The barrel modules of the ATLAS semiconductor tracker
Introduction
The ATLAS experiment [1] is being constructed to explore the physics of 14 TeV proton–proton collisions at the CERN Large Hadron Collider (LHC) [2], with first beam expected in 2007. The ATLAS Inner Detector (ID) [3] tracks charged particles coming from the interaction region, and consists of a pixel detector (Pixel), surrounded by the SemiConductor Tracker (SCT), which is itself surrounded by a gaseous/polypropylene foil transition radiation tracker (TRT). The overall ID is 2.3 m in diameter and 7 m in length. For analysing the momenta of charged particles, a 2 T uniform magnetic field is provided by a superconducting central solenoid [4] which is integrated inside the cryostat of a liquid argon electromagnetic calorimeter. A quadrant view of the ID together with the solenoid is shown in Fig. 1. Because of the high energy of the proton–proton collisions, large numbers of particles are generated in one interaction, and multiple interactions are expected in one crossing of the proton bunches. The main requirements for the ID are precision tracking of charged particles in the environment of numerous tracks, capability of bunch-crossing identification, tolerance to large radiation doses, construction with the least possible material, and a capability for electron identification within the ID.
The ID consists of barrel and endcap regions in order to minimize the material traversed by particles coming from the interaction region at its centre. The barrel region is made of co-axial cylindrical layers and the endcap of disk layers. The Pixel and SCT detectors use silicon semiconductor technology for precision measurement. In the barrel region there are three Pixel and four SCT layers, each of which is able to read out a position in two dimensions. This paper describes the SCT detector modules of the barrel region. The SCT endcap modules are described elsewhere [5].
Section snippets
Overview of SCT barrel module requirements
The four SCT cylinders in the barrel region (termed Barrels 3, 4, 5 and 6) have radii between 299 and 514 mm and a full length of 1492 mm. Their surface areas are tiled with segmented detector elements, the SCT barrel modules, to provide complete four-layer digitization coverage for particles coming from a length of ±76 mm about the nominal interaction point on the central axis. This is the expected ±2 sigma length of the beam interaction point. The barrel cylinder parameters and the numbers of
Silicon microstrip sensors
The 10,650 silicon sensors used in the assembly of the SCT barrel modules were all fabricated by Hamamatsu Photonics [9] and delivered over a 3-year period from 2000 to 2003. They were received by three SCT Institutes [21], where their quality was checked on a sampling basis, and from there they were forwarded for use at the appropriate module assembly site. A small sample of sensors (∼0.3%) was extracted during the series production and irradiated at the CERN PS with 24 GeV/c protons to a
Mechanical precision of the modules
A selection of typical in-plane (XY) survey results is shown in Fig. 19, and of out-of-plane (Z) survey results in Fig. 20 for all modules, measured at room temperature either after the assembly or after the long-term electrical test. In each figure, the horizontal range of the distribution is the tolerance specification of the quantity, as given in Table 12, Table 13. Nearly all parameters are well within the specification, and the standard deviations of their measured distributions are less
Initial results from the assembled SCT barrels
The barrel modules are assembled to four carbon-fibre barrel structures within the SCT (Table 1). The mounting and initial testing of all the 2112 modules on Barrels 3, 4, 5 and 6 took place over the period from June 2004 to August 2005 [36]. The process is described in detail elsewhere [37], [38], [39], [40]. The four completed barrels were shipped to CERN, where they have been assembled together, within a sealed thermal enclosure. The SCT four-barrel assembly was then mounted within the
Summary
The R&D, prototyping and construction phases have been successfully completed for the barrel module project of the ATLAS SCT. A total of 2582 modules have been constructed in four different SCT cluster locations during a 2-year period of series production. The overall yield of modules with satisfactory mechanical and electrical performance is 90.5%. The required 2112 modules, to full ATLAS electrical and mechanical specification, have been mounted on the four barrel structures of the SCT.
Acknowledgements
We are greatly indebted to all the technical staff who worked on the barrel module project from the ATLAS SCT Institutes. We acknowledge the support of the funding authorities of the collaborating institutes including the Spanish National Programme for Particle Physics; the Research Council of Norway; the Particle Physics and Astronomy Research Council of the United Kingdom; the Polish Ministry of Education and Science; the German Ministry of Science; the Swiss National Science Foundation; the
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