A proportional counter is a type of Gaseous ionization detector - it works on the same principle 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. At the same time, the positive ions drift towards the cathode, at much lower speed; in practical devices, the drift times are measured in microseconds and milliseconds, respectively.
A proportional counter differs from an ionization chamber in that the operating voltage is sufficiently high that the 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.
The geometry of the electrodes and the voltages on them are chosen such that in most of the volume of the counter the electric field strength is not enough to produce a Townsend avalanche. The electrons just drift until they get close to the anode, where a strong field allows avalanche multiplication to occur. In this way each electron is multiplied by approximately the same factor (up to about a million) independent of the distance it has covered in the low-field 'drift region'. If the field strength everywhere is below a critical value, Townsend avalanches do not occur at all, and the detector operates as an ionization chamber. If the voltage (and therefore the field strength) 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. The energy resolution of a proportional counter, however, is limited because both the initial ionization event and the subsequent 'multiplication' event are subject to statistical fluctuations.
- U.S. Patent 3,092,747, S. Fine, "Proportional counter"
- U.S. Patent 2,499,830, E. W. Molloy, "Air proportional counter"
- G.Charpak and F.Sauli (1984). "High-resolution Electronic Particle Detectors". Annual review of Nuclear Science. Annual Reviews Inc. 34: 285–350.
- E. Mathieson, Induced charge distributions in proportional detectors, http://www.inst.bnl.gov/programs/gasnobledet/publications/Mathieson's_Book.pdf