Gaseous and Liquid Ionisation Detectors

Abstract

Ionisation is one of the most common processes used today for radiation detection and measurement. Ionisation of gases and liquids forms the basis for a large number of devices that have been developed over a span of several decades. This Chapter presents the physics principles of ionisation, the properties of transport of electrons and ions in gases, and how these properties are exploited in common devices from the simplest, the Geiger counter, to the Time Projection Chamber.

Glossary

Charge division Operating mode of a proportional chamber, e.g. in a single wire device, that allows the measure of the longitudinal coordinate via charge collection at both ends of the anode

Dead time Time interval over which the detector is not capable of forming a response to an event

Drift chamber Device based on the proportional chamber principle plus the use of a time to distance relationship to locate the distance of ionisation from the anode

Drift velocity Average velocity of electrons and ions after ionisation under the effect of an external electric field

Ionisation chamber Ionisation-based detector operating with unit-gain

Ionisation detector An instrument, based on the ionisation of the gas or the liquid forming the sensitive volume, capable of measuring the presence and possibly some of the properties of incident radiation

Mean free path of ionisation Average length between subsequent ionisation processes:

$$\begin{aligned} \lambda _I = \frac{1}{n_e\sigma _I}, \end{aligned}$$

where $n_e$ is the density of electrons and $\sigma _I$ is the ionisation cross section

MSGC Multi Strip Gas Chamber, a device based on the proportional chamber principle and constructed using anode strips rather than wires to reduce the pitch of the anodes sequence

MWPC Multi Wire Proportional Chamber, a device based on the proportional chamber principle and constructed placing a sequence of anode wires to achieve particle trajectory tracking

Primary ionisation Electrons freed by the direct interaction of an incident radiation particle with an atom or molecule

Proportional chamber Ionisation-based detector operating with high gain, typically with multiplicative factors of order 10$^6$–10$^7$

Recombination Process that leads to the attachment of an electron and an ion to form a neutral atom

Resolution Ratio between the full width at half maximum of the response to mono-energetic (E) incident radiation and the value of E:

$$\begin{aligned} \mathrm{Resolution} = \frac{\Delta E}{E}, \end{aligned}$$

Response Relationship between the amount of released ionisation charge and the incident energy

Response distribution Distribution of the response when the detector is subject to monochromatic (i.e. single energy) radiation

Response time Time interval that the detector requires to form the response signal

RPC Resistive Plate Chamber, a device based on the proportional chamber principle and constructed with two planes of a mixture of phenolic resins kept at high potential voltage difference The signal is induced on pick-up copper strips on the outside of the resistive plates

Secondary ionisation Electrons freed as a result of atomic de-excitation or ionisation of nearby atoms by electrons from primary ionisation