  Compact neutron imagers using double-scatter kinematic reconstruction are
being designed for localization and characterization of special nuclear
material. These neutron imaging systems rely on scintillators with a rapid
prompt temporal response as the detection medium. As n-p elastic scattering is
the primary mechanism for light generation by fast neutron interactions in
organic scintillators, proton light yield data are needed for accurate
assessment of scintillator performance. The proton light yield of a series of
commercial fast plastic organic scintillators---EJ-200, EJ-204, and
EJ-208---was measured via a double time-of-flight technique at the 88-Inch
Cyclotron at Lawrence Berkeley National Laboratory. Using a tunable deuteron
breakup neutron source, target scintillators housed in a dual photomultiplier
tube configuration, and an array of pulse-shape-discriminating observation
scintillators, the fast plastic scintillator light yield was measured over a
broad and continuous energy range down to proton recoil energies of
approximately 50 keV. This work provides key input to event reconstruction
algorithms required for utilization of these materials in emerging neutron
imaging modalities.
