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The Enriched Xenon Observatory (EXO) is a particle physics experiment searching for double beta decay of xenon-136. The experiment uses a large amount of xenon, isotopically enriched in xenon-136. This isotope is theorized to undergo ordinary double beta decay (with the emission of two neutrinos) to barium-136, though past experiments have only placed limits on the half-life.
If neutrinoless double beta decay is detected for the first time, it will be definitive proof of the Majorana nature of neutrinos. EXO intends to measure the effective Majorana neutrino mass (if it exists) with a sensitivity close to 0.01 eV. The actual measurement will be the rate of events, which is equivalent to a measurement of the half-life. Currently only lower limits exist for both the 2-neutrino and neutrinoless double beta decay modes of xenon-136. Observation of the 2-neutrino mode does not provide information about neutrinos, though it is interesting for nuclear theory. Measurement of the half-life of the neutrinoless mode can be converted to an effective neutrino mass using calculated nuclear matrix elements. If the neutrinoless mode is not seen, a lower limit can be placed on the half life, which corresponds to an upper limit on the neutrino mass.
If a limit on the effective neutrino mass is placed at the 0.01 eV mass range, it answers the question of the ordering of neutrino masses. While the differences between neutrino masses is known, it is not known which neutrino is the heaviest. The effective neutrino mass is dependent on the lightest neutrino mass in such a way that a limit at the 0.01 eV level indicates the neutrino masses lie in the normal hierarchy.
EXO currently consists of two facets: a 200-kilogram liquid time projection chamber dubbed "EXO-200" and R&D efforts into a ton-scale xenon experiment. While EXO-200 serves as a testing ground for liquid xenon techniques, the ton-scale experiment may take a different form.
EXO-200 uses a cylindrical time projection chamber design in order 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 has been designed with a goal of less than 40 events per year within two standard deviations from the expected energy. In order to accomplish this, all materials were selected and screened based on radiopurity. Originally the vessel was to be made of Teflon, but the final design of the vessel uses thin, ultra-pure copper.
The relocation of EXO-200 from Stanford to WIPP began in the summer of 2007. Further assembly and commissioning is expected to continue to the end of 2009 with data taking beginning in 2010. Photos of the EXO-200 laboratory and cryostat installed underground at the WIPP site are shown here.
A ton-scale experiment 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.
^ M. Danilov et al. (2000). "Detection of very small neutrino masses in double-beta decay using laser tagging". Physics Letters B 480: 12. arXiv:hep-ex/0002003. Bibcode 2000PhLB..480...12D. doi:10.1016/S0370-2693(00)00404-4.