Speaker
Description
During the LHC Long Shutdown 3 (2026-2030), the ALICE Inner Tracking System (ITS) upgrade to ITS3 will see the replacement of the innermost 3 layers (also referred to as Inner Barrel, or IB) of the current ITS2 with 6 large-area (26 cm long), flexible, stitched 65 nm CMOS sensors. The ITS3 sensors will be air cooled, with integrated power and signal transmission, and for the first time in a High Energy Physics experiment, they will be bent into a truly half-cylindrical shape, with no more than lightweight carbon fibre foam supports to hold the structure in place. The greatest advantage brought by the new ITS3 detector will be the expected low material budget of 0.09% X$_{0}$ per layer on average, compared with the current 0.36% X$_{0}$ per ITS2 IB layer. This will improve ITS tracking and vertexing capabilities, especially at low momenta.
In the complex ITS3 R&D process, stitched test devices such as the MOnolithic Stiched Sensors (MOSS) and their smaller variants (babyMOSS) were developed to evaluate the wafer yields and assess stitched sensor performances. The babyMOSS is a ~14×30 mm$^{2}$ chip, including 8 digitally read out pixel matrices (regions) arranged in 2 rows, or half-units. This design replicates that of one of the 10 MOSS repeated sensor units, in a more compact and easier to handle version. The babyMOSS performance has been extensively characterised in terms of detection efficiency, spatial resolution and fake-hit rate in charged particle beams at the CERN Proton Sychotron and ELSA facilities.
In this contribution, we will present results for babyMOSS sensors irradiated to various fluences. We will show that, consistent with full MOSS devices, babyMOSS sensors can achieve a detection efficiency higher than 99%, a fake-hit rate lower than 10$^{-6}$ hits per pixel and event, and a spatial resolution finer than 5 μm. We also show that the babyMOSS sensor maintains this performance up to the expected ITS3 radiation level of 4·10$^{12}$ 1 MeV n$_{\mathrm{eq}}$ cm$^{-2}$ NIEL (Non-Ionising Energy Loss) + 4 kGy TID (Total Ionising Dose), in full compliance with the ITS3 sensor requirements.