The new Fast Beam Condition Monitor using poly-crystalline diamond sensors for luminosity measurement at CMS

The Fast Beam Condition Monitor (BCM1F) is used in CMS to measure beam induced background and precision luminosity. The system in its current implementation was first installed in 2014, equipped with single-crystalline (sCVD) diamond sensors. After some radiation damage, erratic occurrence of HV instabilities required lowering of the operational HV and the system became inefficient, resulting in unstable measurements. The system was replaced in 2017 with poly-crystalline (pCVD) diamond sensors and silicon (Si) sensors. Both new sensor types show significantly better operational stability. The luminosity performance of pCVD and Si based luminosity measurement is discussed.