  This paper presents the model-based design and evaluation of an instrument
that estimates incident neutron direction using the kinematics of neutron
scattering by hydrogen-1 nuclei in an organic scintillator. The instrument
design uses a single, nearly contiguous volume of organic scintillator that is
internally subdivided only as necessary to create optically isolated pillars.
Scintillation light emitted in a given pillar is confined to that pillar by a
combination of total internal reflection and a specular reflector applied to
the four sides of the pillar transverse to its long axis. The scintillation
light is collected at each end of the pillar using a photodetector. In this
optically segmented design, the (x, y) position of scintillation light emission
(where the x and y coordinates are transverse to the long axis of the pillars)
is estimated as the pillar's (x, y) position in the scintillator "block", and
the z-position (the position along the pillar's long axis) is estimated from
the amplitude and relative timing of the signals produced by the photodetectors
at each end of the pillar. For proton recoils greater than 1 MeV, we show that
the (x, y, z)-position of neutron-proton scattering can be estimated with < 1
cm root-mean-squared [RMS] error and the proton recoil energy can be estimated
with < 50 keV RMS error by fitting the photodetectors' response time history to
models of optical photon transport within the scintillator pillars. Finally, we
evaluate several alternative designs of this proposed single-volume scatter
camera made of pillars of plastic scintillator (SVSC-PiPS), studying the effect
of pillar dimensions, scintillator material, and photodetector response vs.
time. Specifically, we conclude that an SVSC-PiPS constructed using EJ-204 and
an MCP-PM will produce the most precise estimates of incident neutron direction
and energy.
