ARTIFICIAL ACTIVITY OK HAFNIUM AND SOME OTHER ELEMENTS 71 



In view of 1 he softness of 1 he /:?-rays emilled we used tliin scan- 

 dium oxide layers ; about 50 nigm/cm'^. In spite of the thin layers 

 used the soft components were absorbed in the preparation to an appreci- 

 ably greater extent than were the hard ones ; accordingly we have to 

 leckon with the possibility that the radiation emitted by scandium 

 is still softer than indicated by the figures of Table 6. 



THE RADIOACTIVITY OF EUROPIUM AND ITS ANALYTICAL 



APPLICATION 



In their fundamental research on the action of neutrons Fermi and 

 his collaborators (1) investigated also the activity of a gadolinium pre- 

 paration bombarded by neutrons and found an activity decaying with 

 a period of 8 hours. Afewyearslater,SuGDEN (7), investigating the radio- 

 activity of europium, discovered a very strong activity decaying with 

 a period of 9.2 h. and, at that time, interpreted the above mentioned 

 period of decay of gadolinium to be due to the presence of some europium 

 in the sample investigated. Investigations carried out by us, in whicli 

 we made use of different gadolinium samples prepared by Prof. Rolla 

 and partly by Prof. Prandtl and the late Baron Auer v. Welsbach, 

 confermed completely the conclusion arrived at by Sugden, and this 

 induced us to make use of the radioactivity of europium produced under 

 the action of neutrons to determine the amount of europium present 

 in gadolinium preparations. Prof. Rolla, being engaged in the prepara- 

 tion of large amounts of pure gadolinium compounds, sent us several 

 samples, the europium content of which he wished ascertained. We 

 describe in the following the analytical procedure used by us. 



Thin layers of the gadolinium oxide samples to be investigated were 

 fixed between two glass plates and placed within a paraffin block. 

 Usually we investigated simultaneously the activation of 4 symmetrically 

 placed preparations. It is of importance to bombard layers having the 

 same thickness anrl to bombard them with neutrons in such a way that 

 each preparation is hit by the same number of neutrons ; the latter 

 was achieved by arranging the sources in the block circularly. We used 

 in these experiments radium-beryllium sources containing 600 mgm 

 of radium, the neutron emission of which corresponds to that from about 

 400 millicuries of radium emanation ; in addition a beryllium-emanation 

 mixture containing 300 millicuries emanation was also present. After 

 irradiating the samples for 3 days they were homoginized and each 

 sample placed in a small aluminium dish having a surface of 1.2 cm- 

 and put below the window of a Geiger-counter. The intensity of the 

 activity of the different gadolinium samples investigated is proportio- 

 nal to their europium content. In order to arrive at a figure stating 



