Development of the scintillating fibre tracker technology for the LHCb upgrade and the LHC beam profile monitoring system

The LHCb detector located at the large hadron collider (LHC) at CERN will undergo a major upgrade in 2019-2020. The detector will be prepared to operate at an increased instantaneous luminosity and to read out full events at 40MHz. This requires many changes in the detector hardware as well as the full software implementation of the event selection. A large tracker has been developed and will be installed for particle tracking downstream of the magnet. It is based on 2.5m-long scintillating fibres (SciFi) with 250um diameter read out by silicon photomultiplier (SiPM) multichannel arrays. A significant R&D effort has been devoted to tailor the technology to the large dimension and the radiation environment. I have developed the instrumentation for the characterisation of SiPMs with a focus on the measurement of the radiation-induced damage. We achieve a precision of 3% on the measurement of the gain and photon detection efficiency (PDE) of SiPMs irradiated to 610¹¹ Neq/cm², equivalent to the expected accumulated radiation in the LHCb SciFi tracker. The SiPM photodetector selected for the SciFi tracker features very low correlated noise (enabling a wide operation range) and a high PDE. The radiation damage is characterised by a dramatic increase in the dark count rate (typically 35MHz/mm² at 3.5V and -40°C for a device irradiated to 610¹¹ Neq/cm²) which is counteracted by cooling (-40°C), short signal integration time length (40MHz read-out) and an efficient noise suppression algorithm (clustering). No significant change in gain, correlated noise and PDE due to irradiation has been observed. The expected light yield given by the fibres at the end of lifetime together with the estimated SiPM noise are in agreement with the required hit detection efficiency. The beam-gas vertex (BGV) detector is an instrument dedicated to beam profile measurements at the LHC. A demonstrator based on the SciFi technology, installed in 2015, provides promising results and validates this detector concept for the high-luminosity LHC project.