Gaseous ionization detector

thumb|300px|Plot of variation of ion pair current against applied voltage for a wire cylinder gaseous radiation detector.

Gaseous ionization detectors are radiation detection instruments used in particle physics to detect the presence of ionizing particles, and in radiation protection applications to measure ionizing radiation.

They use the ionising effect of radiation upon a gas-filled sensor. If a particle has enough energy to ionize a gas atom or molecule, the resulting electrons and ions 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

thumb|right|Families of ionising radiation detectors

The three basic types of gaseous ionization detectors are 1) ionization chambers, 2) proportional counters, and 3) Geiger–Müller tubes

All of these have the same basic design of two electrodes 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

[[File:Ion chamber operation.gif|thumb|right|300px|Schematic diagram of ion chamber, showing drift of ions. Electrons typically drift 1000 times faster than positive ions due to their much smaller mass.

Proportional counter

thumb|300 px|The generation of discrete Townsend avalanches in a proportional counter.

Proportional counters operate at a slightly higher voltage, selected such that discrete avalanches 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.

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 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 (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, the gas electron multiplier and the micromegas detector.

Geiger–Müller tube

thumb|300px|Visualisation of the spread of Townsend avalanches by means of UV photons

Geiger–Müller tubes are the primary components of Geiger counters. 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. 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 detectors are gaseous ionization detectors in widespread use. A small source of radioactive americium 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.