LHCb: Radiation Damage in the LHCb VELO

LHCb is a dedicated experiment to study new physics in the decays of beauty and charm hadrons at the Large Hadron Collider (LHC) at CERN. The beauty and charm hadrons are identified through their flight distance in the Vertex Locator (VELO), and hence the detector is critical for both the trigger and offline physics analyses. The 88 VELO sensors are all n-on-n type but one, which is made from n-on-p silicon, and is the only n-on-p module silicon sensor operated at the LHC. The sensors have an inner radius of only 7 mm from the LHC beam and an outer radius of 42 mm, consequently the sensors receive a large and non-uniform radiation dose. The LHCb is planned to record an integrated luminosity up to 5 $fb^{-1}$ with collision energies between 7 and 14 TeV before 2018. The leakage current in the sensors has increased significantly following the delivered luminosity, and decreased during shutdown periods due to annealing. The effective depletion voltage of the sensors is measured from the charge collection efficiency and from studying the noise versus voltage behavior. Clear differences in behaviour, as expected, are observed between n-on-n and n-on-p sensors. The n-on-p sensors shows an initial drop in EDV before it starts to increase at a much lower fluence than n-on-n. Type inversion is observed in the n-on-n sensors, the first sensors to undergo type inversion at the LHC. The depletion voltage of the n-on-p sensors is observed to first reduced and is now increasing. Studies of the cluster finding efficiency have revealed a radiation induced charge loss due to the second metal layer on the sensors. The second metal layer on the R sensors reads out the strips to the outer radius of the sensors. Analysis of the distribution show that the drop only when a particle passes through the sensor close to a routing line, and when not shielded by the metal line of a strip.