The LHCb VELO Upgrade

The VErtex LOcator (VELO) subdetector of the LHCb experiment will undergo a major transformation during the next shutdown to a hybrid pixel detector capable of full event readout at 40 MHz. This forms part of the general strategy to upgrade the entire experiment to run at higher luminosity and enhanced trigger efficiency after 2020, so it can continue to make crucial measurements in the flavour sector and beyond. The VELO surrounds the interaction region, and its role is to reconstruct and trigger on the primary and secondary vertices of the events. The new detector is designed to cope with the increased occupancies and radiation levels expected for Run 3 and 4. The upgraded VELO is composed of 52 modules placed along the beam axis, divided into 2 retractable halves. Each module is equipped with 4 silicon pixel tiles, each read out by 3 VeloPix ASICs. The pixels have a square pitch of 55 microns and the sensors are produced in 0.2 mm thick n-in-p type silicon. The sensors must withstand an integrated fluence of up to 8x10151 MeV neq/cm2, a roughly equivalent dose of 400 MRad. The highest occupancy ASICs will have pixel-hit rates of 800 Mhits/s, with a total rate of 1.6 Tbit/s for the whole detector. The VELO upgrade tiles are mounted onto a cooling substrate made of thin silicon plates with embedded micro-channels that allow the flow of liquid CO2. The secondary vacuum in which the modules are located is separated from the beam vacuum by a thin custom-made foil. This foil is manufactured through a novel milling process and possibly thinned further by chemical etching. The upgraded VELO is currently under construction.