The CpFM, an in-vacuum Cherenkov beam monitor for UA9 at SPS

The use of bent crystals for beam manipulation in particle accelerators is a well-assessedconcept rapidly evolving into practical application. The experiments of the UA9 collaboration atthe CERN-SPS have played a key role for a quantitative understanding of channeling and volumereflection mechanisms.Investigation of the channeling process close to a circulating beam ideally requires in vacuumdetectors resolving the single particle, which should be located inside the vacuum pipe itself.Cherenkov radiators are potential candidates for such functionality due the reduced electro-magneticinteraction of the radiator material with the circulating charges and their compatibility with vacuumrequirements in the beam pipe.For this purpose, we developed a device called Cherenkov detector for proton Flux Measurement(CpFM) that aims at counting the number of deflected protons of the beam halo surrounding thecirculating beam with an accuracy of 15%.After a detailed description of the detection chain and its simulation, we present results ofbeam tests of different configurations, the calibration of the final device and the results of its firstoperation in the SPS.Keywords:Beam-line instrumentation (beam position and profile monitors; beam-intensity mon-itors; bunch length monitors); Cherenkov detectors; Vacuum-based detectors; Photon detectors forUV, visible and IR photons (vacuum) (photomultipliers, HPDs, others)