Measurement of the scintillation time spectra and pulse-shape discrimination of low-energy β and nuclear recoils in liquid argon with DEAP-1

Abstract

The DEAP-1 low-background liquid argon detector was used to measure scintillation pulse shapes of electron and nuclear recoil events and to demonstrate the feasibility of pulse-shape discrimination down to an electron-equivalent energy of 20 keV ee .In the surface dataset using a triple-coincidence tag we found the fraction of β events that are misidentified as nuclear recoils to be < 1.4 × 10 −7 (90% C.L.) for energies between 43-86 keV ee and for a nuclear recoil acceptance of at least 90%, with 4% systematic uncertainty on the absolute energy scale. The discrimination measurement on surface was limited by nuclear recoils induced by cosmic-ray generated neutrons. This was improved by moving the detector to the SNOLAB underground laboratory, where the reduced background rate allowed the same measurement to be done with only a double-coincidence tag.The combined data set contains 1.23 × 10 8 events. One of those, in the underground data set, is in the nuclearrecoil region of interest. Taking into account the expected background of 0.48 events coming from random pileup, the resulting upper limit on the level of electronic recoil contamination is < 2.7 × 10 −8 (90% C.L.) between 44-89 keV ee and for a nuclear recoil acceptance of at least 90%, with 6% systematic uncertainty on the absolute energy scale.We developed a general mathematical framework to describe pulse-shape-discrimination parameter distributions and used it to build an analytical model of the distributions observed in DEAP-1. Using this model, we project a misidentification fraction of approximately 10 −10 for an electron-equivalent energy threshold of 15 keV ee for a detector with 8 PE/keV ee light yield. This reduction enables a search for spin-independent scattering of WIMPs from 1000 kg of liquid argon with a WIMP-nucleon cross-section sensitivity of 10 −46 cm 2 , assuming negligible contribution from nuclear recoil backgrounds.

Extracting nuclear form factors with coherent neutrino scattering

“…We, however, learned from experiments like DEAP-1 and DEAP-3600 using LAr where the PSD method provides a suppression factor of ∼ 10 −8 to identify nuclear recoils among the overwhelmingly dominant electromagnetic recoils [47,48]. We believe that it is not a problem for PSD to distinguish the CEνNS signal from 39 Ar beta decays.…”

Direct detection of WIMP dark matter: concepts and status

“…two time constants differ by three orders of magnitude: τ ↑↓ = 6 ns, τ ↑↑ = 1.5 µs [113]. By comparing the fast part of the scintillation signal F prompt to the total pulse area ERs can be rejected to a very high level (< 2 × 10 −8 at 90% NR acceptance [79]). However, the need for a large number of detected photons for pulse shape discrimination increases the analysis threshold to ∼30 keV nr .…”

Scintillation and ionization ratio of liquid argon for electronic and nuclear recoils at drift-fields up to 3 kV/cm

“…In this paper, we focus on the drift-field dependence of S2/S1 properties up to 3.0 kV/cm. Although liquid argon (LAr) scintillation has strong pulse shape discrimination (PSD) power [8], to simplify, S2/S1 discrimination power is separately discussed from PSD property in this paper.…”