A measurement of Lorentz angle and spatial resolution of radiation hard silicon pixel sensors

Silicon pixel sensors developed by the ATLAS collaboration to meet LHC requirements and to withstand hadronic irradiation to fluences of up to 1015 neq/cm2 have been evaluated using a test beam facility at CERN providing a magnetic field. The Lorentz angle was measured and found to alter from 9.0° before irradiation, when the detectors operated at 150 V bias at B=1.48 T, to 3.1° after irradiation and operating at 600 V bias at 1.01 T. In addition to the effect due to magnetic field variation, this change is explained by the variation of the electric field inside the detectors arising from the different bias conditions. The depletion depths of irradiated sensors at various bias voltages were also measured. At 600 V bias 280 μm thick sensors depleted to ≈200 μm after irradiation at the design fluence of 1×1015 1 MeV neq/cm2 and were almost fully depleted at a fluence of 0.5×1015 1 MeV neq/cm2. The spatial resolution was measured for angles of incidence between 0° and 30°. The optimal value was found to be better than 5.3 μm before irradiation and 7.4 μm after irradiation.