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A proportional counter is a measurement device to count particles of ionizing radiation and measure their energy.

A proportional counter works on the same principal as the Geiger-Müller counter, but uses a lower operating voltage. An incoming ionizing particle, if it has sufficient energy, liberates electrons from the atomic orbitals of the gas atoms (see ionization potential), leaving an electron and positively charged atom, commonly known as an ion pair. As the charged particle travels through the chamber it leaves a trail of ion pairs along its trajectory. The electrons created in this process drift toward a readout electrode, known as the anode, under the influence of an applied electric field. A proportional counter differs from an ionization chamber in that the operating voltage is sufficiently high that these drifting electrons gain enough energy over a mean free path to create further ion pairs when they collide with other neutral atoms of the gas. The electrons created in these new events also drift toward the readout electrode and can create further ion pairs themselves. In this manner, a cascade of ion pairs can be created, this is known as a Townsend avalanche. If the operating voltage is chosen carefully, each avalanche process occurs independently of other avalanches which derive from the same initial ionizing event. Therefore, even though the total number of electrons liberated can increase exponentially with distance, the total amount of charge created remains proportional to the amount of charge liberated in the original event.

By measuring the total charge (time integral of the electric current) between the electrodes, we can find out the particle's kinetic energy, because the number of ion pairs created by the incident ionizing charged particle is proportional to its energy. So long as the potential difference over which the electrons drift is chosen carefully, this proportionality is preserved.

If the voltage is lowered below a critical value, the electrons do not gain sufficient energy as they drift to create further ion pairs so the detector operates as an ionization chamber. If the voltage is too high, the degree of charge amplification tends to a maximum value, and all pulses from the chamber have the same amplitude, so the detector operates as a Geiger-Müller counter.

This process of charge amplification can improve the signal-to-noise ratio of the detector and also reduce the amount of amplification required from external electronics. The proportionality between the energy of the charged particle travelling through the chamber and the total charge created makes proportional counters useful for charged particle spectroscopy.

They are also useful for detection of high energy photons, such as X-rays or gamma-rays, provided these can penetrate the entrance window.