Real-time investigation of temporal and spatial correlations in fast
      neutron assay from spontaneous and stimulated fission
Creators: name: Sarwar, Rashed
Publisher: name: Lancaster University
Year: 2019
Subjects: nuclear,
scintillation
fpga
neutron
gamma
multiplicity
time-of-flight

description: A study of the use of digital techniques for the real-time, fast neutron
      coincidence analysis of time- and space-correlated radiations emitted by
      californium-252 and uranium-235 is described. These radiations have been
      measured with detectors based on the organic liquid scintillant, EJ-309.
      Time-synchronized neutron and gamma-ray event-trains, separated with pulse
      shape discrimination, have been sampled with a field-programmable gate
      array programmed with an algorithm developed in this research. This
      approach has been used to extract the interval time distribution of this
      event-train, with a time resolution of 5 ns, to investigate the temporal
      correlation between the neutrons and/or gamma rays emitted in the
      spontaneous fission of californium-252. The established model for the
      characterization of the interval-time distributions of correlated thermal
      neutron events, used widely in thermal neutron coincidence assay, has been
      extended to fast neutrons. The influence of geometry and the surroundings
      on these distributions has been investigated and quantified: the temporal
      coefficients for the die-away of the distributions for neutrons and gamma
      rays are 3.18\pm0.09 ns and 1.49\pm0.06 ns, respectively. It has been
      observed that 99.7% of the correlated neutrons and gamma rays are detected
      within 27 ns and 21 ns of each other, respectively, when a low-scatter
      geometry is examined. The spatial distribution of fast neutrons emitted in
      spontaneous fission (californium-252) has also been investigated to yield
      the evidence for the angular distribution of higher-order, correlated
      neutrons presented in this thesis; this infers a dipolar trend for third
      (triplet) and fourth (quadruplet) neutrons consistent with that known for
      second (doublet) neutrons. The gamma-ray emission has been used to provide
      time-of-flight information and hence the neutron spectrum for fission
      neutrons from californium-252. A technique for the determination of the
      foreground and background coincidence distribution of the emitted fast
      neutrons and/or gamma rays for passive and active neutron coincidence
      counting methods has been developed. Finally, two models have been
      developed to correct for erroneous coincidence events which might
      otherwise limit the use of organic scintillators in coincident assay: one
      for photon breakthrough and one for detector crosstalk. These models have
      been validated using californium-252 indicating that photon-breakthrough
      constitutes a 20% increase in the neutron count rates whilst crosstalk can
      result in increases of 10% and 35% on first-and second-order coincident
      events, respectively, for the investigated geometries. The
      instrumentation, techniques and results reported in this thesis extend our
      understanding of the fundamental temporal characteristics of nuclear
      fission, and are of direct relevance to the application of organic
      scintillators with pulse shape discrimination to nuclear safeguards and
      non-proliferation verification.