Enriched Xenon Observatory
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The Enriched Xenon Observatory (EXO) is a proposed particle physics experiment designed to search for neutrinoless double beta decay of Xenon-136. The experiment will require a large amount of xenon, isotopically enriched in xenon-136. This isotope is known to undergo 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 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.
In order to search for neutrinoless double beta decay, the energy of the two electrons emitted in the decay must be measured to high precision. To separate genuine double beta decay events from background, the experiment intends to tag the resulting barium atom using laser spectroscopy.
Two possible designs are being considered for the EXO experiment:
- A high-pressure gaseous xenon experiment. The two electrons emitted in the double beta decay will be tracked and their energy deposition measured using time projection chamber techniques. The barium atom will be laser tagged in situ. The disadvantage is that the experiment will be quite large in order to contain enough mass of xenon in order to achieve interesting results in a reasonable amount of run time.
- A liquid xenon experiment, which has the advantage of being compact. The energy of the two electrons emitted in the double beta decay will be measured using ionization and scintillation. The disadvantage is that in order to laser tag the barium atom, it must first be physically extracted from the liquid xenon volume.
EXO is one of the candidate experiments to be sited deep underground in SNOLAB; the deep location would provide shielding from background events caused by cosmic rays.
At present, EXO is undergoing testing to determine the precision of its scintillation and ionization detection methods. In April a 200 kilogram version of the experiment without barium tagging, known as EXO-200, will be placed underground in the WIPP facility in New Mexico.
