Enriched Xenon Observatory
The Enriched Xenon Observatory (EXO) is a particle physics experiment searching for neutrinoless double beta decay of xenon-136 at WIPP. Neutrinoless double beta decay(0νββ) detection would prove the Majorana nature of neutrinos and impact neutrino masses.
EXO measures the rate of neutrinoless decay events above the expected background of similar signals, to state or limit the half-life, and is converted to an effective neutrino mass using nuclear matrix elements. A limit on effective neutrino mass ≤ 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 a limit ≤ 0.01 eV indicates the neutrino masses lie in the normal hierarchy.
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.
The prototype EXO-200 uses a cylindrical time projection chamber designed to gather information about the decay. Xenon is a scintillator, so the prompt light provides time information of the event. A large electric field is set up to drive ionization electrons to wires for their collection. The difference in time between the light and the first ionization collection determines the z coordinate of the event, while a grid of wires determines the radial and angular coordinates. Scintillation light is collected by avalanche photodiodes.
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 2010.
In August 2011, EXO-200 was the first experiment to observe double beta decay of 136Xe, with a half life of 2.11 × 1021 years. This is the slowest directly observed process. An improved half life of 2.165 +- 0.016 (stat) +- 0.059 (sys) x 1021 years was published in 2014. In July 2012, EXO-200 set a limit
0νββ of 1.6 × 1025 years, with an upper limit on the neutrino mass of 140–380 meV, depending on nuclear matrix elements.
A ton-scale experiment, nEXO ("next EXO"), must overcome many backgrounds. The EXO collaboration is exploring many possibilities to do so, including barium tagging in liquid xenon. Any double beta decay event will leave behind a daughter barium ion, while backgrounds, such as radioactive impurities or neutrons, will not. Requiring a barium ion to be present at the location of the event eliminates all backgrounds. Tagging of a single ion of barium has been demonstrated and progress has been made on a method for extracting ions out of the liquid xenon. One method is using a probe that freezes a layer of xenon, containing the ion, onto its tip. Tagging of barium in gaseous xenon is also being developed.
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- "EXO project equipment successfully placed underground at WIPP" (Press release). DOENews. 24 July 2007.
- N. Ackerman et al. (2011). "Observation of Two-Neutrino Double-Beta Decay in 136Xe with EXO-200". Physical Review Letters 107 (21): 212501. arXiv:1108.4193. Bibcode:2011PhRvL.107u2501A. doi:10.1103/PhysRevLett.107.212501.
- "An improved measurement of the 2νββ half-life of Xe-136 with EXO-200". Phys. Rev. C 89, 015502 (2014). 2014. doi:10.1103/PhysRevC.89.015502.
- M. Auger et al. (2012). "Search for Neutrinoless Double-Beta Decay in 136Xe with EXO-200". Physical Review Letters 109 (3): 032505. arXiv:1205.5608. Bibcode:2012PhRvL.109c2505A. doi:10.1103/PhysRevLett.109.032505.
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