Enriched Xenon Observatory

Coordinates: 32°22′18″N 103°47′37″W / 32.37167°N 103.79361°W / 32.37167; -103.79361The EXO-200 cryostat installed underground at WIPP.The EXO-200 cleanrooms installed underground at WIPP. Coordinates: 32°22′18″N 103°47′37″W / 32.37167°N 103.79361°W / 32.37167; -103.79361 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 (EXO-200) with R&D efforts on a ton-scale experiment (nEXO). 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 low, 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. EXO-200 was designed with a goal of less than 40 events per year within two standard deviations of expected decay energy. This background was achieved by selecting and screening all materials for radiopurity. Originally the vessel was to be made of Teflon, but the final design of the vessel uses thin, ultra-pure copper. EXO-200 was relocated from Stanford to WIPP in the summer of 2007. Assembly and commissioning continued until the end of 2009 with data taking beginning in May 2011. Calibration was done using 228Th, 137Cs, and 60Co gamma sources. 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. A large electric field drives ionization electrons to wires for collection. The time between the light and first collection determines the z coordinate of the event, while a grid of wires determines the radial and angular coordinates. The background from earth radioactivity(Th/U) and 137Xe contamination led to ≈2×10−3 counts/(keV·kg·yr) in the detector. Energy resolution near Qββ of 1.53% was achieved.