As voltage is increased in the ionization region (Region II), there is no appreciable
increase in the pulse height. The field strength is more than adequate to ensure collection
of all ions produced; however, it is insufficient to cause any increase in ion pairs due to
gas amplification. This region is called the ionization chamber region.
As voltage increases to the proportional region (Region III), the pulse height increases
smoothly. The voltage is sufficient to produce a large potential gradient near the anode,
and it imparts a very high velocity to the electrons produced through ionization of the gas
by charged radiation particles. The velocity of these electrons is sufficient to cause
ionization of other atoms or molecules in the gas. This multiplication of electrons is
called gas amplification and is referred to as Townsend avalanche. The gas amplification
factor (A) varies from 103 to 104. This region is called the proportional region since the
gas amplification factor (A) is proportional to applied voltage.
Limited Proportional Region
In the limited proportional region (Region IV), as voltage increases, additional processes
occur leading to increased ionization. The strong field causes increased electron velocity,
which results in excited states of higher energies capable of releasing more electrons from
the cathode. These events cause the Townsend avalanche to spread along the anode. The
positive ions remain near where they were originated and reduce the electric field to a
point where further avalanches are impossible. For this reason, Region IV is called the
limited proportional region, and it is not used for detector operation.
The pulse height in the Geiger-Müller region (Region V) is independent of the type of
radiation causing the initial ionizations. The pulse height obtained is on the order of
several volts. The field strength is so great that the discharge, once ignited, continues to
spread until amplification cannot occur, due to a dense positive ion sheath surrounding
the central wire (anode). V4 is termed the threshold voltage. This is where the number
of ion pairs level off and remain relatively independent of the applied voltage. This
leveling off is called the Geiger plateau which extends over a region of 200 to 300 volts.
The threshold is normally about 1000 volts. In the G-M region, the gas amplification
factor (A) depends on the specific ionization of the radiation to be detected.