Neutron beam monitoring for time-of-flight facilities with gaseous detectors

Triple Gas Electron Multipliers (GEM) for slow and fast neutrons were employed at the n_TOF facility at CERN as online beam imaging monitors and for energy spectra measurements via the time-of-flight technique. The detectors were exposed to the neutron spectrum ranging from thermal to 1 GeV, produced by spallation of 20 GeV/c protons in a lead target with a maximum intensity of 7·1012 protons per pulse. The spectrum and the 2D count distribution of the neutron beam were measured and compared at two distances from the target, 185 m and 200 m. The detectors showed radiation hardness, linear response and the ability to monitor the beam profile online with high spatial resolution.