Neutrinos, the only neutral elementary fermions, have provided us with many surprises.  Flavor oscillations reveal the non-conservation of the lepton flavor number and demonstrate that, although neutrino masses are finite, yet they are surprisingly smaller than those of other fermions (by at least six orders of magnitude!). It is then natural to ask if the mechanism providing the mass to neutrinos is the same that gives masses to the other (charged) elementary particles and if neutrinos are described by 4-component Dirac wavefunctions or, as possible for neutral particles, by 2-component Majorana ones.

The hypothetical phenomenon of neutrino-less double-beta decay can probe the Majorana nature of neutrinos and the conservation of the overall lepton number.   It may also help elucidating the origins of mass in the neutrino sector.

Current limits on the half-lives for neutrino-less double-beta decay are in the 1025 -1026 years range (or 1015-1016 times the age of the Universe!) making experiments rather challenging.    I will describe nEXO, an experiment using 5 tonnes of Xenon enriched to 90% in the isotope 136 in a liquid phase TPC.  The nEXO detector uses a well-proven design, deriving from that of the very successful EXO-200, the first 100 kg-scale experiment to take data.  At the same time, a number of state-of-the-art technical solutions make the nEXO instrumentation quite novel and interesting in its own right.   The nEXO scientific collaboration and construction team includes over 200 scientists and engineers from 32 institutions and nine countries.

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