development of micro-pattern gaseous detectors – gem - LMU

3.3. The Gas System 31
copper-cladded sides of one foil which rises higher than a few nA results in a shutdown of the high
voltage supply. With this procedure, an essential tool is implemented to avoid damage on the foils,
due to (continuous) discharges. At the latest 30 seconds after a trip of the HV supply is recognized
by the control code, an entry with time stamp and name of the tripped foil(s) is added to the output
file, followed by the gradually re-increasing of the voltages to the operation value. The potentials on
the GEM foils are not identical in the exemplary configuration of Fig. 3.6, but slightly increase from
first to last GEM, since it was experienced that discharge rates can be minimized this way (compare
Ch. 2.3 and [Bach 02]).
3.3 The Gas System
3.3.1 Benefits of Noble Gases - The Gas Choice
In general, there are no exclusions on the choice of gas or gas mixture for the operation of any
gaseous detector. Since the initiation of gas amplification of a corresponding gas depends on the
kinetic energy of the incident particle, the high voltage limit of the suppling device is the only
constraint, theoretically.
However, a main point in the considerations for a detector are low working voltages, as they directly
affect the design and material constraints. Furthermore, one should think of a short dead time or the
capability of high particle rates, as it is also a motivation for this triple GEM detector.
Looking for gas amplification at low electric field, noble gases are the best candidates, due to their
shell configuration. For the desired detection of MIPs in the drift region of the detector, one needs
a specific minimal amount of ionized gas particle, that increases with the atomic number of the
corresponding gas. Since the noble gases Krypton and Xenon are among the rarest elements on earth
and therefore extremely expensive, Argon is the filling gas, chosen for the GEM detector, as for many
other gaseous detectors.
Due to primary ionization in the drift region and gas amplification inside the GEMs, the gas volume is
filled with ions and excited atoms, that are not collected by the readout structure. Ions are guided on
the field lines to neutralize at the GEMs surfaces or the cathode, maybe followed by the emission of a
photon. The gas atoms descend to their ground state by either emitting an electron from an inner shell
(Auger effect, see Ch.1.1.2) or by emission of another photon. Taking into account that the minimum
energy of these photons is EAr γ = 11.6 eV and therefore higher than the ionizing potential of copper
(ECu ion = 7.7 eV), they may hit metal-clad surfaces in the detector and release new electrons which may
undergo amplification processes as well. To prevent these “wrong” electrons from being detected and
added to the real signal, one applies so-called quencher gases to the detector volume. They consist
of polyatomic molecules and are inserted to absorb the trouble causing photons before reaching any
surface and displace their energy in rotation and vibration modes [Saul 77]. In our case CO2 was used
which is a neither flammable nor a polymerizing molecular gas.
We chose to operate the triple GEM detector with a mixture of Ar/CO2 at a ratio of 93/7.
The properties of this mixture are well understood and studied for long time in our group, as it
is the gas mixture, chosen for the Monitored Drift Tube detectors at ATLAS. Intense studies on
these gas filled tubes were done at the Cosmic Ray Measurement Facility in Munich, see [Bieb 03]
and [Engl 09] for detailed discussion.