About the Experiment
EXO is an experiment looking for neutrinoless double beta decay in the 136 isotope of xenon. The experiment currently consists of two facets:

1. EXO-200, a 200-kilogram prototype experiment currently operating at WIPP. It has measured for the first time the two-neutrino mode of double beta decay of Xenon 136. It has also set the most stringent limit on the rate of neutrinoless double beta decay. It continues to collect data in order to improve on this limit or potentially discover the decay.
2. nEXO, ("next EXO"), a tonne scale experiment using Xenon 136 to search for neutrinoless double beta decay. The collaboration is undergoing extensive R&D to design the xenon detector and a way to "tag" the barium daughter ion produced by the decay in order to eliminate all backgrounds.

There are many advantages to using a noble element, specifically xenon. It is relatively easy to purify the LXe, which allows it to be reused in different detectors. The 136 isotope can be enriched using the same centrifuge techniques used for fissile nuclear isotopes, which makes processing large quantities feasible, while putting the centrifuges to peaceful use. The xenon 136 Q-value — the energy of the decay — is 2.48 MeV, which is high enough to be above many of the uranium gamma lines. Gamma rays from naturally-occurring radioactive isotopes are a background that can make the decay we're interested in difficult or impossible to detect. Nobel liquids like LXe are natural radiation detectors, and so we avoid the need for excess materials that could generate extra radioactive backgrounds. Furthermore, we are able to achieve great energy resolution through collecting both ionization electrons and scintillation light from the xenon. Finally, xenon potentially allows for complete background rejection through tagging of the daughter Barium ion. This unique property motivates much of the work done by our collaboration.

Our Collaboration
The project involves scientists from

• University of Alabama
• Universität Bern
• Caltech
• Carleton University
• Colorado State University
• Drexel University
• Duke University
• IHEP
• Indiana University
• UC Irvine
• Institute of Basic Science (Republic of Korea)
• ITEP (Moscow)
• Laurentian University
• McGill University
• University of Erlangen-Nuremberg
• University of Illinois at Urbana-Champaign
• University of Maryland
• University of Massachusetts - Amherst
• University of North Carolina Wilmington
• University of South Dakota
• SLAC National Accelerator Laboraotory
• Stanford University
• Stony Brook University
• Technische Universität München
• TRIUMF
• Yale University

Neutrinoless Double Beta Decay
Neutrinoless double beta decay is a special case of beta decay. Beta decay is a common form of nuclear decay which occurs when a neutron in an unstable nucleus emits an electron and an antineutrino and becomes a proton.

Double beta decay occurs when a nucleus is energetically or spin forbidden to decay through single beta decay. While it has been predicted to exist for a long time, the long half-life makes it difficult to observe. Double beta decay was only first observed in 1986. Many isotopes undergo double beta decay, including xenon 136. EXO-200 was the first experiment to observe this decay in xenon. In normal double beta decay, two electrons and two antineutrinos are ejected from the nucleus when two neutrons become protons. The half-lives of double beta decay isotopes are very long, above 10²⁰ years. This is more than a billion times longer than the age of the universe! So if you started with 8 billion atoms that can undergo double beta decay at the beginning of the universe, you would expect about 1 to have decayed by now.

Neutrinoless double beta decay has not yet been seen, but some theories predict it. It is like normal double beta decay, but because of special properties of the neutrino, no neutrinos would be emitted from the nucleus. The electrons would carry all the energy of the decay, unlike normal double beta decay, in which the antineutrinos carry away energy. Thus, neutrinoless double beta decay has a unique, observable signature. In order for this to occur, the neutrino would have to be its own antiparticle. If neutrinos are their own antiparticles, which are known as "Majorana" particles, then this admits many elegant theories to explain how neutrinos acquire mass and why their mass is so much smaller than any other particle we know.

We want to see neutrinoless double beta decay for two reasons. First, we don't know if the neutrino is its own antiparticle or not, and seeing it would answer this question for sure. Second, we don't know the exact mass of the neutrino and a measurement of the neutrinoless double beta decay half life would allow us to measure the neutrino mass. Even if we don't see neutrinoless double beta decay, a limit on the half life places a limit on the neutrino mass.

If you're still curious, Physics World has a nice article on neutrinoless double beta decay that is more detailed, but still accessible to a lay person.

EXO-200
EXO-200 is a prototype to develop techniques for working with liquid xenon in a time projection chamber (TPC). One possibility for tonne-scale EXO is a liquid TPC, so familiarity with EXO-200 technologies will contribute to the design of tonne-scale EXO. Additionally, EXO-200 provides a testing ground for developing and procuring extremely radiopure materials and removing backgrounds. EXO-200 has provided fundamental measurement of the double beta decay of xenon 136 and will provide improved limits on the rate of (or perhaps observation of) neutrinoless double beta decay.

We are using 200 kg of liquid xenon (LXe) enriched to 80% of the 136 isotope for EXO-200. The LXe fills our TPC vessel. When a particle deposits energy in the liquid xenon, it ionizes the xenon atoms, knocking electrons off. We apply an electric field to the xenon, which pushes many of the electrons to wire grids where they are collected. The grid position provides a 2D location, and the number of electrons is related to the event's energy. But some xenon ions recombine with the electrons before they can drift away. This puts the xenon atoms into excited states. When the excited atoms relax, they release ultraviolet light, known as scintillation, which we collect on avalanche photodiodes (APDs). The time between the light signal (which comes nearly instantaneously) and the ionization signal (which must drift and takes microsecond to arrive) allows us to reconstruct the full 3D location of the event when combined with the 2D position from the wire grids. Furthermore, the amount of light is also related to the event's energy. Combining the ionization and light signals allows a better energy menasurement than using either signal on its own.

The TPC vessel is contained within a cryostat system to help keep the xenon at liquid temperature. The vessel is contained in a volume of HFE-7000, a synthetic fluid that is liquid from room temperature down to LXe temperatures. The HFE is within a large copper cryostat, which is then inside another coper cryostat with a vacuum gap in between for insulation. The cryostat is shielded with lead and contained in a class-100 cleanroom located 2150 ft underground at the Department of Energy's Waste Isolation Pilot Plant. All of this is necessary to shield from radioactive backgrounds and cosmic rays. On top of that, materials contained within the lead have been extensively counted for radiopurity. The materials are low in radioactive isotopes and contamination. The majority of the material is ultrapur, copper, teflon, phosphorbronze, and acrylic.