium 

 samples. To reach a decision, we extracted the small calcium content 

 of an old potassium-rich mica. If '^iR were the active isotope, then 

 the calcium isolated should contain ^-Ca. Aston could not, however, 

 find any ^^Ca in our sample. Thus ^^K does not disintegrate and 

 is not radio- active. Baxter was right. A few years later Fermi and 

 collaborators observed the production of an artificial potassium isotope 

 when bombarding potassium with neutrons. We obtained with Miss 

 Hilde Levi Fermi's product by bombarding scandium and also calcium 

 with neutrons. As scandium has only one stable isotope, we could 

 conclude from our investigations that Fermi's radiopotassium has the 

 mass number of 42. 



Activation Analysis 



Auer von Welsbach was very cautious in giving away his very valuable 

 rare-earth samples, but one day when I was staying with him he was 

 in a generous mood and told me to choose one of his samples, of which 



