Radon backgrounds in the DEAP-1 liquid-argon-based Dark Matter detector

TL;DR

This paper analyzes background sources in the DEAP-1 liquid argon detector, focusing on radon decay rates and event discrimination, to inform the design and background reduction strategies for the larger DEAP-3600 Dark Matter detector.

Contribution

It provides a detailed analysis of background events in DEAP-1 and quantifies radon levels, informing improvements for the DEAP-3600 detector.

Findings

Radon decay rate in liquid argon measured between 16 and 26 μBq/kg.

Backgrounds from radon daughters, misidentified electromagnetic events, and external leakage account for observed events.

Backgrounds will be significantly reduced in DEAP-3600 due to higher light yield and simpler geometry.

Abstract

The DEAP-1 \SI{7}{kg} single phase liquid argon scintillation detector was operated underground at SNOLAB in order to test the techniques and measure the backgrounds inherent to single phase detection, in support of the \mbox{DEAP-3600} Dark Matter detector. Backgrounds in DEAP are controlled through material selection, construction techniques, pulse shape discrimination and event reconstruction. This report details the analysis of background events observed in three iterations of the DEAP-1 detector, and the measures taken to reduce them. The $^{222}$Rn decay rate in the liquid argon was measured to be between 16 and \SI{26}{\micro\becquerel\per\kilogram}. We found that the background spectrum near the region of interest for Dark Matter detection in the DEAP-1 detector can be described considering events from three sources: radon daughters decaying on the surface of the active volume, the expected rate of electromagnetic events misidentified as nuclear recoils due to inefficiencies in the pulse shape discrimination, and leakage of events from outside the fiducial volume due to imperfect position reconstruction. These backgrounds statistically account for all observed events, and they will be strongly reduced in the DEAP-3600 detector due to its higher light yield and simpler geometry.