The Enriched Xenon Observatory (EXO) is a particle physics experiment searching for neutrinoless double beta decay of xenon-136 at WIPP near Carlsbad, New Mexico, U.S. Neutrinoless double beta decay (0νββ) detection would prove the Majorana nature of neutrinos and impact the neutrino mass values and ordering.
EXO currently has a 200-kilogram xenon liquid time projection chamber (EXO-200) with R&D efforts on a ton-scale experiment (nEXO).
EXO measures the rate of neutrinoless decay events above the expected background of similar signals, to find or limit the double beta decay half-life, which relates to the effective neutrino mass using nuclear matrix elements.
The prototype EXO-200 uses a copper cylindrical time projection chamber filled with 150 kilograms (331 lb) of pure liquid xenon.
Xenon is a scintillator, so decay particles produce prompt light which is detected by avalanche photodiodes, providing the event time.
[7] A revised analysis of run 2 data with 100 kg·yr exposure, reported in the June issue of Nature reduced the limits on half-life to 1.1×1025 yr, and mass to 450 meV.
No evidence of neutrinoless double beta decay has been found in the combined Phase I and II data, giving the lower bound of
Any double beta decay event will leave behind a daughter barium ion, while backgrounds, such as radioactive impurities or neutrons, will not.
Diameter would be increased to 130 cm and a water tank would be added as shielding and muon veto.
[10]: 17 [11]: 7 An Oct. 2017 paper details the experiment and discusses the sensitivity and the discovery potential of nEXO for neutrinoless double beta decay.