Radiation damage in the LHCb vertex locator

The LHCb Vertex Locator (VELO) is a silicon strip detector designed to reconstruct charged particle trajectories and vertices produced at the LHCb interaction region. During the first two years of data collection, the 84 VELO sensors have been exposed to a range of fluences up to a maximum value of approximately 45 × 10¹² 1 MeV neutron equivalent (1 MeV n_eq). At the operational sensor temperature of approximately −7 °C, the average rate of sensor current increase is 18 μA per fb⁻¹, in excellent agreement with predictions. The silicon effective bandgap has been determined using current versus temperature scan data after irradiation, with an average value of E_g = 1.16±0.03±0.04 eV obtained. The first observation of n⁺-on-n sensor type inversion at the LHC has been made, occurring at a fluence of around 15 × 10¹² of 1 MeV n_eq. The only n⁺-on-p sensors in use at the LHC have also been studied. With an initial fluence of approximately 3 × 10¹² 1 MeV n_eq, a decrease in the Effective Depletion Voltage (EDV) of around 25 V is observed. Following this initial decrease, the EDV increases at a comparable rate to the type inverted n⁺-on-n type sensors, with rates of (1.43±0.16) × 10⁻¹² V/ 1 MeV n_eq and (1.35±0.25) × 10⁻¹² V/ 1 MeV n_eq measured for n⁺-on-p and n⁺-on-n type sensors, respectively. A reduction in the charge collection efficiency due to an unexpected effect involving the second metal layer readout lines is observed.