# First Demonstration of a Scintillating Xenon Bubble Chamber for Dark Matter and CE$ν$NS Detection [CL]

A 30-gram 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 liquid xenon. This chamber is instrumented with a CCD camera for near-IR bubble imaging, a solar-blind PMT 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. The observed single- and multiple-bubble rates when exposed to a $^{252}$Cf neutron source indicate that, for a thermodynamic “Seitz” threshold of 8.3 keV, the minimum nuclear recoil energy required to nucleate a bubble is between 11 and 25 keV. This is consistent with the observed scintillation spectrum for bubble-nucleating events. We see no evidence for bubble nucleation by gamma rays at the thresholds studied, setting a 90% CL 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 WIMP and CE$\nu$NS detection.

D. Baxter, C. Chen, M. Crisler, et. al.
Wed, 1 Mar 17
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