Long-Range Alpha Particles from Thorn 



385 



only a partial explanation. From the close analogy of the 

 " 0" products of radium, thorium, and actinium, it is probable 

 that further examination will show an analogous complexity 

 in the modes of breaking up of radium and actinium C. 

 The loss of energy in the form of expelled alpha particles is 

 very different in the four modes of transformation of thorium 

 C, and in consequence it does not seem likely that the 

 resulting products can be the same in all cases. The differ- 

 ences in the energies emitted by the two branch products of 

 thorium C formed a serious difficulty in the original expla- 

 nation given by Marsden and Darwin of the two main modes 

 of transformation of thorium C, and this difficulty is now 

 further increased. A more detailed discussion on these 

 interesting points will be reserved until further experimental 

 information is available. The relation found by Geiger 

 between the range of the expelled alpha particles and the life 

 of the radioactive product, suggests that the average life of 

 the atoms which expel the long-range alpha particles must be 

 exceedingly short, and of the order of 10 ~ 13 and 10 ~ 16 sec. 

 for the products emitting alpha particles of range 10*2 and 

 ]1*3 cm. respectively. 



The following table gives the velocity of the four groups 

 of alpha particles from thorium C, taking as the basis of 

 calculation the measurements of Rutherford and Robinson 

 that the velocity V of the alpha particles from radium C of 

 range 6*94 cm. at 15° C. is 1*922 x 10 9 cm. per second, and 

 assuming Geiger's relation Y 3 = 7cR where II is the range. 





Kange at 15° C. 



Eatio of 

 velocities. 



Calculated 

 velocity. 



Thorium C 



4'95 cm. 



1 



1-205 



1-275 



1-32 



l-71Xl0 9 cm. 

 206 „ „ 

 2-18 „ „ 

 2'26 „ „ 



Thorium o 



8-6 „ 

 102 „ 

 11-3 „ 



New product (C 3 ) 



New product (C 4 ) 



Recently one of us * showed by direct measurement that 

 the velocities in two main groups of alpha particles from 

 thorium C were in good agreement with the calculated 

 values if the range of the alpha particles from thorium Ci is 

 4'95 cm. instead of 4*80 cm. — the value usually taken. From 

 our measurements, there appears to be no doubt that the 

 higher value is more correct. 



* A. B. Wood, Phil. Mag. xxx. p. 702 (1915). 

 Phil. Mag. S. 6. Vol. 31. No. 184. April 1916. 2 D 



