Recent renewed interest in international treaties for nuclear arms reductions is accompanied by a renewed need to address the technical challenges associated with reliably verifying the participants' adherence to the agreements. A variety of imaging techniques have been proposed to verify the absence or presence of nuclear weapons in items presented for inspection or in situ. To accomplish this, gamma-based techniques are often viewed as too "invasive," and most neutron techniques suffer from significant shortcomings: fast-neutron scatter-based techniques have low efficiency and poor angular resolution, and thermal-neutron techniques image moderator rather than the neutron sources themselves. These shortcomings are remedied by imaging fission spectrum neutrons with the coded-aperture technique. This paper reviews recent results obtained from imaging with a system based on plastic scintillator detectors that are sensitive to neutrons, and it reports on progress in the design and construction of a new imaging system based on pixelated liquid scintillator detectors that can distinguish neutrons and gamma rays. Both highly pixelated detector arrays view sources through a modified uniformly redundant array polyethylene mask, but the elimination of gamma-ray background in the new detector array enables neutron imaging to be performed correspondingly faster.
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