IONIZATION OF A GAS BY X-RADIATION 39 



method of determining the intensity of an x-ray beam under practical 

 conditions. 



In order to evaluate the intensity of the x-radiation in terms of the 

 ionization produced in a gas, it is necessary to have some understanding 

 of this process. Thus, when a beam of x-rays traverses matter, three 

 things are observed : a certain fraction of the incident energy is trans- 

 mitted; a second fraction of the energy is absorbed with the resulting 

 emission of photoelectrons; a third fraction is scattered in all directions 

 and then absorbed and additional photoelectrons are emitted. The 

 volume ionized by these photoelectrons is much larger than that pene- 

 trated by the primary rays. The total number of ions which the photo- 

 electrons in turn produce in the gas is far greater than their own number. 

 X-ray absorption by the gas is to a great extent, therefore, an indirect 

 process. It is the absorbed portion that is responsible for ionization. 



Let us first examine an ideal condition: A very large volume of gas, 

 oxygen for example, is contained in a metal vessel, at 760 mm pressure 

 and 0° C. A monochromatic beam of x-radiation of frequency v in 

 passing through the gas will lose part of its energy through absorption. 

 The energy extracted from the beam manifests itself by the appearance 

 of a large number of atoms, ions, and electrons mixed with the gas mole- 

 cules. The electrons are photoelectrons which are emitted by an atom 

 when it absorbs a photon of energy hv. Their kinetic energy of emission 

 is proportional to the frequency of the absorbed photon and is 

 \rrw 2 = hv. Using x-radiation of 1.0- A wavelength, we can calculate 

 the velocity of emission to be of the order of 6.5 X 10 9 cm/sec. When 

 these high-speed photoelectrons traverse the gas they may collide with 

 neighboring molecules and atoms and ionize them, each electron making 

 many collisions. At each collision additional ions are produced, until 

 after frequent collisions the energy is so reduced that the power of ioniz- 

 ation is lost. This is the fundamental energy exchange process involved 

 in ionization of a gas, and it is essentially an indirect process because of 

 the emission of photoelectrons when x-radiation is the instigator of the 

 process. 



This ionization process is even more complicated when hard x-rays 

 are used. Under these circumstances a quantum of energy hv may 

 collide with an electron, resulting in Compton scattering accompanied 

 by the usual recoil electrons. These recoiling electrons may possess 

 enough kinetic energy to ionize a molecule or atom with which they 

 collide before their available ionization energy is dissipated. X-radia- 

 tion excited at 200 kv or more may produce ionization of greater abun- 

 dance by means of these recoil electrons than through the liberated photo- 

 electrons. A third effect, though a practically negligible one, is the 



