Studies for the LHCb SciFi tracker

Abstract

The LHCb detector will undergo a major upgrade during the Long Shutdown 2 of the LHC, which will take place from 2019 to 2020. As part of the upgrade, the main tracker will be replaced by a SciFi Tracker, based on scintillating fibres read out by SiPMs. Over 100000km of fibres are needed to build the SciFi Tracker, which will extend over an area of 360m2. It will provide a spatial resolution of better than 85 μm and a detection efficiency of 98:7%. This thesis presents a series of studies on scintillating fibres for the LHCb detector. A set of tests to ensure the quality of the fibres are described. Both the radiation resistance and the optical and geometrical properties are systematically monitored. The reported results refer to 6700 km of fibres, corresponding to 61% of the ordered quantity. The fibres performances are stable in time and within the specifications required for the SciFi Tracker. The average attenuation length amounts to 351 cm, the light yield to 13:8 photoelectrons and the degradation of the attenuation length after exposure to a dose of 1 kGy from 40 keV X-rays to Λ'=Λ 0 = 0:72. The fibres have a nominal diameter of 250 μm. Occasionally occurring irregularities in the fibres shape are refined with a thermal procedure, conceived in order to reduce the diameter to a maximum of 350 μm, without harming the fibre. The success rate is close to 100% for bumps smaller than 500 μm. In case of failure of the shrinking procedure, which occurs on average once every 12:5km of analysed fibres, the bump is manually cut away and the fibre glued with UV curing glue. The radiation hardness of the fibres, which is one of the major hurdle faced by the SciFi Tracker, is a major topic of this manuscript. The dose profile in the Tracker rapidly changes over the length of each fibre: after 10 years of operations it will range from 40 Gy at one fibre's extremity to a maximum of 35 kGy at the other extremity. Several irradiation tests were performed on single fibres. The samples were irradiated up to different doses and the results used as input to simulate the ageing of the Tracker. A full SciFi module was irradiated with the same dose profile as in LHCb, confirming the simulated result. At the end of the Tracker's lifetime a signal loss of 40% is expected in the most irradiated region. Finally, an ongoing R&D program will be presented, which aims at the development of a new class of scintillating fibres through the exploitation of the NOL mechanism. The prototype fibres do not yet excel in light yield nor in attenuation length. However, with their decay times constants of τ = 1:2 - 1:4 ns, they outperform the commercially available traditional fibres, the fastest of which provides τ = 2:4 ns.