Measurement of the Quantum Efficiency of Electrode Materials for VUV Photons in Liquid Xenon

TL;DR

The study addresses photoelectric-backgrounds in dual-phase LXe TPCs by quantifying the vacuum ultraviolet quantum efficiency (QE) of candidate electrode materials. It implements a two-subsystem measurement using a deuterium lamp to count incident photons and a cathode–anode setup to count emitted electrons, defining QE as $QE = N_e/N_{ph}$, across vacuum, GXe, and LXe with fields up to $6$ kV/cm. The main findings show Pt has the highest LXe QE, SUS304 is intermediate, and MgF$_{2}$-coated Al has the lowest ($QE = (7.2 \pm 2.3) \times 10^{-5}$ in LXe, about a factor of $\sim 4.4$ lower than SUS304); the observed medium dependence is attributed to backscattering and interface work-function effects, with LXe reducing backscattering relative to GXe. These results suggest using low-QE electrode surfaces to suppress photoelectric backgrounds in S2-based dark matter searches, while highlighting potential issues with surface charging on insulating coatings and the need for optimized, low-resistivity coatings for practical deployment.

Abstract

Light dark matter searches using ionization signals in dual-phase liquid xenon (LXe) time projection chambers (TPCs) are limited by low-energy ionization backgrounds, including those from the photoelectric effect on the electrodes. To address this, we measured the quantum efficiency (QE) of various electrode materials for vacuum ultraviolet (VUV) photons in LXe, including platinum (Pt), stainless steel (SUS304), and magnesium fluoride (MgF$_{2}$)-coated aluminum (Al). Our results show that MgF$_{2}$-coated Al exhibits the lowest QE among the tested materials. The QE for VUV photons with a mean wavelength of 179.5~nm was measured to be $(7.2 \pm 2.3) \times 10^{-5}$, corresponding to a reduction by a factor of 4.4 compared to SUS304, a commonly used electrode material in direct dark matter experiments with LXe. These findings suggest that employing low-QE electrodes can help mitigate photoelectric-induced backgrounds, potentially improving the sensitivity of LXe TPCs in light dark matter searches.

Figures (5)

• Figure 1: Experimental setup used to measure the QE of electrode materials.
• Figure 2: Expected deuterium lamp spectrum after applying the two VUV narrow bandpass filters (blue). The emission spectrum of LXe lxe_wave is also shown (orange).
• Figure 3: Xenon gas handling system used in GXe and LXe measurements. The line highlighted in red is the gas line used during the measurements in LXe.
• Figure 4: Quantum efficiency measured in vacuum, GXe and LXe for Pt (top), SUS304 (middle), and MgF$_{2}$ (bottom), respectively.
• Figure 5: Comparison of quantum efficiency measured in LXe for Pt, SUS304, and MgF$_{2}$. QE of SS304 measured by the LUX experiment LUXelectronbkgs is also shown.