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is confirmed below 4 MeV. α particles of energy more than 4 MeV are so energetic that they can penetrate a CCD detection volume thickness of 20 µ m. 

When the neutron rate is 100 events/frame, more than one event is ob- served inside one signal region. 

4. Neutron beam test 

The performance of neutron detectors with a $^{10}$B converter and a $^{6}$Li converter are tested with neutrons from reactors. 

4.1. Neutron detection efficiency 

The neutron detection efficiency is measured using UCNs supplied at the PF2 beam line of Institut Laue–Langevin in France. Fig. 3 shows the energy 

