Surface background suppression in liquid argon dark matter detectors using a newly discovered time component of tetraphenyl-butadiene scintillation

TL;DR

This paper introduces a novel pulse shape discrimination method based on a newly discovered long-lived scintillation component in TPB, significantly reducing surface backgrounds in liquid argon dark matter detectors without compromising sensitivity.

Contribution

The study uncovers a previously unknown long-lived scintillation component in TPB and demonstrates its use for effective background suppression independent of position cuts.

Findings

Surface backgrounds can be reduced by over 1000 times using pulse shape discrimination.

The long-lived scintillation component varies with excitation type, enabling background identification.

The technique can be adapted for other detector types beyond liquid argon.

Abstract

Decays of radioisotopes on inner detector surfaces can pose a major background concern for the direct detection of dark matter. While these backgrounds are conventionally mitigated with position cuts, these cuts reduce the exposure of the detector by decreasing the sensitive mass, and uncertainty in position determination may make it impossible to adequately remove such events in certain detectors. In this paper, we provide a new technique for substantially reducing these surface backgrounds in liquid argon (LAr) detectors, independent of position cuts. These detectors typically use a coating of tetraphenyl-butadiene (TPB) on the inner surfaces as a wavelength shifter to convert vacuum ultraviolet (VUV) LAr scintillation light to the visible spectrum. We find that TPB scintillation contains a component with a previously unreported exceptionally long lifetime ($\sim$ms). We discovered that this component differs significantly in magnitude between alpha, beta, and VUV excitation, which enables the use of pulse shape discrimination to suppress surface backgrounds by more than a factor of $10^3$ with negligible loss of dark matter sensitivity. We also discuss how this technique can be extended beyond just LAr experiments.