Long-Range Alpha Particles from Thorium. 383 



of the total. This suggests that two groups of homogeneous 

 rays are present, one-third of maximum range 10*2 cm. and 

 two-thirds of range 11*3 cm. The slope of the scintillation 

 curve to be expected on this hypothesis agrees within the 

 experimental error with the observed curve. This division of 

 the alpha particles into two homogeneous groups may be 

 compared with the two well-known groups of alpha particles 

 emitted from thorium C, for it is known that one-third have 

 a range 5*0 cm. and two-thirds a range 8' 6 cm. This 

 suggests that the new groups of alpha particles have their 

 origin in thorium C, and that one-third of range 10*2 cm. 

 accompany the alpha particles of range 5*0 cm., and the 

 remainder of range 11*3 cm. accompany the alpha particles 

 of range 8'6 cm. While it is very difficult to prove the 

 correctness of such a deduction, the numerical agreement in 

 the divisions of alpha particles of different ranges is certainly 

 striking. 



We have not so far examined experimentally whether the 

 new alpha particles are expelled from the alpha ray product 

 thorium C, but this seems very probable. To settle this 

 point, it will be necessary to prepare a strong preparation 

 of thorium G and to determine whether the period of 

 transformation, measured by the new alpha particles, is in 

 agreement with the accepted value for thorium C, viz. half 

 value in 60 minutes. 



Number of Long-range Alpha Particles. 



Since the number of long-range particles decreases at the 

 same rate as the alpha-ray activity of the active deposit of 

 thorium, it is convenient to express their number as a 

 fraction of the total number of alpha particles emitted per 

 second from thorium C. For this purpose, the number of 

 long-range particles per minute was measured with the 

 source fixed at a known distance from the screen. The 

 active deposit was then allowed to decay in situ for 32 hour, 1 -. 

 The absorbing screen was then removed and the number of 

 alpha particles from thorium C measured at distances from 

 6 to 7 cm., so as to include only the longer range alpha 

 particles (8*6 cm.) from thorium C. In this way it was 

 found that the fraction of long-range alpha particles was 

 1/6700. Taking into consideration that the alpha particles 

 of range 8'6 cm. from thorium C are two-thirds of the total, 

 the fraction becomes 1/10000. 



Preliminary observations by different methods gave a 

 somewhat lower value, but the above number cannot be much 

 in error. We thus see that the long-range alpha particles 



