Ionization chamber

An ionization chamber is a device used for two major purposes: detecting particles in air (as in a smoke detector), and for detection or measurement of ionizing radiation.

In any ionization chamber, two conducting metal plates (or two electrodes of some shape) are separated by a gap. There is a voltage difference between the two.

Smoke detectors

In a smoke detector, the gap between the plates is exposed to open air. The chamber contains a small amount of americium-241, which is an emitter of alpha particles. These alpha particles carry a substantial amount of energy, and when they collide with gas in the ionization chamber (mostly nitrogen and oxygen) the momentum transferred can ionize the gas molecules—that is, the uncharged gas molecules will lose one or more electrons and become charged ions.

Since the plates are at different voltages (in a typical smoke detector, the voltage difference is a few hundred volts) the ions and electrons will be attracted to the plates. This small flow of ions between the plates represents a measurable electric current. If smoke enters the detector, it disrupts this current. Ions strike smoke particles and are neutralized. This drop in current triggers the alarm.

Radiation detectors

A Geiger-Müller tube—used in a Geiger counter—is another type of ionization chamber. In such a tube, one plate is wrapped into a cylinder. In place of the other plate is a wire placed along the cylindrical plate's axis. This type of tube is usually sealed and filled with an inert gas. In this device, no current normally flows between the two electrodes even though they are held at different potentials.

If a particle of ionizing radiation enters the tube (an alpha particle, a beta particle, or a gamma ray) then a trail of ions will be created. These ions will briefly allow a conducting path to form between the electrodes, triggering a brief pulse of current. If this output is connected to headphones, one hears the familiar staccato pops of a Geiger counter.

Many different types of radiation counters and detectors are based on Geiger-Müller tube-like devices. Some contain different fill gases; some are filled with liquids; some are open to air. Different measurements are possible depending on the type of window in the device (a glass window will not pass alpha particles, while a mica window will) or the potential difference between the electrodes.