  A 30-g xenon bubble chamber, operated at Northwestern University in June and
November 2016, has for the first time observed simultaneous bubble nucleation
and scintillation by nuclear recoils in a superheated liquid. This chamber is
instrumented with a CCD camera for near-IR bubble imaging, a solar-blind
photomultiplier tube to detect 175-nm xenon scintillation light, and a
piezoelectric acoustic transducer to detect the ultrasonic emission from a
growing bubble. The time of nucleation determined from the acoustic signal is
used to correlate specific scintillation pulses with bubble-nucleating events.
We report on data from this chamber for thermodynamic "Seitz" thresholds from
4.2 to 15.0 keV. The observed single- and multiple-bubble rates when exposed to
a $^{252}$Cf neutron source indicate that, for an 8.3-keV thermodynamic
threshold, the minimum nuclear recoil energy required to nucleate a bubble is
$19\pm6$ keV (1$\sigma$ uncertainty). This is consistent with the observed
scintillation spectrum for bubble-nucleating events. We see no evidence for
bubble 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 a
4.2-keV thermodynamic threshold. This indicates stronger gamma discrimination
than in CF$_3$I bubble chambers, supporting the hypothesis that scintillation
production suppresses bubble nucleation by electron recoils while nuclear
recoils nucleate bubbles as usual. These measurements establish the
noble-liquid bubble chamber as a promising new technology for the detection of
weakly interacting massive particle dark matter and coherent elastic
neutrino-nucleus scattering.
