RADIO-ACTIVITY 201 



That bombardment by a rays is the most 

 promising mode of attack, is clear from figures. 

 These particles are projected with speeds some 

 twenty thousand times greater than that of a rifle 

 bullet, while mass for mass their kinetic energy 

 is four hundred million times that of a bullet. 



If a particles, that is helium nuclei of atomic 

 mass 4, be fired into hydrogen gas, occasional 

 collisions give rise to fast moving hydrogen nuclei, 

 that is charged hydrogen atoms, which will 

 penetrate some 30 centimetres of air and produce 

 scintillations on a zinc sulphide screen. If oxygen 

 or carbon dioxide be substituted for hydrogen, 

 a few of these scintillations, due to traces of 

 hydrogen impurities, are still seen. But, if dry 

 air, or still better nitrogen, be used, many more 

 scintillations appear, and still persist if the screen 

 be moved far beyond the 30 centimetres, or be 

 covered with a screen equivalent to this thickness 

 of air, that is, if the screen is out of shot for the 

 nuclei produced by collision between a rays and 

 hydrogen molecules. 



Their magnetic deflections indicated that the 

 projectiles were still singly charged hydrogen 

 nuclei, but moving with much higher speeds than 

 those obtained by collision in the free hydrogen 

 molecules, thus confirming the evidence of their 

 great power of penetration. It seems to follow 

 that these fast moving hydrogen atoms can only 

 be obtained by an explosion of nitrogen atoms, 

 induced by the impact of the a particles. 



The effect of a particles on solids was then 

 investigated by firing them at very thin films 

 with oxygen gas beyond. Of all the elements 

 examined, the following alone give the same 



