Hans Kraus

Abstract:

We describe the design and assembly of the LUX-ZEPLIN experiment, a direct detection search for cosmic WIMP dark matter particles. The centerpiece of the experiment is a large liquid xenon time projection chamber sensitive to low energy nuclear recoils. Rejection of backgrounds is enhanced by a Xe skin veto detector and by a liquid scintillator Outer Detector loaded with gadolinium for efficient neutron capture and tagging. LZ is located in the Davis Cavern at the 4850’ level of the Sanford Underground Research Facility in Lead, South Dakota, USA. We describe the major subsystems of the experiment and its key design features and requirements.

Abstract:

Deep underground environments are ideal for low background searches due to the attenuation of cosmic rays by passage through the earth. However, they are affected by backgrounds from γ-rays emitted by 40K and the 238U and 232Th decay chains in the surrounding rock. The LUX-ZEPLIN (LZ) experiment will search for dark matter particle interactions with a liquid xenon TPC located within the Davis campus at the Sanford Underground Research Facility, Lead, South Dakota, at the 4850-foot level. In order to characterise the cavern background, in-situ γ-ray measurements were taken with a sodium iodide detector in various locations and with lead shielding. The integral count rates (0–3300 keV) varied from 596 Hz to 1355 Hz for unshielded measurements, corresponding to a total flux from the cavern walls of 1.9 ± 0.4 γ cm−2s−1. The resulting activity in the walls of the cavern can be characterised as 220 ± 60 Bq/kg of 40K, 29 ± 15 Bq/kg of 238U, and 13 ± 3 Bq/kg of 232Th.

Abstract:

Gallium oxide has recently been identified as a promising scintillator. To assess its potential as a detector material for ionizing radiation at low temperatures, we measured the luminescence and scintillation properties of an undoped Ga2O3 crystal over the 7–295 K temperature range. The emission of the crystal is due to the radiative decay of self-trapped excitons and donor-acceptor pairs and peaks at a wavelength of 380 nm. The scintillation light output of the undoped Ga2O3 increases with a decrease in temperature, reaching a maximum value of 19 300 ± 2200 ph/MeV at 50 K. The measured luminescence kinetics has a recombination character with specific decay time (τ0.1) increasing from 1 to 1.8 μs at cooling. Since radiative decay in the crystal competes with nonradiative processes, material optimization could lead to the scintillator achieving a yield of 40800 ph/MeV, a figure considered to be an upper limit.