Abstract
ALICE detector was constructed to study the properties of hot and dense hadronic matter formed in relativistic nuclear collisions. During the second long LHC shutdown in 2019-2020, the collaboration plans to upgrade the current vertex detector, the Inner Tracking System (ITS), in order to increase the reconstruction accuracy of secondary vertices and to lower the threshold of particle transverse momentum measurement. The upgrade strategy of ITS is based on the application of new Monolithic Active Pixel Sensors (MAPS) designed in 0.18 μm CMOS technology. The 50 μm thick chip consists of a single silicon die incorporating a 0.18 μm high-resistivity silicon epitaxial layer (sensor active volume) and matrix of charge collection diodes (pixels) with readout electronics. Radiation hardness of the upgraded ITS is one of the crucial moments in the overall performance of the system. A wide set of MAPS structures with different read-out circuits was produced and is being studied by the ALICE collaboration to optimize the pixel sensor functionality. An overview of the ALICE ITS upgrade and the expected performance improvement will be presented together with selected results from a campaign that includes several irradiation and beam tests.
Highlights
For what concerns the environment in which the new Inner Tracking System (ITS) will operate, recent simulations have shown that the overall dose expected for the innermost layer can reach up to 2.7 Mrad (TID) and 1.7 ×1013 1 MeV neq/cm2 (NIEL), including a safety factor of 10, for the full integrated luminosity
An intrinsic spatial resolution of 5 Ñm for the innermost layer requires a pixel pitch of ∼ 30 Ñm
Based on the STAR Pixel experience [4], the monolithic CMOS Monolithic Active Pixel Sensors (MAPS) technology has been chosen for the pixel detectors
Summary
2. The new Inner Tracking System The goals of the ITS upgrade [3] are to improve the track impact parameter resolution (by at least a factor of 5 in z direction and by a factor of 3 in rφ direction), the tracking efficiency and the transverse momentum resolution at very low pT . In order to achieve these improvements the first layer of silicon detectors will be placed closer to the interaction point (figure 1). The spatial resolution and the tracking performances will gain from a reduction of the material budget, in particular in the inner layers.
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