Enriched Xenon Observatory
The Enriched Xenon Observatory (EXO) is a particle physics experiment searching for neutrinoless double beta decay of xenon-136. The experiment uses a large amount of xenon, isotopically enriched in xenon-136. This isotope undergoes ordinary double beta decay (with the emission of two neutrinos) to barium-136.
If neutrinoless double beta decay is detected for the first time, it will be 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 the neutrinoless double beta decay modes of xenon-136, but the two neutrino decay has been observed, with a halflife of 2.11 ± 0.04 (stat.) ± 0.21 (sys.) × 1021 years. 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 ≤ 0.01 eV, it answers the question of the ordering of neutrino masses. While the differences between neutrino masses are 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 ≤ 0.01 eV 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 was 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 was expected to continue until 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.
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. Besides the Standard Model process of 2 neutrino double decay, if neutrinos are Majorana fermions (i.e are their own antiparticle; non Standard Model because lepton number would be violated by 2.) 136Xe would be able to decay releasing only beta particles. The rate of this decay can then be used to calculate the Majorana neutrino mass. In July 2012 the EXO-200 limit on this decay was 1.6 × 1025 years, which relates to an upper limit on the neutrino mass of 140–380 meV, depending which nuclear matrix element is used.
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.
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