Between February 18 and 21, 2019 a team of engineers and technicians from INFN Padova, coordinated by Ing. Adriano Pepato, accessed the core of the ALICE experiment, one of the four large experiments at the CERN LHC particle accelerator. The goal of the intervention was the disassembly of the Silicon Pixel Detector (SPD), which is the innermost among the eighteen detectors that compose the ALICE apparatus. The detector has come to the end of its operational life at the end of 2018, after 10 years of data-taking at the LHC, and enters now its “outreach life”, within an exhibition of scientific tools.
The ALICE experiment is located about 30 m underground, at the border between France and Switzerland, and it is run by an international collaboration of 1800 scientists from 41 countries. Since 2010 ALICE uses collisions of lead nuclei provided by the LHC to study matter under extreme conditions of high temperature and density, “melting” protons and neutrons to form a plasma of quarks and gluons. In this way, physicists study matter in conditions similar to those that characterised the Early Universe a few microseconds after the Big Bang.
The SPD was designed and constructed in the first half of the 2000s with a strong contribution from INFN, in particular from the Padova team. For more than 10 years, the SPD was the first detector crossed by the particles produced in the collisions of LHC proton and lead beams: positioned at only 3.9 cm from the interaction point, the detector measured the position of the particles’ production point with a precision of a few tens of micrometers, thus providing a central contribution to the studies undertaken by the ALICE experiment in these years.
At the end of 2018 the LHC entered a shutdown phase of two years. During the shutdown, scientists and technicians will upgrade the experiments to improve their performance in terms of precision and data-taking speed.
The disassembly of the Silicon Pixel Detector, which is made of two concentric cylinders with diameters of 8 and 15 cm and 28 cm long, instrumented with 9.8 million pixels, was a high precision mechanical engineering operation, which required detailed preparation and a complex team work by the experts from INFN Padova and the other institutes taking part in the project.
SPD will be replaced by new silicon detectors at the frontier of technology, with smaller size and higher spatial precision. The ALICE team in Padova takes part in the design and construction of the new detector, which will be operational in 2021 when lead beams will circulate in the LHC with higher energy and intensity.