ARTIFICIAL ACTIVITY OF HAFNIUM AM) SOME OTHKK ELEMENTS 67 



The activity found by us in the filtrate of scandium precipitate couhl 

 only be that of ^^K, as the presence of active impurities was excluded 

 by the fact that the above mentioned very pure scandium sample was 

 used. The possibility that we measured the half- life of radioactive 

 sodium of 15 hours can be excluded with certainty not only for- the 

 reason mentioned above but also for the following reasons : Sodium, 

 iiNa, can be prepared either from '-^Na by simple n(>utron capture, or 

 from magnesium if the capture is followed by emission of a proton, 

 or from aluminium if the capture is followed by emission of ana-particle. 

 From the first mentioned process with the neutron sources at our disposal 

 only very weak activities can be obtained even when starting with pure 

 sodium. To prepare measurable amounts of radio-sodium from a few 

 grams of impure scandium oxide an appreciable amount of magnesium 

 or aluminium would have had to l)e present in the preparation. 15 mgm 

 of aluminium mixed with 150 mgm of ammonium nitrate, for example, 

 gave after activation to saturation less than 0.5 counts/min. and the 

 activity obtained by similar amounts of magnesium with the same sources 

 as used when activating scandium was still smaller. The amount of 

 radio-sodium obtained from magnesium is less, and that of sodium by 

 neutron capture very appreciably less than that obtained from alu- 

 minium. Scandium having just one stable isotope liSc, only the potassium 

 isotope ^^K can be produced under neutron bombardment according 

 to the equation 



MSc + Sn = liK-f|He. 



In the case of neutron capture by potassium, on the other hand 

 both reactions ^^K + n and ^^K -|- n can occur. While Fermi and his 

 collaborators left it open which of the two last named potassium iso- 

 topes were produced, we could conclude from our experiments that the 

 process witnessed by Fermi was ^^K -f- n = ^^K, and also that the 

 process ^^K very probably leads to the formation of the potassium 

 isotope *°K which is responsible for the natural radioactivity of potassium. 

 Recently, Walke (9), by making use of Lawrence's powerful cyclotron, 

 which supplies a many thousand times stronger neutron beam as ob- 

 tained from our radium-beryllium sources, was able to follow the decay 

 of ^2Jy through ten periods and determined its half life period to be 

 12.4 7^7 0.2 hours, i. e., a somewhat lower value than that following 

 from the investigations of Fermi and from our Fig. 2. 



Besides preparing ^^K according to the equation 



If Sc + in = IfK 4- a . 

 we succeeded (4) also in preparing this isotope by the process 



42/ 

 20*^ 



^^Ca + Jn = ?,K + m 



6* 



