  We describe a technique, applicable to liquid-argon-based dark matter
detectors, allowing for discrimination of alpha-decays in detector regions with
incomplete light collection from nuclear-recoil-like events.
  Nuclear recoils and alpha events preferentially excite the liquid argon (LAr)
singlet state, which has a decay time of ~6 ns. The wavelength-shifter TPB,
which is typically applied to the inside of the active detector volume to make
the LAr scintillation photons visible, has a short re-emission time that
preserves the LAr scintillation timing. We developed a wavelength-shifting
polymeric film - pyrene-doped polystyrene - for the DEAP-3600 detector and
describe the production method and characterization. At liquid argon
temperature, the film's re-emission timing is dominated by a modified
exponential decay with time constant of 279(14) ns and has a
wavelength-shifting efficiency of 46.4(2.9) % relative to TPB, measured at room
temperature. By coating the detector neck (a region outside the active volume
where the scintillation light collection efficiency is low) with this film, the
visible energy and the scintillation pulse shape of alpha events in the neck
region are modified, and we predict that through pulse shape discrimination,
the coating will afford a suppression factor of O($10^{5}$) against these
events.
