Ionization chamber
Encyclopedia
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.
s 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 particle
s, beta particle
s, X-ray
s, 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 electron
s 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.
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.
and have a greater operating lifetime than standard Geiger tubes; in Geiger-Müller tube
s the gas eventually breaks down due to incident radiation.
, 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 particle
s. 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 ion
s.
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.
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.
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
. Conventionally, the term "ionization chamber" is used exclusively to describe those detectors which collect ion pairs from gases.
Construction and operation
An ionization chamber is an instrument constructed to measure charge from the number of ionIon
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s 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
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
and cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
). 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 particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
s, beta particle
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
s, X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s, or other radioactive emission, the ions and dissociated electrons move to the electrodes of the opposite polarity
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
, thus creating an ionization current which may be measured by a galvanometer
Galvanometer
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. .Galvanometers were...
or electrometer
Electrometer
An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical hand-made mechanical instruments to high-precision electronic devices...
.
Each ion essentially deposits or removes a small electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
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 electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s 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 desiccantDesiccant
A desiccant is a hygroscopic substance that induces or sustains a state of dryness in its local vicinity in a moderately well-sealed container....
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.
Nuclear industry
Ionization chambers are widely used in the nuclear industry as they provide an output that is proportional to radiation doseDose (biochemistry)
A dose is a quantity of something that may impact an organism biologically; the greater the quantity, the larger the dose. In nutrition, the term is usually applied to how much of a specific nutrient is in a person's diet or in a particular food, meal, or dietary supplement...
and have a greater operating lifetime than standard Geiger tubes; in Geiger-Müller tube
Geiger-Müller tube
A Geiger–Müller tube is the sensing element of a Geiger counter instrument that can detect a single particle of ionizing radiation, and typically produce an audible click for each. It was named for Hans Geiger who invented the device in 1908, and Walther Müller who collaborated with Geiger in...
s 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 detectorSmoke detector
A smoke detector is a device that detects smoke, typically as an indicator of fire. Commercial, industrial, and mass residential devices issue a signal to a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible and/or visual alarm from the detector...
, the gap between the plates is exposed to the open air. The chamber contains a small amount of americium-241
Americium
Americium is a synthetic element that has the symbol Am and atomic number 95. This transuranic element of the actinide series is located in the periodic table below the lanthanide element europium, and thus by analogy was named after another continent, America.Americium was first produced in 1944...
, which is an emitter of alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
s. These alpha particles carry a substantial amount of energy, and when they collide with gas in the ionization chamber (mostly nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
) the momentum transferred ionizes the gas molecules—that is, the uncharged gas molecules will lose one or more electrons and become charged ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s.
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
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
. If smoke
Smoke
Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...
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 physicsMedical physics
Medical physics is the application of physics to medicine. It generally concerns physics as applied to medical imaging and radiotherapy, although a medical physicist may also work in many other areas of healthcare...
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
National Physical Laboratory, UK
The National Physical Laboratory is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington, London, England. It is the largest applied physics organisation in the UK.-Description:...
in the UK, or will have a factor determined by comparison against a standards-calibrated chamber at the user's site.