Investigation of a Compact Setup for SNM Mass Assay by Active Interrogation and Fast Neutron Detection

The present paper reports on the experimental investigations of an active neutron interrogation method for quantifying fissile material in sealed containers.  The instrumentation includes a pulsed neutron source in a tungsten/graphite assembly for inducing fission in fissile constituents of the sample with subsequent detection of prompt fast fission neutrons in 4He gas proportional counters utilizing the elastic scattering process.  The experimental investigations of this setup are carried out at the Pulsed Neutron Interrogation Test Assembly (PUNITA) of JRC.   In contrast to the standard differential die-away (DDA) instrument, this setup can be made quite compact.  The reason is that the neutron detectors are placed in the vicinity of the sample rather than on the outside of a substantial graphite assembly.  The close proximity of the detectors to the sample compensates to some extend for the low detection efficiency of the 4He fast neutron detectors compared to e.g. 3He thermal detectors.  The present experiments make use of a 14-MeV neutron generator pulsed at 100 Hz.  The 4He detector modules are sensitive to the fast source neutrons, which however quickly lose energy to below 200-300 keV.  At this point the interrogating source neutrons become invisible to the detectors which remain sensitive only to the fast fission neutrons.  By time gating, the data acquisition system remains active in the period when low-energy source neutrons induce fission and only the fast prompt fission are detected.  This is the mechanism applied to separate the interrogating neutrons from the sample response.  An additional advantage is that the 4He detectors remain insensitive to the gamma rays during the data acquisition period.  The present experimental setup includes 21 4He gas detectors of 550 mm length and 50.8 mm diameter. The paper describes the experimental instrumentation and assay procedure.  The results obtained from assay of lowenriched U standards are presented, and the assay sensitivity discussed.  The paper also discusses the safeguards assay pros and cons of using a neutron source with an energy component above cadmium cut-off, and detection of fast rather than thermal neutrons as in the standard DDA technique.