Early Results from Xenon Doping of a Two-Phase Argon Time Projection Chamber

Two-phase liquid argon time projection chambers measure ionization signals by detecting S2 light produced by ionization electrons extracted from the liquid surface into argon gas under a strong electric field.  This 128 nm light excites fluorescent wavelength-shifting coatings, which emit at longer wavelengths that are sensed by SiPMs or photomultipliers.  The doping of argon gas with small (tens of ppm) quantities of xenon shifts the electroluminescence light to 147 nm, which can be sensed directly by recently developed SiPMs.  This improves both the energy and time resolution of ionization signals; this is especially important for detecting the low-energy nuclear recoils of CEnNS and WIMP dark matter events.   We describe early measurements of the ionization response of the S2 signal channel as a function of increasing xenon concentration in the electroluminescence gap above a xenon-doped argon mixture.  We also discuss prediction and measurement (by RGA-instrumented sampling) of the Henry’s law constant that governs partition of xenon between gas and liquid phases.