development of micro-pattern gaseous detectors – gem - LMU

24 Chapter 2 The GEM Principle
mobility of ions is about hundred to thousand times smaller than for electrons, the former stay along
the field lines and are collected by the top electrode of the foil. In an optimal process they will reach
neither the drift nor the induction gap. In reality there may be a fraction of ions which escape into
the gap regions resulting in an induced current at the cathode. This defines the ion feedback to be the
ratio of induced charges at the drift electrode owing to ions and the desired induced signal caused
by electrons at the anode [Simo 01]. In this way space charge effects are reduced to a minimum.
On account of this ion suppression that results in an achievable excellent spatial resolution, GEMs
are used in a wide range of implementations from particle physics over astrophysics to medical
applications [Saul 03].
2.3 Multiple GEMs
Multiple GEM detectors are already used in several experimental setups. For instance, a lot of
research on GEM detectors was done for the COMPASS 2 experiment at CERN since they represent
an important fraction of the installed tracking devices. This detector investigates hadron structure and
hadron spectroscopy by running in high intensity muon and hadron beams [Ketz 04] and [Ketz 03].
Discharges that may occur inside a GEM foil or even propagating all the way from the GEM electrode
to the readout structure are a serious concern. Initiated by heavily ionizing particle, these local
currents cause a voltage breakdown that may result in destruction of detector material or readout
devices.
In order to attain detectable amounts of charges for the readout, one can multiply the effect of the
GEM by using several GEM foils in a stack. In such an assembly, the uppermost GEM acts as an
amplifying stage for the subsequent foils, increasing the overall gas gain to the power of installed
GEMs. For instance, in view of a triple GEM configuration and a given gain G ≈ 20 per foil, the
total primary charge is amplified by a factor of 20 3 = 8000 [Murt 02], depending on the applied
voltages and used gas mixture.
The performance of GEM detectors in high irradiation environment and the corresponding discharge
probability is studied intensely by Bachmann et al., for instance in [Bach 01a] and [Bach 01b].
Measurements with single and multiple GEM foils in a detector under alpha particle irradiation
indicate that for a given gain the discharge probability reduces the most for a setup of three GEM
foils [Bach 02]. Fig. 2.5 shows how the effective gain of a particular GEM detector increases by
approximately an order of magnitude per added foil without affecting the discharge probability
significantly. By considering a desired gain of G = 4000 the double GEM structure reports a
discharge probability of 0.09 × 10 −2 which corresponds to an already serious rate. Taking triple
GEM detector decreases the discharge probability under the measurable limit of 6 × 10 −6 .
These results are based on data taken for detectors filled with Ar/CO2 at a ratio of 70/30 and with
241 Am as an irradiation source that provided collimated α-particles at a frequency of approximately
100 Hz. The electric fields applied in a multiple GEM detector can be decreased in comparison
to single GEM detectors by still reaching higher and safe gain. Additionally it was found that the
total gain can be further increased by cascading the voltage across the foils with the uppermost foil
at the highest voltage difference (see Ch. 3.2). Although the gas volume of the detector has to be
2 COmmon Muon and Proton Apparatus for Structure and Spectroscopy