Surface Background Rejection in Liquid Argon Dark Matter Detectors using Layered Wavelength-Shifting and Scintillating Thin Films

A technique using layered wavelength shifting, scintillating and non-scintillating films is presented to achieve discrimination of surface alpha events from low-energy nuclear recoils in liquid argon detectors. A discrimination power greater than 10^{8}, similar to the discrimination possible for electronic recoils in argon, can be achieved by adding a thin layer of scintillator with a suitably slow decay time to a wavelength-shifter coated surface. The technique allows suppression of surface alpha events in a very large next-generation argon dark matter experiment (with hundreds of square meters of surface area) without the requirement for position reconstruction, and could also be used to suppress surface backgrounds in compact argon detectors of low-energy nuclear recoils, for example in measurements of coherent neutrino-nucleus scattering or for sensitive measurements of neutron fluxes.