Spatial Resolution Studies for the LHCb VELO Upgrade

The Vertex Locator (VELO) detector is a silicon strip detector that surrounds the proton-proton interaction region of the LHCb detector, reconstructing primary and sec- ondary vertices. The detector will be upgraded during Long Shut Down 2 of the LHC as part of a full LHCb detector upgrade program that aims to allow the experiment to operate at higher luminosities. The VELO will replace the current silicon strip sensors with silicon hybrid pixel sensors which consist of a planar silicon sensor bump-bonded to a readout ASIC. Multiple silicon sensor prototypes have been characterised and tested in a laboratory and at the SPS testbeam facility using the TimePix3 Telescope. The TimePix3 telescope was found to have a telescope resolution of 1.69 ± 0.16 μm in x and 1.55 ± 0.16 μm in y. The focus of this thesis is the study of the spatial resolution performance of the sensors before and after exposure to a maximum radiation fluence of 8 × 10^{15}~1MeVn_{eq}cm^{−2}. The TimePix3 ASIC, which has analogue readout, was used for the majority of the prototype testing. However, the VeloPix ASIC designed for the up- grade has a binary readout. Therefore analogue (Centre of Gravity) and binary position reconstruction algorithms were performed on TimePix3 data and the spatial resolutions compared. Binary resolution was found to be better than analogue resolution indepen- dent of the applied bias voltage, both before and after irradiation. This was because of a non-linear charge sharing feature in the silicon sensor that degraded the precision of the analogue position reconstruction and hence the analogue spatial resolution. The Centre of Gravity algorithm assumes linear charge sharing between pixels and therefore requires a non-linear correction to correct for the sensor effects. Without a correction, the binary resolution was generally better over a range of different track angles, where the analogue resolution only became better than binary after 10◦ for non-irradiated sensors and 18◦ for irradiated. One method of correcting for the non-linear feature is introduced but was determined not to be practical. Further work, beyond that presented in this thesis, is required to correct for the non-linear effects. The sensor prototype that best met the requirements of the upgrade was a 200 μm thick n-on-p sensor with an implant size of 39 μm and a guard ring size of 450 μm, produced by Hamanatsu.