A measurement of the scintillation decay time constant of nuclear recoils in liquid xenon with the XMASS-I detector

TL;DR

This paper measures the decay time constant of nuclear recoil scintillation in liquid xenon using the XMASS-I detector, providing key parameters for dark matter detection and pulse shape discrimination.

Contribution

It presents the first in-situ measurement of the NR scintillation decay time constant in LXe with the XMASS-I detector, including the separation of singlet and triplet components.

Findings

Measured the triplet decay time constant as 26.9 ns.

Determined the singlet state fraction as 0.252.

Evaluated pulse shape discrimination performance for background reduction.

Abstract

We report an in-situ measurement of the nuclear recoil (NR) scintillation decay time constant in liquid xenon (LXe) using the XMASS-I detector at the Kamioka underground laboratory in Japan. XMASS-I is a large single-phase LXe scintillation detector whose purpose is the direct detection of dark matter via NR which can be induced by collisions between Weakly Interacting Massive Particles (WIMPs) and a xenon nucleus. The inner detector volume contains 832 kg of LXe. $^{252}$Cf was used as an external neutron source for irradiating the detector. The scintillation decay time constant of the resulting neutron induced NR was evaluated by comparing the observed photon detection times with Monte Carlo simulations. Fits to the decay time prefer two decay time components, one for each of the Xe$_{2}^{*}$ singlet and triplet states, with $\tau_{S}$ = 4.3$\pm$0.6 ns taken from prior research, $\tau_{T}$ was measured to be 26.9$^{+0.7}_{-1.1}$ ns with a singlet state fraction F$_{S}$ of 0.252$^{+0.027}_{-0.019}$.We also evaluated the performance of pulse shape discrimination between NR and electron recoil (ER) with the aim of reducing the electromagnetic background in WIMP searches. For a 50\% NR acceptance, the ER acceptance was 13.7${\pm}$1.0\% and 4.1${\pm}$0.7\% in the energy ranges of 5--10 keV$_{\rm ee}$ and 10--15 keV$_{\rm ee}$, respectively.