Gas Multiplication in Proportional Counters

When the electric field strength inside a gas counter exceeds a certain value, the electrons that move in such a field acquire, between collisions, sufficient energy to produce new ions. Thus, more electrons will be liberated, which in turn will produce more ions. The net effect of this process is multiplication of the primary ionization. The phenomenon is called gas multiplication. To achieve the high field intensity needed for gas multiplication without excessive applied voltage, chambers operating in this mode are usually cylindrical with a very thin wire stretched axially at the center of the counter (Fig. 5.14). The wall of the counter is normally grounded and a positive voltage is applied to the central wire. In 

The field intensity increases rapidly as the wire is approached. Since the radius a of the wire is a few mills of an inch and thousands of times smaller than the radius b of the counter, an extremely strong electric field is produced in a fraction of the chamber s volume. This volume is so small that the probability that the incident radiation will produce an electron ion pair in it is negligible. 

In addition to the secondary electrons produced by collisions, electrons are also produced by two other processes  

The photoelectric interactions are caused by photons that are produced in the counter as a result of the ionization and excitation of the atoms and molecules of the gas. If the chamber is filled with a monatomic gas, these photons produce photoelectrons only when they strike the cathode (wall of cylinder) because they do not have enough energy to ionize the atoms of the gas. If the counter is filled with a gas mixture, however, photons emitted by molecules of one gas may ionize molecules of another. 

Electrons are also emitted when the positive ions, which are produced in the chamber, reach the end of their journey and strike the cathode. The significance of this effect depends on the type of material covering the surface of the cathode and, more important, on the type of the gas filling the chamber. 

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