This chapter reviews the principle of operation of gaseous detectors and their main designs. Historically, avalanche gaseous detectors were the first devices allowing one to detect a few, and even single, primary electrons created by ionization of a gas volume. In all existing designs of gaseous detectors, these primary electrons trigger an avalanche of secondary electrons, ions, and photons in the detector region powered by a sufficiently strong electric field. In this way, a low number of initial electrons produce a large electrical charge, which is collected on an anode. In this chapter, the various processes involved in the operation of the gaseous detectors are described step by step. Initially, it describes the interactions of radiation (charged particles and high-energy photons) with gases leading to the creation of primary electrons, as well as electron diffusion and the avalanche multiplication process. Later, the design of classical gaseous detectors developed before the invention of micropattern detectors are reviewed. These include single-wire counters, parallel-plate chambers (including spark and streamer detectors), multi-wire proportional chambers, and resistive plate chambers. Finally, the physics behind the main breakdown mechanisms in gaseous detectors in general are discussed. The material presented in this chapter give the background necessary for a better understanding of the following chapters.
Top1. Introduction: Electron Multipliers
Electron multipliers play an important role among gaseous detectors as the detector itself amplifies the signal. Actually, it can be made an ideal noise free amplifier which is capable of amplifying a single electron up to 10 million times. In the absence of radiation it produces no output signal at all.
Most of these electron multipliers have a general functional structure illustrated schematically in Figure 1. With rare exceptions they usually consist of the following five subsequent regions: 1) a converter of the incident radiation into primary electrons and ions, 2) a drift region where the primary electrons drift towards a multiplication region, 3) a primary electron multiplication structure where the primary electrons create many secondary electrons (in some designs the convertor is adjacent to the multiplication structure), 4) a transfer region (in some designs), where secondary electrons drift towards the readout electrodes and 5) a collection electrode structure connected to the readout electronics.
Figure 1. Operation principle of a gaseous detector
The basic operational principle of gaseous detectors is as follows. The incident radiation (elementary particle or photon) creates primary electron-ion pairs (in some media, for example semiconductors/photoconverters, electron-hole pairs) in the convertor. The primary electrons, under the influence of an electric field applied between the detector electrodes, drift towards the electron multiplication structure, where each primary electron produces a certain number, A, of secondary electrons. Depending on the detector type, the multiplication factor A ranges typically between 102 and 107. These secondary electrons are then collected on a system of electrodes where they produce fast electronic signals. By measuring these signals one can electronically detect the radiation, obtain position information about the radiation and, if necessary, visualize it (transfer it to a visible image via electronic processing) and measure its characteristics, for example its energy and time of arrival.
The type of detector; “vacuum,” “gaseous,” “solid” or “liquid,” indicates in what media the electron multiplication occurs. For example, in the case of gaseous detectors the electron multiplication occurs in gas media. There are also hybrid detectors combining in one design features of two or more these main detector types.
Top2. Creation Of Primary Electrons Inside Gaseous Detectors By Charged Particles
The interaction of charged particles passing through matter is described in many books on detectors for high energy physics as well as in many articles. For example, an excellent description of the can be found in (Sauli, 1977). For this reason in this chapter we will only present main facts necessary for understanding the operational principle of gaseous detectors.
When a high energy particle with energy Ep>>Ei, where Ei is the ionization potential of the gas, enters the gaseous detector volume it creates along its track a number of delta electrons produced by electrons kicked out from the gas atoms and molecules. These low energy delta electrons in turn produce short and very dense ionization tracks as they slow down in the gas and liberate new electron-ion pairs (see Figure 2). The result is dense clusters of electron-ion pairs along the track of the primary radiation.
Figure 2. Schematic illustration of the trajectory of a high energy particle passing through a gas volume. Dense clusters of electrons and ions are produced along the track by delta electrons kicked out from atoms and molecules of the gas.