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.

An ionization chamber is an instrument constructed to measure the number of ions within a medium (which we will consider to be gaseous, but can also be solid or liquid). It consists of a gas filled enclosure between two conducting electrodes. The electrodes may be in the form of parallel plates (parallel plate ionization chambers: PPIC) or coaxial cylinders to form a convenient portable detector in which case, one of the electrodes may be the wall of the vessel itself.

When gas between the electrodes is ionized by any means, such as by alpha particles, beta particles, X-rays, or other radioactive emission, the ions and dissociated electrons move to the electrodes of the opposite polarity, thus creating an ionization current which may be measured by a galvanometer or electrometer.

Each ion essentially deposits or removes a small electric charge to or from an electrode, such that the accumulated charge is proportional to the number of like-charged ions. A voltage potential that can range from a few volts to many kilovolts is applied between the electrodes, and allows the device to work continuously by mopping up electrons and preventing the device from becoming saturated. The current that originates is called a bias current, and prevents the device from reaching a point where no more ions can be collected.

Ionization chambers are widely used in the nuclear industry as they provide an output that is proportional to dose and have a greater operating lifetime than standard Geiger tubes (in Geiger tubes (cf. Geiger-Müller tube) the gas eventually breaks down). Ionization chambers are used in nuclear medicine to determine the exact activity of radioactive therapeutic treatments. Such devices are called 'radioisotope dose calibrators'. Ion chambers are sometimes microphonic as they are very sensitive devices and non ion related charges can be set up inside due to the piezoelectric effect.

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 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 measurement

In medical physics and radiotherapy, ionization chambers are used to ensure that the dose delivered from a therapy unit or radiopharmaceutical is that intended. Ionization chambers are connected to electrometers and typically report a collected charge in nanocoulombs. A factor is then required to convert this reading into a meaningful dose. Often a chamber will have a factor established by a national standards laboratory such as the NPL in the UK, or will have a factor determined by comparison against a standards-calibrated chamber at the user's site.

See also

• Dosimetry