Performance test of a triple GEM detector at CERN n_TOF facility

The application of a triple GEM (Gas Electron Multiplier) for neutron detection was tested at the n_TOF facility at CERN. n_TOF allows the neutron energy distribution to be measured via a ~185 m time of flight path. A 20 GeV/c proton beam hits a lead target generating a neutron spectrum ranging from thermal energies to the GeV region. Due to their long flight path and short proton pulse, the neutron arrival times at the experimental area define their energy [1]. A triple GEM detector with a 60 11m Polyethylene (PE) neutron converter and 40 11m of Aluminium, filled with an Ar-CO₂ 70-30% mixture, was installed a few meters downstream of the experimental area, just in front of the beam dump. The measurements were purely "parasitic"; they were conducted in parallel and without interfering with the official n_TOF scientific program. Using the n_TOF trigger it is possible to synchronize the GEM data acquisition in order to select a given neutron energy window and measure the detector efficiency as a function of neutron energy. Changing the detector gain, it is possible to perform these measurements with a low γ-background level. Thanks to the pixelated read-out the neutron beam spot and the efficiency of the detector have been measured.