VELO: the LHCb vertex detector

The Vertex Locator (VELO) of the LHCb experiment is designed to measure precisely production and decay vertices of B-mesons. To allow the most accurate measurements the silicon micro-strip detectors are placed within 7 mm of the LHC beams; the mechanics and integration with the LHC machine required for this are described. The close proximity to the beams leads to the operation of silicon microstrip sensors in a harsh and non-uniform radiation environment. Irradiated prototype n-on-n and p-on-n silicon microstrip detectors have been evaluated in a test-beam while being read out at 40 MHz. Measurements of the efficiency of these prototypes are presented. The particular dangers of p-on-n detectors are outlined and the n-on-n technology choice for LHCb is motivated. Finally, the use of the VELO in the second-level of the LHCb trigger is discussed.