942 Radioactivity resulting from a-Ray Bombardment. 

 nuclei produced per minute is 



25x3-7xl0 7 x60 



10 6 X 10 



= 5500. 



Roughly 1 in 250 of the particles which might be emitted by 

 the abnormal atoms would strike the ZnS screen it* the 

 screen were directly over the bombarded portion. Assuming 

 that the number of particles emitted decreases exponentially 

 with the time and that one a minute could be detected, the 

 time between bombardment and observation must have been 

 at least four times the half-period of decay of the substance. 

 This gives a maximum half-life of —^ x \ = 2 x 10" 5 sec. 



The above calculation makes use only of those atoms 

 changed by the ejection of an H-particle. If the variety 

 of disruption observed by Mr. Shimizu is included, the 

 minimum observable half-life is considerably decreased since 

 the number of abnormal nuclei is now 



25x3-7xl0 7 x60 KK 1A6 



10 2 x 10 =°5x 10 b per mm. 



But 55xl0 6 v 1 



"250~ X 2 17 - i ' / * 



Therefore the maximum half-life becomes 



x -— =4*7xl0- 6 sec. 



12,500 17 



In order to examine the possibility of a cumulative effect 

 from a long, heavy bombardment by the a-particles, an 

 arrangement was made by means of which a metal strip, 

 after several minutes' bombardment, could be brought beneath 

 the ZnS screen after an interval of about T J sec. This 

 likewise gave no observable result. 



The negative results obtained in these experiments do not, 

 of course, preclude the possibility of radioactive disintegra- 

 tions taking pkice which involve the emission of /3-particles, 

 ry-radiation, or of mass particles of range less than 2*0 mm. 

 But the evidence is very strong, in the case of aluminium, 

 carbon, and iron, that no mass particles of range greater 

 than 2'0 mm. are produced after an interval of 8 X 10~ 5 sec, 

 and that none of range greater than 6 mm. are produced 

 after an interval of 3*3 xlO -5 sec. The corresponding- 

 intervals for lead are 1*2 x 10~ 4 and 5*0 x 10~ 5 . 



