Mon 02 Sep 2013
Antonella Del Rosso
“Where was a given particle born? How can we tag it precisely enough to be able to then follow it along its track and through its decays? This is the job of the pixel detector installed at the heart of the ATLAS detector, only centimeters away from the LHC collisions. In order to improve its identification and tagging capabilities, the ATLAS collaboration has recently taken a big step towards the completion of the upgrade of its Pixel detector, which will include the insertion of a brand-new layer of 12 million pixels.
The 7 metre long beryllium beam pipe inserted in the carbon-fibre positioning tool is being prepared ready for the new innermost layer of the Pixel detector to be mounted. (Photo courtesy ATLAS Collaboration)
With its three layers and 80 million channels concentrated in 2.2 square metres, the ATLAS pixel detector was already the world’s largest pixel-based system used in particle physics. Its excellent performance was instrumental in the discovery of the Higgs boson in July 2012. However, already at the time of its design, the collaboration had decided to add a fourth layer at a later stage. With the LHC running at full steam, the ATLAS collaboration very quickly decided to go for a tighter schedule and have the new detector installed by the restart of the LHC in 2014. “The tight schedule was a challenge for all the teams involved in the project,” says Beniamino Di Girolamo, ATLAS Technical Coordinator. “About 40 members of the ATLAS collaboration from several institutes worked – and are still working – very hard to meet the deadlines but nothing would have been possible without the help of CERN’s vacuum experts.”
Indeed, in order for the beam pipe to be inserted into the experiment’s structure, it had to be made smaller. The cylindrical tube used up to that point was too large and the new Pixel detector component would not fit inside the existing structure. This was a job for the Vacuum group in the Technology Department (TE-VSC). “In order to redesign the beam pipe for the ATLAS collision point we first had to consult several other groups, including collimation, machine protection and impedance experts,” explains Mark Gallilee, project leader for the TE VSC group responsible for beam pipes for the experiments. “The requirements of the ATLAS beam pipe were very specific. Because of the limited space we had to reduce the thickness of the aerogel layer, which thermally insulates the tube. We also had to develop a new type of removable vacuum flange so that the vacuum chamber could be removed from the new smaller space if necessary.”
At the beginning of August an important part of the job was completed with the insertion of the new beam pipe into the Pixel detector support structure. ‘During this operation, we had very little clearance and the different parts had to slide and move within a few millimetres. In some cases, we only had a 2 mm clearance and we had to align all the components with a precision of 10 microns,’ says Didier Ferrère, deputy project leader for the new Pixel detector. In addition to the mechanical issues, the ATLAS experts also had to resolve some critical thermal issues. ‘230 degrees are needed to activate the NEG that ensures the correct level of vacuum in the beam pipe, while our Pixel detector operates at a temperature of around -20 degrees,’ says Didier Ferrère. ‘We have run tests and simulations and are confident that the two environments – the new module and the new beam pipe – are compatible.’
The result is worth the effort. The new ATLAS pixel system will be able to provide physicists with 40 million ‘3D snapshots’ every second, with an accuracy of 92 million pixels. ‘It will be an amazing tool for us,’ says Didier Ferrère.”
The ATLAS Pixel Detector provides a very high granularity, high precision set of measurements as close to the interaction point as possible. The system provides three precision measurements over the full acceptance, and mostly determines the impact parameter resolution and the ability of the Inner Detector to find short lived particles such as B-Hadrons. The system consists of three barrels at average radii of ~ 5 cm, 9 cm, and 12 cm (1456 modules), and three disks on each side, between radii of 9 and 15 cm (288 modules). Each module is 62.4 mm long and 21.4 mm wide, with 46080 pixel elements read out by 16 chips, each serving an array of 18 by 160 pixels. The 80 million pixels cover an area of 1.7 m^2. The readout chips must withstand over 300 kGy of ionising radiation and over 5×10^14 neutrons per cm^2 over ten years of operation. The modules are overlapped on the support structure to give hermetic coverage. The thickness of each layer is expected to be about 2.5% of a radiation length at normal incidence. Typically three pixel layers are crossed by each track. The pixel detector can be installed independently of the other components of the ID. In the starting phase, only two of the three layers planned for will be installed. Copyright CERN 2009 — ATLAS Experiment
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