Operating a large-diameter dual-phase liquid xenon TPC in the unshielded PANCAKE facility

TL;DR

This work demonstrates stable operation of a large, shallow dual-phase liquid xenon TPC (diameter ~1.33 m, active LXe ~127 kg) in the unshielded PANCAKE facility, validating a cost-effective, above-ground platform for testing XLZD-scale components. By integrating 19 PMTs in the gas phase, a small wire-based TPC, cryogenic cameras, a muon telescope, and a robust slow control/DAQ system, the study shows feasible S1/S2 event reconstruction and measurements of electron lifetime and drift velocity in a high-background environment. Krypton-83m calibrations reveal a measurable S1 threshold in light-only mode, while TPC operation demonstrates event identification, S1/S2 waveforms, and a purity-driven tau_e approaching 25 µs, consistent with improved xenon purification. The results underscore PANCAKE’s potential for large-scale LXe detector R&D, including HV testing and light-collection optimization, with implications for XLZD readiness and future calibration campaigns.

Abstract

Future liquid-xenon (LXe) based observatories for rare processes, such as XLZD, require testing of large components and sub-assemblies in cryogenic liquid or gaseous xenon environments. Here we present results from the stable operation of a shallow dual-phase LXe TPC with an inner diameter of 133.4\,cm and a height of 3.1\,cm in the unshielded PANCAKE platform, without underground suppression of cosmic-ray backgrounds. A total of 340\,kg of xenon was used in the experiment, of which 127\,kg constituted the active TPC mass. Measurements of the LXe purity-dependent electron lifetime and the electron drift velocity in LXe demonstrate that sensitive measurements to characterize the TPC performance are possible in a high-background environment, even with a very basic PMT-based light detection system. Improving this will straightforwardly reduce the TPC threshold, which was observed to be around 15\,keV for electronic recoils in TPC operation.

Figures (19)

• Figure 1: The PANCAKE facility on the right with its surrounding soft-wall clean area and adjacent cleanroom. The outer cryostat vessel was lowered and the (closed) inner vessel is visible. The cryostat is suspended from the blue support structure.
• Figure 2: Schematic side view of the inner cryostat with the installed PMT array (gold) and TPC. The inset depicts the electrode distances; the colored horizontal lines indicate the electrode planes. The uncertainty on the distance between the anode plane and the PMT array is around 10 mm.
• Figure 3: CAD rendering of the TPC setup inside PANCAKE. It was installed inside a cylindrical open-topped vessel of 1460 diameter which contained the entire LXe for the run. PTFE components were used to built a shallow TPC from the three electrodes, cathode (green), gate (blue), and anode (purple). Only the top of the anode was covered with PTFE reflectors. The PMT array was located centrally above the shallow TPC.
• Figure 4: Block diagram of the hardware in the PANCAKE DAQ system: The 19 amplified PMT signals were digitized by three 8-channel waveform digitizers. Coincident muon telescope signals above the discriminator threshold produced a logic signal that was digitized by one the ADC channels. For PMT calibration, a pulser drives an LED and triggers all ADCs, where the trigger signal is distributed by the Busy model which, in standard TPC operation, collects Busy signals from individual ADCs for further distribution. The ADCs are read out by a server via optical fibers.
• Figure 5: Inner cryostat illuminated by the internal LEDs. The images were captured by the overview camera when the cryostat was at room temperature (top) and when it was cold and filled with liquid xenon (bottom). The images show that the camera loses focus when being cold. This is likely due to thermal shrinkage and is reverted once the camera is at room temperature again.