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

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. The threshold was around 15 keV for electronic recoils in TPC operation; it could be straightforwardly lowered by improving the light collection efficiency.