1916] 



on The Genesis and Absorption of X-Rays 



'31 



the type of ionization remains the same this process is independent 

 of the energy in the cathode rays which produced the ionization. So> 

 that as long as a cathode ray has energy enough to ionize an atom 

 in the target, it will produce Rontgen rays of the same type as those 

 due to cathode rays with greater energy ; it is only when the energy 

 increases to such an extent that the cathode ray is able to produce a 

 new type of ionization — to separate, for example, an electron from a 

 ring of electrons nearer the centre of the atom, and not merely from 

 the outside ring — that we should expect to find marked changes in the 

 quality of the Rontgen radiation. When, however, the energy in the 

 cathode ray reaches a critical value of this kind there W'ill be a rapid 

 change in the type of the Rontgen radiation. We should expect that 

 these critical values would depend upon the material of which the 

 target is made, and a reference to Tables I. and II. indicates that while 

 the most marked change with a silver target takes place between 

 3000 and 3500 volts, with an aluminium target the change take& 

 place at a smaller voltage, between 2000 and 2500. The type- 

 of radiation given out on this view is a " characteristic " radiation 

 of the material of which the target is made. The voltages at 

 wliich rapid changes in the absorption of the rays occur are those 

 which are necessary to excite the various types of "characteristic- 

 radiation." 



I have made a considerable number of experiments on the 

 character of the Rontgen rays emitted by different substances when 

 bombarded by cathode rays of different velocity, measuring the 

 absorption of the rays produced by different numbers of layers of 

 aluminium foil. The absorption produced by the first layer is always- 

 very much greater than that produced by a second layer behind it,, 

 showing that the main part of the radiation is so soft as to be 

 completely absorbed by one layer of the foil ; when this very soft 

 radiation is eliminated constant values of the absorption are obtained 

 for considerable variation in the velocities of the cathode rays. It is 

 the absorption by one layer of aluminium foil after the soft rays 

 have been abstracted which is given in the following table ; the rays 

 were generated by cathode rays striking against celluloid, lead and 

 aluminium targets respectively. 



