Ionization chamber

The ionization chamber is the simplest of all gas-filled radiation detectors, and is used for the detection or measurement of ionizing radiation. Conventionally, the term "ionization chamber" is used exclusively to describe those detectors which collect ion pairs from gases.

Construction and operation

Ion chamber, showing drift of ions. Incident radiation is the dotted line.

An ionization chamber is an instrument constructed to measure charge from the number of ions within a medium (which we will consider to be gaseous, but can also be solid or liquid). It usually consists of a gas filled enclosure between two conducting electrodes (the anode and cathode). 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 some cases 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 have a wide range from a few volts to many kilovolts, depending on the application, can be applied between the electrodes. The applied voltage 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.

Practical Considerations

Because of the very low currents generated (in the order of nanoamperes), the stray leakage current between anode and cathode must be kept to a minimum. Hygroscopic moisture effects on the surface of chamber connection insulators can be sufficient to cause leakage currents which will swamp any radiation-induced ion current. For higher voltage ion chambers this requires scrupulous cleaning and avoidance of moisture at the chamber connections and at both ends of any cabling. Often in industrial applications the chamber is housed in an outer chamber which contains a desiccant to remove airborne moisture. Guard rings are often used to reduce leakage through or along the surface of connection insulators. Ion chambers are sometimes microphonic due to their high impedance, and non-ion related charges can be set up inside due to the piezoelectric effect. To overcome this in practice, where the chamber is a long distance from the measuring electronics, a local converter module is often used to translate the very low ion chamber currents to a pulse train having a frequency related to the incident radiation.

Applications

Nuclear industry

Ionization chambers are widely used in the nuclear industry as they provide an output that is proportional to radiation dose and have a greater operating lifetime than standard Geiger tubes; in Geiger-Müller tubes the gas eventually breaks down due to incident radiation.

Smoke detectors

The ionization chamber has found wide and beneficial use in smoke detectors. In a smoke detector, the gap between the plates is exposed to the 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 ionizes 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 because ions strike smoke particles and are neutralized. This drop in current triggers the alarm.

Medical radiation measurement

In medical physics and radiotherapy, ionization chambers are used to ensure that the dose delivered from a therapy unit or radiopharmaceutical is what is intended. Such devices are called "radioisotope dose calibrators". Ionization chambers are connected to electrometers, and they typically report a collected charge in nanocoulombs. A correction 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
• Sievert chamber