Bubble-assisted liquid hole multipliers: "local dual-phase noble-liquid detectors" (LHM).

Large-volume noble-liquid TPCs, particularly dual-phase devices, are leading tools in rare-event searches and neutrino-physics experiments. Their scaling up in future experiments, however, calls for innovative detector solutions. We recently introduced a new concept of charge and light recording in noble liquid. This so-called bubble-assisted Liquid Hole-Multiplier (LHM) is a “local dual-phase” detection element, designed for recording both ionization electrons and primary scintillation photons induced by particle interactions within the noble liquid. The LHM comprises a perforated electrode (e.g. Thick Gas Electron Multiplier – THGEM, or Gas Electron Multiplier - GEM) immersed in the liquid, with a bubble of the noble gas supported underneath. Ionization electrons deposited in the liquid and scintillation-induced photoelectrons extracted from a CsI UV-photocathode, deposited on the electrode, are focused into the electrode's holes; they induce electroluminescence (EL) in the bubble, with a copious yield of EL photons emitted per drifting electron. Precise reconstruction of the event topology can be performed with pixilated photon detectors, e.g. silicon photomultipliers (SiPM) or cryogenic Gaseous Photomultipliers (GPM). The LHM concept has been investigated thoroughly in liquid xenon (LXe), in dedicated cryostats (WILiX and MINIX). Different LHM perforated electrodes were studied (THGEM, regular bi-conical GEM and single-conical GEM) with bubble trapped in “horizontal” and “vertical” electrode configurations. Visual inspection with a CMOS camera permitted understanding the bubble formation, stability and control. The detector performance was studied with Alpha-particles, detecting EL UV-light in the xenon-gas bubble - resulting from both ionization electrons (collected from LXe) and UV-photoelectrons (extracted in the liquid from the CsI-coated LHM-electrode). The LHM provides very competitive energy resolutions. Further experimental studies accompanied by model simulations aim at validating this new "local dual-phase noble-liquid detector" concept, as a potential tool for future large-mass detectors of dark-matter and other rare-event searches. The R&D has been carried out within the DARWIN large LXe-volume dark-matter observatory project.