Study of the radiation induced effects in the LHCb Vertex Locator

LHCb is a dedicated heavy-flavour physics experiment at the Large Hadron Collider at CERN. The VErtex LOcator (VELO) is a critical part of a LHCb tracking system, enabling the full topological reconstruction of beauty and charm mesons’ decays and providing essential input for the High Level Trigger (HLT) system used by the experiment to select events. The VELO comprises 42 modules made of two $n^{+}$-on-$n~300~\mu$m thick half-disc silicon sensors with $R$- and ${\mit\Phi}$-measuring micro-strips, arranged in two retractable halves, operating only about 8 mm from the proton beams. In these paper, selected aspects of the VELO performance during the Run 1 data-taking period is shortly summarised along with the radiation damage studies. The track finding efficiency is typically greater than 98\%. An impact parameter resolution of less than $35~\mu$m is achieved for particles with transverse momentum greater than 1 GeV/$c$. An overview of all important performance parameters will be given. The VELO sensors have received a large and non-uniform radiation dose of up to $1.2 \times 10^{14}$ 1 MeV neutron equivalent cm$^{-2}$. Silicon-type inversion has been observed in regions close to the interaction point.