Neutron beam profile measurements with a triple GEM for thermal neutrons at the CERN n_TOF facility

A triple GEM detector for thermal neutrons, with a side-on geometry, was tested at the CERN n_TOF facility as beam profile detector. The n_TOF, neutron time-of-flight facility, provides a neutron beam from thermal energy to 1 GeV with a flight path of 185 m. The triple GEM detector used in this measurement has a cathode specifically conceived for thermal neutron conversion, with a series of thin strips of ¹⁰B deposited on glass supports. Filled with an Ar-CO₂ 70-30% mixture, it was installed in the experimental area, using a step motor to change the position and to image the whole beam area. Furthermore, using the n_TOF trigger it was 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 between thermal and 2 eV. Working at low gain, it was possible to perform measurements with a low γ-background level. The neutron beam spot and the efficiency of the detector were measured.