Doping Liquid Argon with Xenon in ProtoDUNE Single-Phase: Effects on Scintillation Light

TL;DR

This study tests xenon doping in a kiloton-scale liquid argon TPC (ProtoDUNE-SP) to enhance scintillation light collection and reduce nonuniformities in a large-volume detector. Using $720~\mathrm{t}$ of LAr and six xenon injections totaling $13.6$ kg to reach $18.8~\mathrm{ppm}$, the team deployed dedicated X-ARAPUCA detectors alongside the standard PDS to quantify argon-to-xenon energy transfer and its interaction with nitrogen quenching. The results show the Xe light fraction approaching $0.65$ at $16~\mathrm{ppm}$, with the slow component increasing and the fast component decreasing, and light collection improving notably at larger drift distances; the total light recovers to near the pure Ar level despite nitrogen contamination. Importantly, the TPC charge signal remained stable throughout the doping campaign, indicating xenon mixing does not hinder charge collection and validating xenon doping as a viable enhancement for the DUNE FD2 photon-detection strategy, provided high-purity Xe and careful purification to avoid fluorinated contaminants.

Abstract

Doping of liquid argon TPCs (LArTPCs) with a small concentration of xenon is a technique for light-shifting and facilitates the detection of the liquid argon scintillation light. In this paper, we present the results of the first doping test ever performed in a kiloton-scale LArTPC. From February to May 2020, we carried out this special run in the single-phase DUNE Far Detector prototype (ProtoDUNE-SP) at CERN, featuring 720 t of total liquid argon mass with 410 t of fiducial mass. A 5.4 ppm nitrogen contamination was present during the xenon doping campaign. The goal of the run was to measure the light and charge response of the detector to the addition of xenon, up to a concentration of 18.8 ppm. The main purpose was to test the possibility for reduction of non-uniformities in light collection, caused by deployment of photon detectors only within the anode planes. Light collection was analysed as a function of the xenon concentration, by using the pre-existing photon detection system (PDS) of ProtoDUNE-SP and an additional smaller set-up installed specifically for this run. In this paper we first summarize our current understanding of the argon-xenon energy transfer process and the impact of the presence of nitrogen in argon with and without xenon dopant. We then describe the key elements of ProtoDUNE-SP and the injection method deployed. Two dedicated photon detectors were able to collect the light produced by xenon and the total light. The ratio of these components was measured to be about 0.65 as 18.8 ppm of xenon were injected. We performed studies of the collection efficiency as a function of the distance between tracks and light detectors, demonstrating enhanced uniformity of response for the anode-mounted PDS. We also show that xenon doping can substantially recover light losses due to contamination of the liquid argon by nitrogen.

Figures (20)

• Figure 1: Schematic representation of the production process of scintillation light in pure liquid argon, and the way it is affected by xenon doping and nitrogen quenching. The time constants of the non-radiative energy-transfer processes $\tau_{AX}$ and $\tau_{XX}$ depend on the xenon concentration in LAr.
• Figure 2: 3D model of the ProtoDUNE-SP detector with labelling of all major components and definition of coordinate system used (bottom left).
• Figure 3: The three technologies of PDS modules shown inside the APA frame and individually for comparison.
• Figure 4: X-ARAPUCA detectors installed on a dedicated support and ready for insertion in the ProtoDUNE-SP cryostat.
• Figure 5: 3D model of the two X-ARAPUCA detectors inside the ProtoDUNE-SP cryostat. Left: front view. In green, the frame of APA-5, in red the frame of APA-6, in blue the PDS bars. Right: side view, showing the position of the X-ARAPUCAs with respect to the APA frames and PDS.