The LHCb VELO: status and upgrade developments

The LHCb vertex locator (VELO) is a silicon based vertexing sub-detector which has active silicon positioned only 8 mm from the LHC beams and will operate in an extreme and non-uniform radiation environment (up to 1.3 × 10¹⁴ 1 MeV neutron equivalents/cm²/year). The complex design of the VELO silicon sensors exploits oxygenated silicon and n⁺-on-n technology. Research has been carried out into new materials which could significantly extend the lifetime of silicon detectors at the LHC, these would have particular application in a VELO upgrade. Promising new results on the first test beam of a large, high resistivity Czochralski silicon detector with 50 μm pitch and 40 MHz electronics will be presented. The performance was studied before and after irradiation with high energy protons. A signal to noise of over 20:1 was obtained from the detector and after a fluence of 4.3 × 10¹⁴ 1 MeV n_eq, significant charge collection efficiencies were measured at relatively modest voltages. Studies using the transient current technique, TCT, probed the electric field within MCz test detectors and proved that MCz silicon does not type invert up until a radiation level of at least 5 × 10¹⁴ 24 GeV/c p/cm². This would mean the VELO could replace, in a potential upgrade. n⁺-on-n DOFZ sensors and the complicated processing involved, for standard p⁺-on-n processing with MCz - if MCz sensors prove to be sufficiently radiation hard.