Recent advances in bubble-assisted Liquid Hole-Multipliers in liquid xenon

We report on recent advances in the operation of bubble-assisted Liquid Hole-Multipliers (LHM). By confining a vapor bubble under or adjacent to a perforated electrode immersed in liquid xenon, we could record both radiation-induced ionization electrons and primary scintillation photons in the noble liquid. Four types of LHM electrodes were investigated: a THGEM, standard double-conical GEM, 50 mu m-thick single-conical GEM (SC-GEM) and 125 mu m-thick SC-GEM-all coated with CsI photocathodes. The 125 mu m-thick SC-GEM provided the highest electroluminescence (EL) yields, up to similar to 400 photons per electron over 4 pi, with an RMS pulse-height resolution reaching 5.5% for events comprising similar to 7000 primary electrons. Applying a high transfer field across the bubble, the EL yield was further increased by a factor of similar to 5. The feasibility of a vertical-mode LHM, with the bubble confined between two vertical electrodes, and the operation of a two-stage LHM configuration were demonstrated for the first time. We combine electrostatic simulations with observed signals to draw conclusions regarding the location of the liquid-gas interface and suggest an explanation for the observed differences in EL yield between the investigated electrodes.