Development of a dual-phase xenon time projection chamber prototype for the RELICS experiment

TL;DR

The RELICS project targets coherent elastic neutrino-nucleus scattering from reactor antineutrinos using a dual-phase xenon TPC, requiring sub-keV thresholds and robust background control. This work documents a dedicated prototype that validates the detector concept, including the TPC, cryogenics and xenon purification, slow control, and DAQ, and demonstrates a sub-keV capability via a single-electron gain of $34.30 \pm 0.01$ PE/e$^-$ and direct observation of 0.27 keV L-shell decay from $^{37}$Ar. It also develops and validates data analysis and optical simulation frameworks (RelicsSim) to calibrate light and charge yields, reconstruct 3D positions with ~5 mm accuracy, and perform S2-only analyses that reveal sub-keV signals in the presence of DE backgrounds. The prototype confirms the feasibility of core RELICS technologies and provides a practical experimental basis plus a scalable software and calibration framework for the full-scale detector. These results support the continued development toward the final RELICS detector for precise CEvNS measurements and reactor antineutrino studies.

Abstract

The RELICS (REactor neutrino LIquid xenon Coherent elastic Scattering) experiment aims to detect coherent elastic neutrino-nucleus scattering from reactor antineutrinos using a dual-phase xenon time projection chamber. To validate the detector concept and ensure technical reliability for the full-scale experiment, a dedicated prototype was designed, constructed, and operated. This work presents an overview of the design, construction, and operational performance of the prototype, with emphasis on its major subsystems, including the TPC, cryogenic and xenon purification systems, slow control, and data acquisition. During operation, the detector demonstrated the capability to achieve a sub-keV energy threshold required for the RELICS physics program, as reflected by a measured single electron gain of 34.30~$\pm$~0.01~(stat.)~PE/e$^-$ and the successful detection of 0.27~keV L-shell decay events from $^{37}$Ar. In addition, essential data analysis techniques and simulation frameworks were developed and validated, establishing the methodological foundation for future RELICS operations. The successful construction and operation of this prototype confirm the feasibility of the core technologies and provide a crucial experimental basis for the final RELICS detector.

Figures (15)

• Figure 1: Schematic cross-section of the dual-phase xenon TPC demonstrator designed for the RELICS experiment. The main cutaway view illustrates the outer vessel, inner vessel, and internal support structures, while the enlarged inset highlights the TPC layout. The top and bottom photomultiplier (PMT) arrays detect scintillation and electroluminescence photons. The cathode and gate, together with the field-shaping rings, establish the drift field throughout the inner volume, while the gate and anode establish the extraction field at the liquid-gas interface. Level meters continuously monitor the LXe height. The diving bell ensures a well-defined xenon liquid level, and the bottom screening mesh protects the bottom PMTs from high voltage effects. All inner components are suspended from the top flange by the structural suspension panel.
• Figure 2: The electric field magnitude distribution in the X–Z cross-section within the TPC, simulated using COMSOL Multiphysics. The color scale indicates the field strength in unit of V/cm. The simulation visualizes the high-field extraction region between the gate and anode (top red area), and the drift field in the central volume, maintained by the field-shaping rings. The black lines represent the electric field lines, and the green streamlines trace electron drift paths from various initial positions, demonstrating their successful transport to the liquid-gas interface for extraction.
• Figure 3: The map shows the fractional deviation of the charge yield in the X–Z cross-section within the TPC inner drift volume, defined as $\frac{\Delta CY(x,y,z)}{CY_{\mathrm{ave}}} = \frac{CY(x,y,z)-CY_{\mathrm{ave}}}{CY_{\mathrm{ave}}}$. The simulation incorporates the field-dependent ionization response of LXe, enabling position-dependent corrections to be applied to the measured S2 signals.
• Figure 4: Representative single-photoelectron charge spectrum from one of the PMT Channels obtained during PMT gain calibration. The black histogram shows the experimental data, and the dashed curve represents the fitted model combining Poisson distributed photoelectron statistics with Gaussian amplification response. The yellow line indicates the electronic pedestal, while the blue line denotes the single-photoelectron component. The measured gain of $\mathrm{5.21 \times 10^6}$ provides sufficient charge amplification to ensure reliable detection of individual VUV photons from xenon scintillation, fulfilling the signal-to-noise requirements for low-energy event reconstruction in the RELICS dual-phase xenon TPC.
• Figure 5: Schematic diagram of the RELICS prototype cryogenic system and xenon handling infrastructure. The functional layout illustrates the closed-loop xenon circulation, including the cryocooler (CRY) with its cold compressor and cold head, the heat exchanger, the purification loop with pump and getter, and the calibration source injection lines for $^{37}$Ar and $^{\mathrm{83m}}$Kr. Blue lines denote the LXe flow path, whereas orange lines indicate the GXe circulation used for purification.