development of micro-pattern gaseous detectors – gem - LMU

22 Chapter 2 The GEM Principle
double-conical shape (see Fig. 2.1). This shape is a compromise due to research results, as a nonrealizable
cylindrical shape should be preferred [Benl 98] to keep the walls from charging-up. Due
to irregularities in the material, local defects or even total removal of the kapton layer may occur
after an etching process for cylindrical holes [Altu 02]. The double-conical geometry was introduced
to improve the dielectric rigidity and also to prevent shortcuts between the upper and lower copper
cladded side of the foil under operation at e.g. ∆Ugem ≈ 350 V.
A typical GEM foil, or shortly GEM, as it is designed by the CERN workshop, consists of a 50 µm
kapton layer covered on both sides by 5 µm thick copper films. Since the holes are double-conically
shaped, one can define two diameters, see Fig. 2.1. The inner diameter in kapton is about 50 µm and
the outer one in copper about 70 µm, with a center-to-center distance of 140 µm [Saul 97] and [Bij
05].
The ratio of material liberated area to total area of the foil is defined by the optical transparency
[Simo 01]
τ =
perforated area
unit cell
= π d
2
2
√ 3
2 · P2 = 2 √ 3 · π
2 d
P
where d is the inner diameter and P the pitch of the holes as denoted in Fig. 2.1. The GEMs used
here provide a τ ≈ 0.46. Fig. 2.2 presents a microscopic view of such a foil.
Figure 2.2: Microscopic pictures of the GEM foil. On the left the cross-section of a hole [Altu 02]. Size of
holes and pitch are 70 µm and 140 µm, respectively [Saul 03].
2.2 The GEM Foil Principle
Applying high voltage to the GEM’s copper electrodes generates an electric field inside the holes
as high as 55 kV /cm, for a given voltage difference of ∆UGEM = 350V . Placed in an appropriate
gas-filled environment, this field is able to provide energies for the electrons that are high enough for
gas amplification. The simplest way to build a particle detector including a GEM foil is by inserting
the foil in a parallel plate electric field, generated by a drift electrode and a charge collection anode
(cf. Fig. 2.3).
Electron-ion pairs, created by a traversing ionizing particle, are separated in the electric field of the
upper drift gap. As shown in Fig. 2.3, the electrons drift towards the holes, where they undergo gas
amplification, due to the high field in the GEM. The multiplied charge is guided by the field lines
into the so-called induction region to the anode structure, from where it can be readout.
(2.1)