  We characterized the light output response of a new 140 cm3 stilbene-d12
crystal up to 14.1 MeV neutron energies using a coincidence neutron scattering
system. We also characterized its light output response to alpha particles in
the 5 to 6~MeV energy range. The excellent PSD capability of the
stilbene-d$_{12}$ detector allowed us to select light pulses produced by
particles of increasing ionization density, namely electrons, protons,
deuterium-ions, and alpha particles. The measured fast decay component of the
light pulses is increasingly quenched as the ionization density of the particle
in the crystal increases. Consistently with this finding, the Birks' quenching
parameter of alpha particles is approximately 8.5 times larger compared to the
quenching of deuterium ions, produced by neutron scattering interactions. The
reported experimental characterization will allow high-fidelity modeling of the
detector enabling its application for fast-neutron detection and spectroscopy
in nuclear physics, radiation protection, nuclear security, and
non-proliferation.
