Simulation and measurement of particle detection with silica fibres as a function of impinging angle

Beam instrumentation in particle accelerators is necessary to ensure the quality of the circulating beams. Some of the transfer lines in the North Experimental Area at CERN transport beams of high intensity (109 1013 ⁄), which are slowly extracted during 4.8 or 9.8 seconds. The profile and position of these beams is measured at present with different types of secondary electron emission monitors that suffer from radiation damage due to the high beam intensities involved. We investigate the feasibility of a new beam profile monitor based on the detection of Cherenkov light in silica optic fibres, which would have the required radiation hardness and would be compatible with ultra-high vacuum. We have studied in the laboratory the detection of light from bare silica fibres coupled to photomultiplier tubes, by exciting them with a 90Sr radioactive source. We have performed measurements of the amount of particles detected as a function of the angle between the radioactive source and the fibre, for different fibre thicknesses, and we have compared the experimental data with GEANT4 simulations