Development and characterization of a slow wavelength shifting coating for background rejection in liquid argon detectors

TL;DR

This paper introduces a novel slow wavelength-shifting coating for liquid argon detectors that enhances background rejection by enabling pulse shape discrimination of alpha decays in regions with poor light collection.

Contribution

A new pyrene-doped polystyrene wavelength-shifting film was developed and characterized, improving background discrimination in liquid argon dark matter detectors.

Findings

The film has a decay time constant of 279 ns at liquid argon temperature.

The wavelength-shifting efficiency of the film is approximately 46%.

The coating is predicted to suppress alpha background events by a factor of 10^5.

Abstract

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.