Abstract
The Inner Tracking System (ITS) of the ALICE experiment will be upgraded during the second long LHC shutdown in 2019–2020. The main goal of the ALICE ITS upgrade is to enable high precision measurements of low-momentum particles (<1 GeV/c) by acquiring a large sample of events, benefiting from the increase of the LHC instantaneous luminosity of Pb-Pb collisions to ℒ = 6 ⋅ 1027 cm−2 s−1 during Run 3. Working in this direction the ITS upgrade project is focusing on the increase of the readout rate, on the improvement of the impact parameter resolution, as well as on the improvement of the tracking efficiency and the position resolution. The major setup modification is the substitution of the current ITS with seven layers of silicon pixel detectors. The ALPIDE chip, a CMOS Monolithic Active Pixel Sensor (MAPS), was developed for this purpose and offers a spatial resolution of 5 μm. The use of MAPS together with a stringent mechanical design allows for the reduction of the material budget down to 0.35% X0 for the innermost layers and 1% X0 for the outer layers. The detector design was validated during the research and development period through a variety of tests ensuring the proper operation for the full lifetime inside ALICE. The production phase is close to completion with all the new assembled components undergoing different tests that aim to characterize the modules and staves and determine their qualification level. This contribution describes the detector design, the measurements performed during the research and development phase, as well as the production status.
Highlights
The ALPIDE chipThe ALPIDE chip is the building block of the ALICE tracker [3]
The use of Monolithic Active Pixel Sensor (MAPS) together with a stringent mechanical design allows for the reduction of the material budget down to 0.35% X0 for the innermost layers and 1% X0 for the outer layers
The pixels are arranged in double columns and read out by a priority encoder which sends the addresses of the pixels that recorded a hit to the chip periphery [5]
Summary
The ALPIDE chip is the building block of the ALICE tracker [3]. The total ionizing dose tolerance requirement is achieved with small thickness gate oxides whereas the 25 μm p-type epitaxial layer with a resistivity >1 kΩ cm and the possibility of applying reversed bias voltage to the substrate increase the tolerance to the non-ionizing energy loss. All the above characteristics improve the charge collection and make the ALPIDE chip a unique choice for the ALICE tracker. The ALPIDE chip has dimensions 15 × 30 mm and hosts 512 × 1024 pixels. The pixels are arranged in double columns and read out by a priority encoder which sends the addresses of the pixels that recorded a hit to the chip periphery [5]. Pixels with no hits are not read out making the readout procedure faster and reducing the power consumption which for every pixel is about 40 nW
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