Development of a Sensor-Integrated Cryogenic Probe for Barium-Tagging

Neutrinoless double beta decay is a proposed radioactive process that, if observed, could give further insight into the nature of neutrinos. It would prove that the neutrino is a Majorana particle (its own antiparticle) and be the first observation of lepton number violation. The nEXO experiment aims to observe this phenomenon using 5 tons of liquid xenon enriched to 90% in Xe136. A future upgrade to the experiment is called Barium tagging. By detecting the xenon double decay daughter nucleus barium in situ, one can completely remove radioactive backgrounds, thus significantly increasing the experimental sensitivity. One scheme is to extract the barium from liquid xenon with a cryogenic probe and detect it with fluorescence spectroscopy. At UCSD, we have built such a cryoprobe for freezing thin layers of xenon ice for capturing barium ions. The probe has an interdigitized electrode (IDE) capacitance sensor to monitor the thickness of the xenon ice with an accuracy of 3 μm. We will discuss the design of the probe and sensors. Preliminary results from the cryoprobe in gas and liquid xenon will be reported. In the future, the probe will be incorporated at a radioactive ion beam facility at TRIUMF to demonstrate the efficiency of capturing barium ions in a liquid xenon detector.