Enriched Xenon Observatory
Coordinates: 32°22′18″N 103°47′37″W / 32.37167°N 103.79361°W
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. These are important open topics in particle physics.
EXO currently has a 200-kilogram xenon liquid time projection chamber with R&D efforts on a ton-scale experiment. Xenon double beta decay was detected and limits have been set for 0νββ.
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. A limit on effective neutrino mass below 0.01 eV, would determine the neutrino mass order. The effective neutrino mass is dependent on the lightest neutrino mass in such a way that that bound indicates the normal mass hierarchy.
The expected rate of 0νββ events is very rare, so background radiation is a significant problem. WIPP has 650 metres (2,130 ft) of rock overburden—equivalent to 1,600 metres (5,200 ft) of water—to screen incoming cosmic rays. Lead shielding and a cryostat also protect the setup. The neutrinoless decays would appear as narrow spike in the energy spectrum around the xenon Q-value (Qββ = 2457.8 keV), which is fairly high and above most gamma decays.
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