A near beam silicon microstrip tracking system to test the crystal based collimation

The collimation based on bent crystals represents an opportunity to increase the cleaning efficiency and reduce the beam impedance in high energy accelerators, starting from LHC, whose luminosity is now limited (at the 40% of the nominal value) by the collimation system. The basic idea of the crystal based collimation is to replace the amorphous primary collimator with a silicon bent crystal able to deflect the beam halo outside the beam towards an absorber. The UA9 experiment is testing this scheme on the CERN SPS circulating beam. The 120 GeV/c proton beam is stimulated to create a halo that impinges on the selected crystal; the particles deflected by the crystal are directed on to a tungsten absorber located ~ 70 m after. The experiment combines the standard beam diagnostic system (beam loss monitors and ionisation detectors) with a silicon microstrip tracking system that will be placed in two roman pots located between the crystal and the absorber. The tracking system presented in this paper represents one of the UA9 novelties with respect to the past similar experiment: it will measure the position and angle of the deflected particles allowing one to clearly recognise the crystal behaviour.