16 Integration of Detectors into a Large Experiment: Examples from ATLAS. . .
701
reaching when one goes beyond simple sketches and simulations to the details of
the real experiment:
– successful operation of detectors able to measure the energies of particles with
polar angles as small as one degree with respect to the incoming beams has
required quite some inventiveness in material technology and a lot of detailed
validation work to qualify the so-called forward calorimeters in terms of the
very large radiation doses and particle densities encountered so close to the
beams. Similar issues have been addressed of course very early on for the
trackers, the main concerns being damage to semi-conductors (sensors and
integrated circuits) and ageing of gaseous detectors. Even the muon detectors,
to the initial surprise of the community, were confronted with irradiation and
high-occupancy issues from neutron-induced cavern backgrounds pervading
the whole experimental area;
– avoiding any cracks in the acceptance of the experiment (especially cracks
pointing back to the interaction region) has been a challenge of its own in
terms of minimising the thickness of the LAr cryostats in ATLAS and of
properly routing the large number of cables required to operate the ATLAS
and CMS inner trackers;
– if no particle can escape from the large volumes occupied by the experiments,
then it becomes very hard for human beings to enter for rapid maintenance and
repair. The access and maintenance scenarios for both experiments are quite
complex and any major operation will only be feasible during long shutdowns
of the accelerators. The detector design criteria have therefore become close
to those required for space applications in terms of robustness and reliability
of all the components.
• The high particle fluxes and harsh radiation conditions prevailing in the experimental areas have forced the collaborations to foresee redundancy and robustness
for the measurements considered to be most critical. A few of the most prominent
examples are described below:
– CMS has chosen the highest possible magnetic field (4 T) combined with
an inner tracker consisting solely of Silicon pixel detectors (nearest to the
interaction vertex) and of Silicon microstrip detectors providing very high
granularity at all radii. The occupancy of these detectors is below 2–3% even
at the LHC design luminosity and the impact of pile-up is therefore minimal;
– ATLAS has invested a very large fraction of its resources into three superconducting toroid magnets and a set of very precise muon chambers, constantly monitored with optical alignment devices, to measure the muon
momenta very accurately over the widest possible coverage (|η| < 2.7) and
momentum range (4 GeV to several TeV). This system provides a stand-alone
muon momentum measurement of sufficient quality for all benchmark physics
processes up to the highest luminosities envisaged for the LHC operation;
701
reaching when one goes beyond simple sketches and simulations to the details of
the real experiment:
– successful operation of detectors able to measure the energies of particles with
polar angles as small as one degree with respect to the incoming beams has
required quite some inventiveness in material technology and a lot of detailed
validation work to qualify the so-called forward calorimeters in terms of the
very large radiation doses and particle densities encountered so close to the
beams. Similar issues have been addressed of course very early on for the
trackers, the main concerns being damage to semi-conductors (sensors and
integrated circuits) and ageing of gaseous detectors. Even the muon detectors,
to the initial surprise of the community, were confronted with irradiation and
high-occupancy issues from neutron-induced cavern backgrounds pervading
the whole experimental area;
– avoiding any cracks in the acceptance of the experiment (especially cracks
pointing back to the interaction region) has been a challenge of its own in
terms of minimising the thickness of the LAr cryostats in ATLAS and of
properly routing the large number of cables required to operate the ATLAS
and CMS inner trackers;
– if no particle can escape from the large volumes occupied by the experiments,
then it becomes very hard for human beings to enter for rapid maintenance and
repair. The access and maintenance scenarios for both experiments are quite
complex and any major operation will only be feasible during long shutdowns
of the accelerators. The detector design criteria have therefore become close
to those required for space applications in terms of robustness and reliability
of all the components.
• The high particle fluxes and harsh radiation conditions prevailing in the experimental areas have forced the collaborations to foresee redundancy and robustness
for the measurements considered to be most critical. A few of the most prominent
examples are described below:
– CMS has chosen the highest possible magnetic field (4 T) combined with
an inner tracker consisting solely of Silicon pixel detectors (nearest to the
interaction vertex) and of Silicon microstrip detectors providing very high
granularity at all radii. The occupancy of these detectors is below 2–3% even
at the LHC design luminosity and the impact of pile-up is therefore minimal;
– ATLAS has invested a very large fraction of its resources into three superconducting toroid magnets and a set of very precise muon chambers, constantly monitored with optical alignment devices, to measure the muon
momenta very accurately over the widest possible coverage (|η| < 2.7) and
momentum range (4 GeV to several TeV). This system provides a stand-alone
muon momentum measurement of sufficient quality for all benchmark physics
processes up to the highest luminosities envisaged for the LHC operation;