  Organic scintillators doped with capture agents provide a detectable signal
for neutrons over a broad energy range. This work characterizes the fast and
slow neutron response of EJ-254, an organic plastic scintillator with 5%
natural boron loading by weight. For fast neutrons, the primary mechanism for
light generation in organic scintillators is n-p elastic scattering. To study
the fast neutron response, the proton light yield of EJ-254 was measured at the
88-Inch Cyclotron at Lawrence Berkeley National Laboratory. Using a
broad-spectrum neutron source and a double time-of-flight technique, the EJ-254
proton light yield was obtained over the energy range of approximately 270 keV
to 4.5 MeV and determined to be in agreement with other plastic scintillators
comprised of the same polymer base. To isolate the slow neutron response, an
AmBe source with polyethylene moderator was made incident on the EJ-254
scintillator surrounded by an array of EJ-309 observation detectors. Events in
the EJ-254 target coincident with the signature 477.6 keV $\gamma$ ray
(resulting from deexcitation of the residual $^{7}$Li nucleus following boron
neutron capture) were identified. Pulse shape discrimination was used to
evaluate the temporal differences in the response of EJ-254 scintillation
signals arising from $\gamma$-ray and fast/slow neutron interactions. Clear
separation between $\gamma$-ray and fast neutrons signals was not achieved and
the neutron capture feature was observed to overlap both the $\gamma$-ray and
fast neutron bands. Taking into account the electron light nonproportionality,
the neutron-capture light yield in EJ-254 was determined to be 89.4$\pm$1.1
keVee.
