ivitJiin closed vessels due to Rontgen and Gamma Rays. 435 



nature of the ionization produced by Rontgen and y rays, 

 and from the remarkable experimental verification obtained 

 by C. T. R. Wilson by photographs of ionization effects in 

 an expansion-chamber. 



In the case of Rontgen rays, ionization appears to be due 

 to the expulsion of electrons from air or other substances, 

 and these electrons, having paths about a millimetre in length, 

 produce groups of electrons along those paths. 



In the present experiments the testing vessels cut down 

 the intensity of the Rontgen rays to an extent mainly de- 

 pendent on the thickness and density of the walls. The 

 electrons are projected from the air and from the walls, with 

 velocities of the order 10 9 cm. sec. -1 , and have irregular paths 

 about a millimetre in length. Hence there is least ionization 

 with thick or heavy walls which cut off the primary and 

 secondary Rontgen rays, and prevent them from causing 

 volume ionization of the air within. 



But with 7 rays the ionization is mainly due to the electrons 

 with much higher velocity expelled by or from the 7 rays, 

 chiefly on meeting the walls of the vessel. These electrons 

 have velocities of the order of those of the ft rays from 

 radium C, 10 10 cm. sec." 1 , with an average path of one or 

 two metres. Moreover, many electrons, before absorption, 

 will be reflected again and again from the sides of the vessel, 

 ionizing the air along their paths. 



The extent of ionization due to multiple reflexion may be 

 gauged from the results of Kovarik*, who experimented 

 with ft rays striking reflectors above and below his testing 

 vessel. 



Ionization Measurements. 





Kovarik. 



/3 rays with 



multiple reflectors. 



Testing vessels 

 and y rays. 



Pb 



100 



70-5 



69-5 



65-0 



51 



42 



100 

 69 

 69 



68 

 58 

 49 



Zn 



Cu 



Fe 



Al 





The agreement between his results for ft rays and my 

 results with 7 rays is sufficiently good to indicate that 

 * Phil. Mag-. November 1910. 



