  Gamma-ray emission from special nuclear material (SNM) is relatively easy to
shield from detection using modest amounts of high-Z material. In contrast,
fast-neutrons are much more penetrating and can escape relatively thick high-Z
shielding without losing significant energy. Furthermore, fast neutrons provide
a clear and unambiguous signature of the presence of SNM with few competing
natural background sources. The challenge of detecting fast neutrons is
twofold. First, the neutron flux from SNM are only a fraction of the
corresponding gamma-ray flux. Second, fast neutrons can be difficult to
differentiate from gamma rays. The ability to discriminate gamma rays from
neutrons combined with neutron imaging can yield large benefit to isolate the
localized SNM neutron source from background. With the recent developments of
pulse-shape-sensitive plastic scintillators that offer excellent
gamma-ray/neutron discrimination, and arrays of silicon photomultipliers
combined with highly scalable and fast positron-emission-tomography (PET)
multi-channel readout systems, field-deployable neutron imagers suitable for
SNM detection might now be within reach. In this paper, we present a
characterization of the performance of a recently available commercial
PET-scanner readout, including its sensitivity to pulse-shape differences
between fast neutrons and gamma rays, energy and timing resolution, as well as
linearity and dynamic range. We find that, while the pulse-shape discrimination
is achievable with stilbene, further improvement of the readout is required to
achieve it with the best available plastic scintillators. The time and energy
resolution appear to be adequate for neutron imaging in some circumstances.
