﻿Ionization Ranges of tlie a Rays of Actiniuum. 245 



In the case of actinium, the disturbances due to its emana- 

 tion were even greater than in the case of thorium, since the 

 decay period of actinium emanation (half value in 3*7 seconds) 

 is so rapid that by far the greater part of it is transformed 

 before it can be removed even by a very strong air-current. 

 Consequently, disturbances due to the emanation were always 

 encountered, and the only way of avoiding this difficulty 

 completely was to cover the emanating preparation by a mica 

 screen and to determine the u stopping power " of the covering 

 screen by a special experiment. 



Ionization Curve of the Active Deposit of Actinium. 



It was of special interest to determine the ionization- 

 distance curve of the active deposit of actinium, as previous 

 experiments with the active deposit of thorium had conclusively 

 shown that the latter consisted of two well-marked a ray 

 products instead of one as formerly supposed. 



The investigation of the active deposit of actinium does 

 not present any of the above-mentioned difficulties, as it can 

 easily be obtained in a pure state in the form of an extremely 

 thin film. 



The active deposit was obtained on a thin copper wire, 

 about 1 cm. long, by making it the only negatively-charged 

 body exposed to the emanation of a fairly large amount of 

 actinium. The measurements of the ionization curve were 

 begun about 10 or 15 minutes after removal from the emana- 

 tion. After that interval, the activity of the active deposit 

 decays nearly according to an exponential law with the period 

 of 36 minutes. The values of the current obtained at different 

 times had, consequently, to be corrected for their decay of 

 activity, and this was easily done by extrapolation from the 

 known decay curve. 



Since the activity of the wire was initially not very great 

 and decayed rapidly during the experiment, only about one- 

 half of the ionization curve could be accurately obtained with 

 one active wire before the activity became very small. The 

 curve was, consequently, completed by using another active 

 wire. In this way the ionization curve could be obtained 

 with accuracy. 



Fig. 1 shows the curve obtained for the active deposit of 

 actinium. 



From the shape of the curve it is obvious that the active 

 deposit of actinium contains only one a, ray product. The 

 ionization of the a, rays begins at a distance of 5'50 cms., 

 increases very rapidly, reaches a maximum at about 3'6 cms., 

 and decreases slowlv with the distance from the source. The 



