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XVII. The Transformation of the Actinium Emanation. 

 By H. Geiger, Ph.D.* 



IT was first shown by H. L. Bronsonf that the actinium 

 emanation in breaking up emits two a particles for each 

 a particle from the active deposit. This peculiarity was 

 examined later by E. Marsden and myself t, in order to 

 settle whether the two a particles are expelled together. By 

 means of a double screen arrangement it was found that a 

 large proportion of the scintillations, produced by the 

 a. particles from the emanation, appeared as doublets, and 

 the number of these apparent doublets was far greater than 

 in the case of other radioactive substances. It was there- 

 fore concluded that either the decay of the emanation is 

 connected with the simultaneous emission of two a particles, 

 or that the emanation is followed by another short-life a-ray 

 product of less than -^ Q second period. 



In order to throw further light on this question I have 

 investigated the ranges of the a particles from the actinium 

 emanation by the scintillation-method in the following way. 

 A flat glass ring, about 1 mm. thick, was covered airtight on 

 one side with a thin sheet of mica, whose stopping power 

 was equivalent to 8 mm. of air. This ring could be pressed 

 airtight against a plane metal plate, and in this way a shallow 

 box was formed. The metal plate had two small holes which 

 made it possible to circulate air carrying actinium emanation 

 through the box. 



In order to count the scintillations at different distances a 

 zinc-sulphide screen was fixed to a microscope, which could 

 be moved along a graduated scale. The scintillations were 

 counted within the last 2 to 3 cms. of the range at intervals 

 of 1 to 2 mm. The change in number which was due to the 

 increase of distance was corrected for by multiplying each 

 figure by the square of the distance at which the reading 

 w r as taken. 



Curve I. in fig. 1 shows the result obtained for the 

 actinium emanation and its active deposit. The curve brings 

 out clearly that three products of ranges 5*4, 5*7, and 6*5 cm. 

 are present. The shortest range, viz. 5*4 cm., is obviously 

 that of the a particles from the active deposit which is 

 generally given as 5*5 cm. To ascertain the correctness of 

 this value an experiment was carried out for the active 

 deposit alone, and the curve obtained is drawn on the same 



* Communicated by Prof. E. Rutherford. 



f Ei. L. Bronson, Phil. Mag. xvi. p. 291 (1908). 



X H. Geiger and E. Marsden, Phys. Zeitschr. xi. p. 7 (1910). 



