Speaker
Markus Ball
(Technische Universitaet Muenchen (DE))
Description
The ALICE (A Large Ion Collider Experiment at CERN) collaboration plans
an upgrade of the detector during the second long shutdown of the LHC,
where the interaction rate will be increased to 50 kHz for Pb-Pb collisions.
This demands an operation in an ungated continuous mode of the Time
Projection Chamber (TPC). Therefore, a gating grid can not be used to
block the ion back-flow (IBF) to the drift volume. Gas Electron Multipliers
(GEM) offer an intrinsic suppression of ions although not on the same level
as a gating grid. In order to keep the distortions due to space charge on a
manageable level, an IBF of 5 to 10 back drifting ions per incoming elec-
tron is required. To measure the IBF several gas detectors with a GEM
amplication system have been built and set up. In order to arrive at the
target IBF, different TPC gases such as Ar/CO2 (90/10), Ne/CO2 (90/10)
and Ne/CO2/N2 (90/10/5) are compared. In our studies a high impact of
the incident X-ray rate as well as the direction on the IBF has been ob-
served suggesting a large influence of space charge on the IBF. To clarify
this, systematic measurements over a wide range of different charge densi-
ties has been measured. In parallel detailed simulations using ANSYS and
Garfield have been performed to complement the measurements and find
the optimal set of parameters. Different hole geometries leading to different
optical transparencies and their impact on the ion collection as well as the
usage of a fourth GEM has been simulated to further reduce the ion back-
flow. A first prototype of an ALICE Inner Read-Out Chamber (IROC) was
equipped with three large GEM foils as an amplication stage to demon-
strate that the requirements both in detector performance as well as the
stability of operation can be reached. The dE/dx resolution of the proto-
type was measured at the CERN PS and compared to the resolution of the
MWPC IROC. Stability under LHC conditions has been evaluated during
the ALICE p-Pb beamtime with the prototype mounted close to the inter-
action point. Preliminary results as well as the further R&D program will
be presented.
