Performance of an irradiated p-on-n micro prototype VELO detector

A 200 micron thick p-on-n design LHCb PR-02 prototype VELO detector was irradiated and equipped with LHC-speed electronics. This detector was successfully operated in a test-beam and its performance assessed. After an irradiation dose corresponding to between one and two years of LHCb operation the depletion voltage was found to be 320 V, significantly greater than that predicted. The signal-to-noise, at full charge collection efficiency, was found to be 15. The maximum irradiation received was 10^15 protons/cm^2: even after this very high irradiation, charge was still collected albeit with reduced cluster finding efficiency. Particular risks associated with a p-on-n fine pitch detector with a double metal layer are identified. It was shown that at underdepletion the fine pitch p-on-n detector rapidly loses efficiency due to both charge spread and charge loss to the double metal layer.