Detection of neutron-induced events and neutron/γ-ray discrimination with an imaging capability of a P-channel X-ray CCD

We report a direct detection method of fast neutrons and neutron/γ-ray discrimination using a P-channel X-ray CCD. The CCD has a depletion layer of ∼300μm and has a pixel size of 15μmx15μm. With such a thick depletion layer, this device has a capability of being a neutron imager without any external converters. We conducted neutron irradiation experiments with the P-channel X-ray CCD by using a fast-neutron source 252Cf, which also emits γ-rays with an average energy of 0.85 MeV. We found two types of events in CCD images; one has a small point-like shape and the other has an elongated shape with a head-tail structure. Electrons scattered by the MeV γ-rays travel much longer than silicon nuclei kicked by the fast neutrons. Therefore it is likely that the point-like events are produced by the neutrons, while the head-tail events are due to the γ-rays. By analyzing the event shapes quantitatively, we conclude that the P-channel X-ray CCD can detect fast neutrons and the imaging capability of the CCD makes it possible to distinguish neutron-induced events from γ-ray-induced events. The detection efficiency of the neutrons and the rejection factor of the γ-rays with the CCD detector are about 0.4% and 70%, respectively.