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

Abstract

A : 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 τ S = 4.3±0.6 ns taken from prior research, τ 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±1.0% and 4.1±0.7% in the energy ranges of 5-10 keV ee and 10-15 keV ee , respectively.

Search for WIMP-129Xe inelastic scattering with particle identification in XMASS-I

“…The γ-ray calibration data is also used for determining the scintillation time profile for the β-rays' and γ-rays' events. A 252 Cf neutron source is used to determine the timing parameters for the Nuclear Recoil (NR) events [16]. The neutron source was installed at the end of a pipe, which penetrates the water region of the OD and reaches the vacuum vessel that thermally isolates the detector from the water.…”

“…Before the start of period 4, we recovered the xenon from the detector in liquid phase to an external reservoir. Then we filled again the detector [16]. The blue and the red shaded spectra are from our MC with different cross section libraries, ENDF-B/VII [23,24] and G4NDL3.13 based on ENDF/B-VI, respectively.…”

“…The blue and the red shaded spectra are from our MC with different cross section libraries, ENDF-B/VII [23,24] and G4NDL3.13 based on ENDF/B-VI, respectively. Widths of these spectra represent the ±10% uncertainty of the neutron tagging efficiency [16].…”

“…Since a γ-ray is converted into lower energy electrons in LXe, its time constant is shorter than that of a β-ray. The scintillation light from the NR has a shorter time constant than that of a β-ray and a γ-ray since its ionization density is higher and the ion-electron pairs recombine faster [16]. Since inelastic scattering has contributions from both NR and a γ-ray, it has a shorter time constant than a pure β-ray event.…”

“…The change of the signal efficiency and this change in the NR timing parameters were used to evaluate the systematic uncertainty. The relevant timing parameter values were obtained from the 252 Cf calibration [16].…”