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Fast neutron coincidences from induced fission as a method for detection of SNM

A method for the detection of special nuclear materials (SNM) in shielded containers which is both sensitive and easily applicable under field conditions is presented. The method applies neutron induced fission in SNM by means of an external pulsed neutron source with subsequent detection of the fast prompt fission neutrons. Liquid scintillation detectors surrounding the container under investigation are able to discriminate gamma rays from fast neutrons by the so-called pulse shape discrimination technique (PSD). One advantage of these detectors, besides the ability to do PSD analysis, is that the analogue signal from a detection event is of very short duration (typically few tens of nanoseconds). This allows the use of very short coincidence gates for multiple detectors for the detection of prompt neutrons from the same fission event while benefiting from a low accidental (background) coincidence rate. These features result in a relatively low detection limit of the fissile mass. Another principal advantage of this method derives from the fact that the external neutron source is pulsed. By proper time gating the interrogation can be done by either epi-thermal or thermal neutrons. These source neutrons do not appear in the neutron signal following the PSD analysis thus providing a fundamental method for separating the interrogating source neutrons from the response in form of fast fission neutrons. The paper describes laboratory tests with multiple detectors at the Pulsed Neutron Interrogation Test Assembly (PUNITA) for the purpose of investigating the measurement principle. Results of thermal and epi-thermal neutron interrogations are shown and discussed.