The NA60 silicon vertex spectrometer

The NA60 experiment studies the production of open charm and prompt dimuons in proton-nucleus and nucleus-nucleus collisions at the CERN SPS. To access the kinematics of charged particles already at the vertex level, a radiation tolerant silicon pixel detector has been placed in a 2.5 T magnetic field near the target. This vertex spectrometer was built from 96 ALICE1LHCB pixel chips arranged in 12 tracking planes. The vertex spectrometer was successfully operated in a run with a 158 GeV/nucleon indium ion beam incident on indium targets in October-November 2003. During the five-week-long run it was exposed to high levels of radiation distributed inhomogeneously over the detector. The most exposed regions of the silicon sensors underwent type inversion. With only a fraction of the total statistics analysed, the vertex spectrometer can already be seen to have dramatically enhanced the physics performance of NA60 with respect to that of its predecessors