Ionization-based detectors

Ionization detectors were some of the earliest detectors developed in the study of high energy physics. While they have fallen out of widespread use in favor of more recent technology such as silicon detectors, they remain a simple, easy to maintain, and inexpensive option for tracking charged particles. Furthermore, descendents of the original ionization detectors, like the multi-wire proportional chamber and the time projection chamber, continue to push the boundaries of particle detection and are now being used in dark matter searches and collider detectors. 

The simplest ionization detector looks a bit like a straw. The straw has a wire fed through its center, and a high voltage is applied. That central wire is the anode, and conducting material along the inside of the straw itself is connected to ground and serves as the cathode. The straw is filled with gas, which generally cycles through the straw. The basic principle is that when a charged particle passes through, it ionized some of the gas molecules inside. The applied voltage generates an electric field that causes the yanked-off electrons to accelerate towards the central wire. They hit the wire, and we measure their presence as current.

Naturally, though, the whole business is slightly more complicated than that; there's actually a variety of regions in the behavior of the detector based on the voltage you apply to the central wire.

The various regions in ionization detector behavior. Figure from W.R. Leo, chapter 6.

Each of these regions has something different going on physically.

• Recombination region: The electric field in the detector is low enough that ionized atoms are able to recombine (electrons find ions and re-merge) with some probability. As a result, not all the primary ionization electrons are captured, so as you increase the voltage on the wire, the number of ionization electrons you capture increases fairly dramatically.
• Ionization chamber region: All ionized atoms remain ionized and travel to the anode/cathode. As such, there's a plateau here: increasing or decreasing the voltage applied to the central wire doesn't change how many of the ions are caught.
• Proportionality region: The voltage in this range is high enough that it attracts those primary electrons very strongly - so strongly that they in turn ionize more atoms on their way to the central wire. This causes an avalanche, and in this region at least, the number of captured electrons is proportional to the number that were ionized in the first place, with a constant of proportionality dependent on the voltage.
• Region of limited proportionality: Increase the voltage higher still, and the cascades of electrons develop such high charge densities near the anode that they distort the electric field, which reduces the proportionality of the detector.
• Geiger-Muller counter: Beyond the region of limited proportionality is another plateau in the number of detected electrons. Physically, at this point, the cascades don't stop, and photons emitted by excited atoms ionize more and more atoms. These detectors end up with a self-sustaining chain reaction, so that even the slightest ionization in the gas results in exactly the same current as a massive, highly charged particle passing through. To avoid getting a constant current after just a single hit, these detectors have to have a quenching gas inside, in order to capture and disperse the energy from these emitted photons. 

If you're interested in learning more about ionization detectors (as well as a variety of other interesting detectors and techniques), I highly recommend W.R. Leo's Techniques for Nuclear and Particle Physics Experiments, which gives an excellent explanation of the theory behind many detector types.