IONIZATION OF A GAS BY X-RADIATION 41 



It has been found that several factors such as distance between the 

 plates, their shape, their size, and their enclosure influence the potential 

 at which the saturation current value is attained. For a specific design 

 of apparatus, however, the potential at which saturation takes place 

 depends only on the degree of ionization, which in turn depends on the 

 intensity of the x-radiation. 



As a practical guide for determining the distance between the electrode 

 plates one must possess some information about the distance high-speed 

 photoelectrons may travel in a gas. 



Kulenkampff 's [1926] experiments have shown that the energy of 

 ionization is nearly independent of the wavelength of the absorbed 

 x-radiation in the region 0.56 to 2.0 A and is equal to 35 volts per ion 

 pair. The range of the electron in its passage through the gas before 

 its ionization ability has been exhausted is shown in Table 1-8. These 

 data also show that, when the energy of a photon of wavelength 0.12 A 

 (100-kv) is absorbed, the emitted electron has a velocity of 16.45 X 10 9 

 cm/sec and ceases to ionize after having attained a range of 21 cm. 



These data were taken into consideration in designing the " standard 

 parallel-plate ionization chamber " when the international unit of x-ray 

 quantity, the roentgen (r), was established. 



A source of error encountered in ionization measurements is attribut- 

 able to the x-radiation striking the walls of the chamber. The radiation 

 falling on the metal produces an emission both of high-speed photo- 

 electrons and of x-radiations characteristic of the metal. These effects, 

 if not eliminated, are measured as part of the ionization recorded by 

 the current-measuring instrument. 



In order that the ionization current recorded shall be a true measure of 

 the intensity of the x-radiation, three conditions must be fulfilled by an 

 ionization measurement : 



1. No radiation must strike the walls of the ionization chamber. 



2. Sufficient volume of a gas must be interposed into the path of 



