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How does gas multiplication help a proportional counter distinguish particle energies?

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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.

A proportional counter distinguishes particle energies by utilizing gas multiplication to amplify the signal generated by incident radiation while maintaining a direct relationship between the output pulse and the initial energy deposition.12

When a particle enters the detector, it creates a specific number of initial ion pairs. In a proportional counter, the electric-field near the anode is intense enough to accelerate these primary electrons, causing them to trigger secondary ionization and form a Townsend avalanche.1

The detector is designed so that each original electron creates its own individual avalanche of a consistent size. Because these avalanches are uniform, the total charge collected at the anode remains directly proportional to the number of original ion pairs formed along the particle's track. Since the number of initial ion pairs is proportional to the energy lost by the incident radiation in the gas, the final output pulse size serves as a reliable measure of that energy.12