Particle physicists in the UK have completed the first critical element of the largest semiconductor tracker ever built, the ATLAS SCT, which is to form the innermost core of the giant detector that will operate at CERN’s Large Hadron Collider (LHC). The SCT will track charged particles to an accuracy of better than 20 μm over its diameter of 1 m. In all, more than 200 physicists and engineers from 12 countries have taken 10 years to bring the project to this stage.
The ATLAS SCT consists of 60 m2 of silicon detectors in the form of four concentric barrels and two end-caps. Each of the four barrels is tiled with hundreds of silicon detector modules measuring about 6 x 12 cm, 710 of which were built by a collaboration involving the Rutherford Appleton Laboratory (RAL), the Universities of Birmingham and Cambridge, UK, and Queen Mary College, London. The remaining modules have been provided by similar collaborations in Japan, the US and Scandinavia. All four barrels of the SCT are being assembled in the UK using components from a total of 37 institutes worldwide.
The first completed barrel has now been populated with all of its 384 silicon modules. Each module contains four silicon wafers mounted on a thermal pyrolytic graphite baseboard with a flexible circuit hybrid strip containing 12 application-specific integrated circuits. The three outer barrels will carry progressively more modules, with the fourth and final barrel containing 672 silicon modules. Overall, the SCT is divided into 6 million channels and every channel has its own amplifier and data buffer.
The fabrication of the ATLAS barrel modules has been a technically challenging project. The very stringent mechanical, thermal and electrical performance demanded by the ATLAS physics programme is already tough, but on top of that the modules will have to survive in a very-high-radiation environment at -10 °C for 10 years, providing continuous operation without failure. The team at RAL has been working in purpose-built cleanrooms, with separate areas for the construction of the mechanical components, for the state-of-the-art bonding facility where the components are integrated, and for electrical test and characterization. The completed modules are then sent from RAL to collaborating UK universities for burn-in testing, and finally to Oxford for mounting onto the barrel.
The UK team has also been working on the barrel structure. These lightweight structures were built in Germany and came to RAL via the University of Geneva. At RAL, the electrical services, optical read-out fibres, custom bi-phase cooling circuits and alignment systems are added to each barrel. The barrels are then sent to Oxford in custom refrigerated transporters where the modules are mounted onto the barrels with remarkable accuracy.
