A SCIENTIFIC CAREER 2 



Ziy 



he said he was willing to give me a larger amount. 1 chose dysprosium 

 without having any special reason to do so. Ten years later, after the 

 discovery of artificial radioactivity, we exposed Auer's dysprosium to 

 slow neutrons and succeeded in producing an exceedingly strongly active 

 radiodysprosium. No element is known that can be actived more inten- 

 sively than dysprosium and europium. Exposure of Auer's europium to a 

 neutron beam also led to the formation of a very strongly active radio- 

 europium, while no active gadolinium could be prepared by using radium- 

 beryllium as the source of neutrons. At that time my friend Professor 

 Rolla, of the University of Florence, who prepared a few kilograms of 

 gadolinium oxide, sent me samples of this material which he wished us to 

 analyze for europium by X-ray spectroscopy. We had earlier analyzed 

 several of his samples quantitatively applying secondary X-rays, a 

 method which was worked out in the Freiburg laboratory. Having no 

 access to a Roentgen spectroscope in Copenhagen at this time, we tried 

 together w-ith Miss Hilde Levi to analyze the samples by exposing 

 them to a flux of slow neutrons. All the samples contained some euro- 

 pium . 



By preparing standards containing a known amount of pure gado- 

 linium and pure europium, we could arrive at quantitative figures for 

 the europium content of Rolla's samples. This was the start of activa- 

 tion analysis, which has since become an important tool in analytical 

 chemistry. It was possible by this method to determine, for example,, 

 the minute amounts of sodium and potassium in a nerve fiber. 



Deuterium as a Tracer 



Urey's epochal discovery of deuterium took place while I worked in 

 Freiburg. Most kindly he promptly supplied us with some liters of 

 waters containing 0.6 per cent of deuterium oxide. This low heavy- 

 water concentration sufficed to study the interchange of the water 

 molecules between goldfish and the surrounding water and also to 

 determine the water content of the human body, making use of the 

 principle of isotope dilution already introduced a few years earlier 

 (1931) when we determined the lead content of rocks. The mean lifetime 

 of the w^ater molecules in the human body was determined as well. 

 When I returned to Copenhagen in the fall of 1934, August Krogh called 

 on me immediately upon my arrival. He wished to apply labelled water 

 in his permeability studies. 



I initially intended, upon return to Copenhagen, to do work with 

 deuterium on similar lines as later published by Schoenheimer and 

 Rittenberg. The possibility of obtaining artificial radioactive isotopes, 

 however, induced me to abandon this plan and to concentrate on the 

 application of radiophosphorus in biological studies. 



