168 l)r. 0. Halm on Radioactinium. 



radioactini uni preparation completely free from emanation, 

 but a preparation can easily be obtained which initially shows 

 only a very small emanating power compared with the value 

 observed two or three weeks later. 



The amount ot emanation and of ft ray activity observed 

 rise proportionally, since actinium B which emits the ft rays 

 is a product of the emanation. 



The a and especially the ft ray activity of the radioactinium 

 preparation increase rapidly for a week or more, and then 

 more slowly as the maximum is reached (see figs. 1 & 2, 

 pp. 173 & 175). The activity measured by the a rays reaches 

 a maximum slightly earlier than the activity measured by the 

 ft rays or by the amount of emanation present. 



b. Decay Period of Radioactinium. 



It was very soon recognized that the period of the decay 

 was not the 10*2 day period of actinium X but considerably 

 slower. The activity, whether measured by the a or ft rays 

 or the amount of emanation present, finally decays according 

 to an exponential law. It generally does not decay quite to 

 zero, but a small residual activity of a few percent, was 

 observed, indicating that the preparation contained small 

 traces of actinium. By using actinium preparations carefully 

 freed from radium, the possibility of this residual activity 

 being due to radium was excluded. 



After subtracting the small residual activity, the exponential 

 nature of the decay curve was easily to be observed. The 

 following values of the decay constant \( sec - _1 ) were found 

 for several decay curves of radioactinium, which had been 

 prepared by different methods and using both the actinium 

 of Debierne and the actinium of Giesel : — 









0-425 



X 



io- G 









0*413 



X 



lO" 6 









0-399 



X 



io- 6 









0-407 



X 



io- 6 





Av< 



3 rage 



0*412 



X 



io- 6 





0-411 



X 



io- 6 



From 



this 



it follow 



s that the time 



radioactinium to be 

 half transformed is 1*68 X IO 6 sees, or 19*5 days. 



This value is a little lower than the value of 20 days, 

 which I stated in my first paper on the subject. The period 

 19*5 days may still be slightly in error. Further measure- 

 ments are in progress to determine the period with more 

 accuracy. 



