This chapter describes another popular micropattern detector, the Gas Electron Multiplier (GEM). The GEM belongs to the family of hole-type detectors made of a dielectric sheet metalized on both sides with a matrix of holes through it. When a voltage is applied between the metalized electrodes, a strong electric field is created inside the holes. The electric field is sufficiently strong for avalanche multiplication of primary electrons produced by radiation in a drift region adjacent to the plate. In contrast to other hole-type detectors, GEM is manufactured by a photolithographic technology from thin metalized Kapton sheets. This detector has several unique features (e.g. the possibility to operate in cascade mode to increase the maximum achievable gain or to combine a GEM with other gaseous detectors, MSGC, MICROMEGAS, etc.). Cascaded GEMs are used today in several experiments at CERN and elsewhere. A modified robust version of the GEM, called a “thick GEM,” can operate at gas gains higher than ordinary GEMs and is used in various designs of photodetectors.
Top1. Early Work On Hole-Type Electron Multipliers
In the previous chapters we described gaseous detectors in which the electron multiplication occurs in a strong electric field formed either between two parallel electrodes, or near small anode wires, strips or dots where the field lines experience a focusing effect.
In this chapter we will introduce a new multiplication structure with a hole between two parallel metallic electrodes. Similar structures had been used earlier, e.g. as electrostatic lenses in TV, vacuum tubes, linear accelerators etc. (see Figure 1).
Probably the first authors who observed multiplication in hole-type structures in a gas atmosphere were Fujieda (1986) and Del Guerra (1987). This team studied various glass capillary tubes with ends coated with a thin conductive layer. A photograph of one of their capillary plates is shown in Figure 2.
Figure 2. Photograph of one of glass capillary plates with metalized planar surfaces between which the HV was applied
When a voltage is applied across the capillary (via the conductive electrodes) the field lines experience a focusing effect exactly as in an electrostatic lens (see Figure 3).
Figure 3. Schematic drawing illustrating the operational principle of a glass capillary plate with conductive layers on the outer surfaces. As in an electrostatic lens, the field lines experience a focusing effect and thus a region of high electric field is created inside the tube (Del. Guerra, 1987).
Fujieda and Guerra managed to drift electrons though capillaries and at high enough voltage they even observed a gas multiplication (Figure 4).
Figure 4. Avalanche multiplication in capillary tubes as a function of electric field strength for different gas mixtures and pressure. a) Ne+He +10%C2H6 (40 Torr), b) Ne+He +10%C2H6 (70 Torr), c) Isobutane (40 Torr), d) Ne+He +4%C2H6 (760 Torr).
In 1991, independently from Del. Guerra, multiplication in a micro-hole structure was observed by A. Oed (1991).
The next step forward was done by Bartol et al,. who developed a simplified hole-type structure made by drilling holes in a dielectric sheet with metalized surfaces, see Figure 5 (Bartol, 1996). They called this detector CAT (recall that this is the French acronym for “Compter a trou”).
Figure 5. Shematic drawing of the CAT and calculated equipotential lines (from Bartol, 1996)