Performance and perspectives of the diamond based Beam Condition Monitor for beam loss monitoring at CMS.

At CMS, a beam loss monitoring system is operated to protect the silicon detectors from high particle rates, arising from intense beam loss events. As detectors, poly-crystalline CVD diamond sensors are placed around the beam pipe at several locations inside CMS. In case of extremely high detector currents, the LHC beams are automatically extracted from the LHC rings. Diamond is the detector material of choice due to its radiation hardness. Predictions of the detector lifetime were made based on FLUKA monte-carlo simulations and irradiation test results from the RD42 collaboration, which attested no significant radiation damage over several years. During the LHC operational Run1 (2010-2013), the detector efficiencies were monitored. A signal decrease of about 50 times stronger than expectations was observed in the in-situ radiation environment. Electric field deformations due to charge carriers, trapped in radiation induced lattice defects, are responsible for this signal decrease. This so-called polarization effect is rate dependent and results in a non-linearity of the detector response. Measurements using the transient current technique reveal the electric field distribution. Online measurements and laboratory analysis of polarization effects in diamond sensors are presented. Electric field calculations and estimations of TCT pulse shapes show good agreement with measurements.