A 30-g xenon bubble chamber, operated at Northwestern University in June andNovember 2016, has for the first time observed simultaneous bubble nucleationand scintillation by nuclear recoils in a superheated liquid. This chamber isinstrumented with a CCD camera for near-IR bubble imaging, a solar-blindphotomultiplier tube to detect 175-nm xenon scintillation light, and apiezoelectric acoustic transducer to detect the ultrasonic emission from agrowing bubble. The time of nucleation determined from the acoustic signal isused to correlate specific scintillation pulses with bubble-nucleating events.We report on data from this chamber for thermodynamic "Seitz" thresholds from4.2 to 15.0 keV. The observed single- and multiple-bubble rates when exposed toa $^{252}$Cf neutron source indicate that, for an 8.3-keV thermodynamicthreshold, the minimum nuclear recoil energy required to nucleate a bubble is$19\pm6$ keV (1$\sigma$ uncertainty). This is consistent with the observedscintillation spectrum for bubble-nucleating events. We see no evidence forbubble nucleation by gamma rays at any of the thresholds studied, setting a 90%C.L. upper limit of $6.3\times10^{-7}$ bubbles per gamma interaction at a4.2-keV thermodynamic threshold. This indicates stronger gamma discriminationthan in CF$_3$I bubble chambers, supporting the hypothesis that scintillationproduction suppresses bubble nucleation by electron recoils while nuclearrecoils nucleate bubbles as usual. These measurements establish thenoble-liquid bubble chamber as a promising new technology for the detection ofweakly interacting massive particle dark matter and coherent elasticneutrino-nucleus scattering.
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