The LHCb Vertex Locator (VELO) - 2 half disks

42 modules like this one surround the collision point inside the LHCb detector. Their role is to measure the tracks of short-lived particles spraying out from the collision and to pinpoint the exact spots where they decay into secondary particles. Some exist for just trillionths of a second before decaying! The silicon modules operate so close to the collision point, they can only be moved into position once the circling particle beams are at their most focused. Otherwise, peripheral particles on the outside of the finer-than-a-hair beam would bore a hole right through them.