Gas Electroluminescence in a Dual Phase Xenon-Doped Argon Detector

TL;DR

This work investigates gas electroluminescence in a dual-phase xenon-doped argon detector, addressing the challenge of detecting low-energy signals by redirecting argon excitation energy into xenon-related light. Using the CHILLAX setup with up to ~4% Xe in liquid argon, the authors measure EL gain as a function of Xe concentration and electric field, and observe a roughly 2.5-fold increase in detected EL at around 2% Xe. They reveal a clear Xe2* emission component and develop an analytical energy-transfer model describing Ar2* to Xe* to Xe2* pathways, supported by waveform analyses and optical simulations. The findings demonstrate that Xe-doped liquid argon acts as a wavelength shifter and that gas EL in Xe-doped argon can enhance low-energy ionization detection, with potential implications for dark matter and CEvNS experiments.

Abstract

Noble element detectors using argon or xenon as the detection medium are widely used in the searches for rare neutrino and dark matter interactions. Xenon doping in liquid argon can preserve attractive properties of an argon target while enhancing the detectable signals with properties of xenon. In this work, we deployed a dual-phase liquid argon detector with up to 4% xenon doping in the liquid and studied its gas electroluminescence properties as a function of xenon concentration. At $\sim$2% xenon doping in liquid argon, we measured $\sim$34 ppm of xenon in the gas and observed $\sim$2.5 times larger electroluminescence signals in the detector than those in pure argon. By analyzing signals recorded by photosensors of different wavelength sensitivities, we confirm that the argon gas electroluminescence process is strongly affected by the addition of xenon. We propose an analytical model to describe the underlying energy transfer mechanism in argon-xenon gas mixtures. Lastly, the implications of this measurement for low energy ionization signal detection will be discussed.

Figures (20)

• Figure 1: Left: A schematic of the CHILLAX TPC (not to scale). The top SiPM assembly (a) sits in the gas phase (light green) and looks through the anode (d) into the liquid bath (dark green), where the cathode (e) defines the lower boundary of the active volume. Below the cathode lies a grounded shield (f) that screens the bottom SiPM assembly (g). The liquid level height is set using a two-RTD level meter (b) and the xenon concentration in the gas phase is sampled through a capillary tube (c). Right: A to-scale schematic of the top SiPM assembly. The upper right SiPM is a S13371 model masked with a quartz window, and the upper left SiPM is the same model with the window removed. The lower SiPM is a 2x2 array of windowless S13370 units, but is read out as a single channel in the same way as the S13371s.
• Figure 2: Pulse amplitude distributions corresponding to the smallest amplitude dataset (gray, 10 kV, 0% [Xe], Ch 2) and the largest (orange, 14 kV, 4% [Xe], Ch 0) superimposed with the saturation and trigger threshold (red and black, respectively). The shoulders at 300 ADC and 6000 ADC in the gray and orange spectra respectively correspond to the $^{241}$Am events with the largest amplitude. The counts with amplitudes larger than these shoulders correspond to background events.
• Figure 3: The pulse rise time for $^{241}$Am events at 4.0% [Xe] and 10.0 kV cathode volume as a function of pulse integral as recorded by the sum of all channels. The grayed out region in the left is populated primarily by S1-only or S1-dominated events, due to their faster rise time, and the events with larger rise time are mostly S2 pulses.
• Figure 4: The background-subtracted S2 energy spectra for $^{241}$Am calibration events recorded by SiPM Ch 0 at 12 kV for 0--4% [Xe] (top to bottom). The red curves represent the best fits to extract the endpoint energy (see text).
• Figure 5: The number of EL photons detected by each top SiPM channel at the endpoint of $^{241}$Am gamma energy deposition, as a function of nominal xenon concentration for 3 tested electric fields. The integrated photon area are obtained from fitting the measured S2 energy spectra to a model described in the text.