Radiation damage on the LHCb-VELO

The LHCb Vertex Locator (VELO) is designed to reconstruct charged particle trajectories and vertices produced at the LHCb interaction region. This is a silicon strip detector system operating at the closest distance from the interaction region among all the currently installed detectors at the CERN LHC. The geometry of the sensors implies that every detector region is exposed to a different dose of radiation depending mainly on the radial distance from the beam line. In the first 2 years of operation they have been exposed to a range of fluences up to a maximum value of about 4.5×1013 1 MeV neutron equivalent (neq) cm−2. At the end of 2012 a maximum of about 1.5×1014 neq has been reached; data analysis is ongoing. Sensor currents, full depletion voltage and charge collection efficiencies have been studied at various fluences. Observation of type inversion of the n-type silicon sensors has been made (about after 10-15×1012 neq). The n-type bulk sensors have been compared with the only p-type bulk detectors installed in the LHC. A reduction in the charge collection efficiency due to charge losses to a second metal layer (used for routing the signal to the readout electronics in LHCb-VELO sensors) has been measured. Possible solutions to this undesirable effect are planned to be studied in the framework of the LHCb-VELO upgrade.