34 Mr Crowther, On the Relation between Ionization and 



On the Relation between Ionization and Pressure for Rontgen 

 Rays in different Gases. By J. A. Crowther, B.A., St John's 

 College. [Communicated by Sir J. J. Thomson,] 



[Read 23 November 1908.] 



1. In the absence of any secondary radiation, the amount of 

 ionization per cubic centimetre produced in a given gas by the 

 passage of Rontgen rays of given type and intensity should be 

 proportional to the mass of the gas present per cubic cm. ; that is, 

 if the temperature is constant, to the pressure of the gas. Thus 

 in the absence of any secondary radiation, the ionization-pressure 

 curve should be a straight line passing through the origin. 



If, however, the action of the rays on the gas generates 

 secondary rays, this will no longer be the case. 



It was shewn in a previous paper* that in the cases of air and 

 carbon dioxide, the energy of the penetrating secondary radiation 

 is simply proportional to the pressure of the gas, and this result 

 has subsequently been confirmed for the more powerful radiators. 



Since this type of radiation is sufficiently penetrating to pass 

 through the whole of the gas between the electrodes in the 

 apparatus employed, the ionization produced by it will be pro- 

 portional to the intensity of the secondary radiation and to the 

 pressure of the gas ; that is, to the square of the pressure of the 

 gas. The ionization-pressure curve should thus have the form 



I^Ap^-Bp", 



where / is the ionization and p the pressure. For most gases 

 at the pressures employed in the present experiments the 

 energy of this secondary radiation is too small to make any 

 appreciable alteration in the shape of the curve. Taking Barkla's 

 value ('00024 times the primary) for the energy of secondary 

 radiation from a cubic cm, of air as being approximately correct, 

 it can be shewn that in the case of the most efficient radiator 

 used, namely ethyl bromide, the energy of secondary radiation at 

 a pressure of 160 mm, of mercury is only about 3 °l^ of that of the 

 primary beam; or since the secondary radiation is in this case 

 about three times as absorbable as the primary, the secondary 

 ionization should be about 9 °/^ of that produced by the primary 

 rays at the pressure named. Since ethyl bromide gives off more 

 than five times as much secondary radiation as methyl iodide, and 

 two hundred times as much as air, it is evident that the ionization 

 produced by the penetrating secondary rays in these gases is 

 negligible. 



* Crowther, Phil. Mag. xiv. Nov. 1907, p. 653. 



