Ionization chamber


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 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 since 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 since they are very sensitive devices and non ion related charges can be set up inside due to the piezoelectric effect.

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) it may ionize an atom within the tube. The rapid acceleration of the resulting electron by the electrostatic potential within the tube may result in a Townsend avalanche, or a cascade of electrons from further ionization events. The electrons and ions created by the avalanche then migrate to the electrodes within the Geiger counter inducing 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. For example, a bubble chamber contains a liquid under pressure just below its boiling point. 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.

With the development of cosmic ray physics and nuclear physics it became desirable to alter the simple Geiger counter, an "all or nothing" proposition, so as to discriminate among incident ionizing particles according to the energy. The less sophisticated detector that resulted is the proportional counter.

moke 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 dosimetry.


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