EXO-200 & nEXO
Earlier work on the search for neutrinoless double beta decay of ¹³⁶Xe with liquid xenon time projection chambers: the EXO-200 experiment and its tonne-scale successor, nEXO.
EXO-200
EXO-200 was a single-phase liquid xenon time projection chamber, filled with xenon enriched in the isotope ¹³⁶Xe, that operated at the Waste Isolation Pilot Plant (WIPP) in New Mexico until 2018. By recording both the scintillation light and the ionization charge of each event, it achieved the energy resolution and background rejection needed to search for the neutrinoless double beta decay of ¹³⁶Xe.
The analysis of the complete dataset, corresponding to a ¹³⁶Xe exposure of 234.1 kg yr, found no evidence for the decay and set a lower limit on its half-life of 3.5 × 10²⁵ years (90% C.L.). The same data have been used for a broad range of other searches and measurements, including Majoron-emitting decay modes, double beta decay of ¹³⁴Xe, nucleon decay, and MeV-scale dark matter, as well as detailed studies of the response of liquid xenon. A subset of the calibration data has been released publicly for machine-learning development and education.
nEXO
nEXO is the proposed next-generation experiment: a time projection chamber holding 5000 kg of liquid xenon enriched in ¹³⁶Xe, with a projected half-life sensitivity of 1.35 × 10²⁸ years after ten years of data taking — enough to cover the parameter space of the inverted neutrino mass ordering.
A central element of the design is the detection of xenon’s 175 nm scintillation light with vacuum-ultraviolet-sensitive silicon photomultipliers. Characterizing these sensors — their photon detection efficiency, reflectivity, correlated noise, and behavior in strong electric fields — was a major focus of this work and carries over directly into our current detector R&D .