The Liquid Hole-Multiplier: Lighting Up the Dark with Shining Xenon Bubbles

Large-volume noble-liquid time projection chambers (TPCs) lead the worldwide race for the direct detection of dark matter in the form of Weakly Interacting Massive Particles. While current TPCs are already utilizing ton-scale targets of liquid xenon and liquid argon, the next generation of experiments will employ multi-ton targets to fully cover the relevant parameter space. Since scaling up existing designs to the multi-ton regime may prove to be quite challenging, there is strong motivation to search for new innovative and enabling detection solutions.

We propose a new concept, the bubble-assisted Liquid Hole-Multiplier (LHM) – a “local dual-phase” detection element, designed for the detection of both ionization electrons and primary scintillation photons induced by particle interactions within the noble liquid. The LHM comprises a perforated micro-pattern electrode (e.g. Gas Electron Multiplier – GEM, or Thick-GEM) immersed in the liquid, with a bubble of the noble gas supported underneath. Ionization electrons and scintillation-induced photoelectrons extracted from a cesium iodide (CsI) photocathode drift through the electrode`s holes and induce electroluminescence (EL) in the bubble, with up to several hundred EL photons emitted per drifting electron. Accurate imaging of the EL patterns, resulting in precise reconstruction of the event topology, can be performed with small-pixel photon detectors, e.g. silicon photomultipliers.

We will present the principle of the new concept, as well as measurements of ionization electrons and VUV scintillation photons using CsI-coated LHM prototypes in liquid xenon (LXe). The mechanism, stable over months of operation, yields energy resolutions surpassing those of current dual-phase LXe TPCs, with a time resolution on the nanosecond scale for events comprising a few hundred photoelectrons. We will discuss the near-term experimental plan, and outline the potential advantages of employing LHM modules in future multi-ton dark matter experiments.