  Organic liquid scintillators are used in a wide variety of applications in
experimental nuclear and particle physics. Boron-loaded scintillators are
particularly useful for detecting neutron captures, due to the high thermal
neutron capture cross section of $^{10}$B. These scintillators are commonly
used in neutron detectors, including the DarkSide-50 neutron veto, where the
neutron may produce a signal when it scatters off protons in the scintillator
or when it captures on $^{10}$B. Reconstructing the energy of these recoils is
complicated by scintillation quenching. Understanding how nuclear recoils are
quenched in these scintillators is an important and difficult problem. In this
article, we present a set of measurements of neutron-induced proton recoils in
a boron-loaded organic liquid scintillator at recoil energies ranging from
57--467 keV, and we compare these measurements to predictions from different
quenching models. We find that a modified Birks' model whose denominator is
quadratic in $dE/dx$ best describes the measurements, with $\chi^2$/NDF$=1.6$.
This result will help model nuclear recoil scintillation in similar detectors
and can be used to improve their neutron tagging efficiency.
