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. 252Cf 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 Xe2* singlet and triplet states, with τS = 4.3 ± 0.6 ns taken from prior research, τT was measured to be 26.9+0.7−1.1 ns with a singlet state fraction FS 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 keVee and 10–15 keVee, respectively.
Highlights
We evaluated the performance of pulse shape discrimination between nuclear recoil (NR) and electron recoil (ER) with the aim of reducing the electromagnetic background in Weakly Interacting Massive Particle (WIMP) searches
To analyze waveforms of individual photomultiplier tubes (PMTs), we developed a peak finding algorithm based on a Savitzky-Golay filter [34] to obtain individual photon hit timings
We evaluated the performance of pulse shape discrimination (PSD) in XMASS-I based on our scintillation decay time constant measurement
Summary
The XMASS-I detector is located in the Kamioka mine under 1,000 m of rock (2,700 meter water equivalent). The inner detector (ID) contains 832 kg of LXe inside a spherical, oxygen free high conductivity (OFHC) copper structure with an 80 cm diameter. The inner containment vessel contains the LXe and the PMT holder, while the outer containment vessel holds vacuum for thermal insulation. The OD is a cylindrical tank m in diameter and m in height filled with ultrapure water. 72 Hamamatsu 20-inch R3600 PMTs are mounted on the inner surface of the water tank to provide an active muon veto. Signals from the 642 ID PMTs were recorded by CAEN V1751 waveform digitizers with a. PMT hits are observed in a 200 ns coincidence window, a global trigger is issued to both the ATMs and the waveform digitizers. The OD trigger requires at least 8 PMT hits in a 200 ns coincidence window
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