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XCIX. An Attempt to detect Induced Radioactivity resulting 

 from air-Ray Bombardment. By A. Gr. Shenstone, B.A. * 



IN recent experiments t Sir Ernest Rutherford and 

 Dr. Chad wick have shown that a certain proportion of 

 collisions of a-particles with atoms of some of the lighter 

 elements causes the ejection by the nucleus of charged 

 particles which can be identified as hydrogen nuclei. From 

 the consideration of their energy of emission, it is shown 

 that these particles must have derived a portion of their 

 energy from the internal energy of the original nucleus. 

 Moreover, the work of Dr. Shimizu J in the Cavendish 

 Laboratory on the photography of the tracks of a-particles 

 in saturated air suggest that a much larger proportion of 

 a-particles, of the order of 1 in 300, produces some other 

 form of separation of the constituents of the nucleus. In 

 some cases this action is apparently the ejection of a helium 

 nucleus. 



Such facts have very materially increased our knowledge of 

 the structure of the atomic nucleus. But, because of our 

 almost complete ignorance of the forces existing in and 

 immediately around the nucleus, no definite deductions can 

 be drawn as to the subsequent history of the nucleus from 

 which a portion has been ejected. ■ 



It would seem, however, that such a violent partial dis- 

 memberment of a composite system could not fail to have 

 still more far-reaching results. The stability of the nucleus 

 cannot fail to be upset to some extent ; and lack of stability 

 in the nucleus is, we believe, essentially the cause of radio- 

 activity. The critical point is the degree of that instability. 

 Tt may be so great that the nucleus is completely and 

 instantaneously dismembered ; or it may be so small that 

 the atom almost immediately regains equilibrium in a new 

 form. The existence of isotopes of many elements is strong 

 evidence that the latter condition is quite probable. Between 

 these two extremes, however, is the whole range of degrees 

 of instability exemplified in the radioactive substances. 



If the effect of the condition of the deranged nucleus is 

 the further emission of high-speed particles after a time of 

 the order of hours or minutes, it would almost certainly have 



* Communicated by Prof. Sir E. Rutherford, F.R.S. 



t Roy. Soc. Proc. A. xcvii. p. 374 (1920) ; Phil. Mag. Nov. 1921. 



X Roy. Soc. Proc. A. xcix. p. 432 (1921). 



