BCM Detector Performance Plots showing radiation damage to diamond sensors based on leakage current measurements and fluka simulations

The Beam Condition Monitor (BCM) of the CMS detector at the LHC is a protection device similar to the LHC Beam Loss Monitor system. While the electronics used is the same, poly-crystalline Chemical Vapor Deposition (pCVD) diamonds are used instead of ionization chambers as the BCM sensor material. The main purpose of the system is the protection of the silicon Pixel and Strip tracking detectors by inducing a beam dump, if the beam losses are too high in the CMS detector. By comparing the detector current with the instantaneous luminosity, the BCM detector efficiency can be monitored. Over the LHC running period thus far, a reduction in signal strength has been observed. An explanation for this effect will be discussed in this paper, depending on the sensor type and location in CMS. The number of radiation-induced defects in the bulk material, reduces the average drift length (CCD) of the charge, and hence lowers the signal. The number of these induced defects can be simulated using FLUKA monte-carlo. The cross section for creating defects increases with decreasing energies of the impinging particles. This explains, why diamond sensors mounted close to heavy calorimeters experience more radiation damage, owing to the high number of low energy neutrons produced in these regions. The signal decrease was stronger than expected from the number of simulated defects. Here polarization from trapped charge carriers in the defects is a likely candidate for explaining the difference, as suggested by Transient Current Technique (TCT) measurements. With increasing trap density charge carriers get more frequently trapped, forming a corresponding increase in local polarization effects. Mounted at the same location, the single-crystalline (sCVD) diamond sensor shows a faster relative signal decrease than the pCVD sensor. This is expected, since the relative increase in the number of defects is larger in sCVD than in pCVD sensors.