Gaseous ionization detectors are radiation detection instruments used in

particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

to detect the presence of ionizing particles, and in

radiation protection Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Expos ...

applications to measure ionizing radiation. They use the ionising effect of radiation upon a gas-filled sensor. If a particle has enough energy to

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

ize a

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

atom or molecule, the resulting

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

s and

s cause a current flow which can be measured. Gaseous ionisation detectors form an important group of instruments used for radiation detection and measurement. This article gives a quick overview of the principal types, and more detailed information can be found in the articles on each instrument. The accompanying plot shows the variation of ion pair generation with varying applied voltage for constant incident radiation. There are three main practical operating regions, one of which each type utilises.

Types

The three basic types of gaseous ionization detectors are 1)

ionization chamber The ionization chamber is the simplest type of gas-filled radiation detector, and is widely used for the detection and measurement of certain types of ionizing radiation, including X-rays, gamma rays, and beta particles. Conventionally, the term ...

s, 2)

proportional counter The proportional counter is a type of gaseous ionization detector device used to measure particles of ionizing radiation. The key feature is its ability to measure the energy of incident radiation, by producing a detector output pulse that is ''pro ...

s, and 3) Geiger–Müller tubes All of these have the same basic design of two

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...

s separated by air or a special fill gas, but each uses a different method to measure the total number of ion-pairs that are collected. The strength of the electric field between the electrodes and the type and pressure of the fill gas determines the detector's response to ionizing radiation.

Ionization chamber

Ionization chamber The ionization chamber is the simplest type of gas-filled radiation detector, and is widely used for the detection and measurement of certain types of ionizing radiation, including X-rays, gamma rays, and beta particles. Conventionally, the term ...

s operate at a low electric field strength, selected such that no gas multiplication takes place. The ion current is generated by the creation of "ion pairs", consisting of an ion and an electron. The ions drift to the cathode while free electrons drift to the anode under the influence of the electric field. This current is independent of the applied voltage if the device is being operated in the "ion chamber region". Ion chambers are preferred for high radiation dose rates because they have no "dead time"; a phenomenon which affects the accuracy of the Geiger–Müller tube at high dose rates. The advantages are good uniform response to gamma radiation and accurate overall dose reading, capable of measuring very high radiation rates, sustained high radiation levels do not degrade the fill gas. The disadvantages are 1) low output requiring sophisticated electrometer circuit and 2) operation and accuracy easily affected by moisture.

Proportional counter

Proportional counter The proportional counter is a type of gaseous ionization detector device used to measure particles of ionizing radiation. The key feature is its ability to measure the energy of incident radiation, by producing a detector output pulse that is ''pro ...

s operate at a slightly higher voltage, selected such that discrete

avalanches An avalanche is a rapid flow of snow down a slope, such as a hill or mountain. Avalanches can be set off spontaneously, by such factors as increased precipitation or snowpack weakening, or by external means such as humans, animals, and earth ...

are generated. Each ion pair produces a single avalanche so that an output current pulse is generated which is proportional to the energy deposited by the radiation. This is in the "proportional counting" region.Glenn F Knoll, ''Radiation detection and measurement'', John Wiley and son, 2000. The term "gas proportional detector" (GPD) is generally used in radiometric practice, and the property of being able to detect particle energy is particularly useful when using large area flat arrays for alpha and beta particle detection and discrimination, such as in installed personnel monitoring equipment. The

wire chamber A wire chamber or multi-wire proportional chamber is a type of proportional counter that detects charged particles and photons and can give positional information on their trajectory, by tracking the trails of gaseous ionization. was located via ...

is a multi-electrode form of proportional counter used as a research tool. The advantages are the ability to measure energy of radiation and provide spectrographic information, discriminate between alpha and beta particles, and that large area detectors can be constructed The disadvantages are that anode wires are delicate and can lose efficiency in gas flow detectors due to deposition, the efficiency and operation affected by ingress of oxygen into fill gas, and measurement windows easily damaged in large area detectors.

Micropattern gaseous detectors Micropatterning is the art of miniaturisation of patterns. Especially used for electronics, it has recently become a standard in biomaterials engineering and for fundamental research on cellular biology by mean of soft lithography. It generally ...

(MPGDs) are high granularity gaseous detectors with sub-millimeter distances between the anode and cathode electrodes. The main advantages of these microelectronic structures over traditional wire chambers include: count rate capability, time and position resolution, granularity, stability and radiation hardness. Examples of MPGDs are the

microstrip gas chamber Microstrip is a type of electrical transmission line which can be fabricated with any technology where a conductor is separated from a ground plane by a dielectric layer known as the substrate. Microstrip lines are used to convey microwave-frequ ...

, the

gas electron multiplier A gas electron multiplier (GEM) is a type of gaseous ionization detector used in nuclear and particle physics and radiation detection. All gaseous ionization detectors are able to collect the electrons released by ionizing radiation, guiding them ...

and the micromegas detector.

Geiger–Müller tube

Geiger–Müller tubes are the primary components of

Geiger counters A Geiger counter (also known as a Geiger–Müller counter) is an electronic instrument used for detecting and measuring ionizing radiation. It is widely used in applications such as radiation dosimetry, radiological protection, experimental ph ...

. They operate at an even higher voltage, selected such that each ion pair creates an avalanche, but by the emission of UV photons, multiple avalanches are created which spread along the anode wire, and the adjacent gas volume ionizes from as little as a single ion pair event. This is the "Geiger region" of operation. The current pulses produced by the ionising events are passed to processing electronics which can derive a visual display of count rate or radiation dose, and usually in the case of hand-held instruments, an audio device producing clicks. The advantages are that they are a cheap and robust detector with a large variety of sizes and applications, large output signal is produced from tube which requires minimal electronic processing for simple counting, and it can measure the overall gamma dose when using an energy compensated tube. The disadvantages are that it cannot measure the energy of the radiation (no spectrographic information), it will not measure high radiation rates due to dead time, and sustained high radiation levels will degrade fill gas.

Guidance on detector type usage

The UK

Health and Safety Executive The Health and Safety Executive (HSE) is a UK government agency responsible for the encouragement, regulation and enforcement of workplace health, safety and welfare, and for research into occupational risks in Great Britain. It is a non-depar ...

has issued a guidance note on the correct portable instrument for the application concerned. This covers all radiation instrument technologies and is useful in selecting the correct gaseous ionisation detector technology for a measurement application.

Everyday use

Ionization-type

smoke detector A smoke detector is a device that senses smoke, typically as an indicator of fire. Smoke detectors are usually housed in plastic enclosures, typically shaped like a disk about in diameter and thick, but shape and size vary. Smoke can be detecte ...

s are gaseous ionization detectors in widespread use. A small source of radioactive

americium Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium, and thus by analogy was na ...

is placed so that it maintains a current between two plates that effectively form an ionisation chamber. If smoke gets between the plates where ionization is taking place, the ionized gas can be neutralized leading to a reduced current. The decrease in current triggers a fire alarm.

References

{{DEFAULTSORT:Gaseous Ionization Detectors Particle detectors Ionising radiation detectors