Proportional counter:

If the applied voltage is increased beyond V₂, the primary electrons will be subject to a relatively large electrostatic field and will acquire enough kinetic energy to further ionize filler gas atoms, producing secondary ion-pairs. In the region between V₂ and V₃, the total number of ion-pairs reaching the electrodes is proportional to both applied voltage and the energy of radiation.  Therefore, this region is called the proportional counter.  Further this region is not only useful for measuring energy of radiation but also to discriminate between different radioisotopes emitting different radiations.