Development of a Cryogenic Probe for Barium-Tagging in Neutrinoless Double Beta Decay Searches

Neutrinoless double beta decay (0νββ) is a hypothetical radioactive process which if observed, would prove that the electron neutrino is a Majorana particle (its own antiparticle) and constitute the first observation of lepton number violation, revealing physics beyond the Standard Model. The nEXO experiment aims to observe this decay using five tons of liquid xenon enriched with the isotope Xe-136. A proposed upgrade known as barium tagging would identify the barium daughter nucleus from the Xe-136 decay, eliminating nearly all background events and enhancing experimental sensitivity.

At UCSD, we are developing a cryogenic probe to capture and extract barium ions in a solid xenon matrix for fluorescence spectroscopy identification. The probe employs an interdigitated electrode (IDE) capacitance sensor to control and monitor xenon ice thickness with 3 μm precision. Preliminary tests in gaseous xenon demonstrate stable ice growth and capacitance behavior consistent with xenon’s phase transitions, validating sensor performance. Ongoing work includes testing in liquid xenon and future deployment at a radioactive ion beam facility to measure barium capture efficiency.