Oiiginally published in tho Kgl. Danske Videnskabei'nes Selfikab. Mathemcttisk- 



fijsiskc Meddeldser. 15, II (1938) 



5. ARTIFICIAL ACTIVITY OF HAFNIUM AND 

 SOME OTHER ELEMENTS 



G. Hevesy and Hilde Levi 

 From Tho Institute of Theoretical Physios, University of Copenhagen 



ARTIFICIAL RADIOACTIVITY OF HAFNIUM 



Some time ago we found that under the action of neutron bombardnienl 

 a radioactive isotope of hafnium is produced, the activity decaying 

 with a period of a few months (4). To determine the period of decay 

 more exactly, we activated 280 mgni of hafnium oxide prepared by 

 one of us (3) by placing it in a paraflin block together with radium- 

 beryllium sources, containing 600 mgm of radium element as sulphate 

 and twenty times as much metallic beryllium powder. After irradiation 

 for three months the hafnium oxide was removed from the paraffin 

 block and put into an aluminium dish having a surface of 1.2 cm^ and 

 a height of 2 mm. The dish was placed directly below the aluminium 

 window of our counter, the window having a thickness of about 20 i-i. 

 We followed the decay of the hafnium preparation for 200 days by 

 comparing its activity with that of an uranium standard. The decay 

 curve obtained is seen in Fig. 1 and Table 1. 



From the latter we can conclude that the half life of liafnium is 

 55 + 10 days (standard mean square deviation). Our initial activity 

 was 20 counts per min, the natural effect being about 4 counts per min. 

 We followed the decay curve until we had a net activity of 1 counts per 

 min. From the fact that, in spite of the long activation, such a modest 



Table 1. — Dec.w-measueement of Hafnii'm 



