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How does increasing voltage change an ionization chamber into a proportional counter?

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detector output connected to a measuring circuit — radiation measurement Figure 1: (A) A simple equivalent circuit for the development of a voltage pulse at the output of a detector. R represents the resistance and C the capacitance of the circuit; V(t) is the time (t)-dependent voltage produced. (B) A representative current pulse due to the interaction of a single quantum in the detector. The total charge Q is obtained by integrating the area of the current, i(t), over the collection time, t_c. (C) The resulting voltage pulse that is developed across the circuit of (A) for the case of a long circuit time constant. The amplitude (V_max) of the pulse is equal to the charge Q divided by the capacitance C.

An ionization chamber functions as a proportional counter when the applied voltage is increased to a level that creates an intense electric field near the axial wire.1

In a standard ionization chamber, the electric field is only strong enough to collect the ion pairs created by incident radiation. By increasing the voltage, the field strength becomes sufficient to accelerate electrons to energies high enough to cause further ionization through collisions with gas molecules. This process, known as gas multiplication, triggers a Townsend avalanche, where a single free electron produces a shower of many additional electrons.213

Because each original electron creates its own individual avalanche, the total charge collected remains proportional to the initial ionization produced by the radiation. This multiplication increases the output pulse size by factors of hundreds or thousands, allowing the device to differentiate between particles of various energies and types, which is the defining characteristic of a proportional counter.31