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

1. EXO-200, a 200-kilogram prototype experiment currently being installed and commissioned at WIPP. This will measure the - yet unobserved - two-neutrino mode of double beta decay of Xenon 136 and provide a competitive limit on neutrinoless double beta decay.
2. EXO - a ton scale experiment using Xenon 136 to search for neutrinoless double beta decay. The collaboration is undergoing extensive R&D to develop the Xenon detector and a way to "tag" the products of 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. It is relatively easy to enrich to the 136 isotope using Russian centrifuges. 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 lines. In using LXe as a scintillator in our detector we avoid needing additional materials and we are able to achieve great energy resolution through collecting both ionization electrons and scintillation light. Finally, using Xenon allows for complete background elimination through tagging of the daughter Barium ion.

Our Collaboration
The project involves scientists from

• University of Alabama
• Universität Bern
• Caltech
• Carleton University
• Colorado State University
• UC Irvine
• ITEP (Moscow)
• Laurentian University
• University of Maryland
• University of Massachusetts - Amherst
• SLAC
• Stanford

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. There are version of it where positrons or neutrinos are absorbed, or a proton becomes a neutron.

Double beta decay occurs when a nucleus is energetically forbidden to decay through single beta decay. While it has been predicted to exist for a long time, double beta decay was first observed in 1986. Many isotopes are theorized to undergo double beta decay, including Xenon 136. 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, about 10^20 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 is predicted to exist. It is like normal double beta decay, but because of special roperties of the neutrino, no neutrinos would be emitted from the nucleus. In order for this to occur, the neutrino must be its own antiparticle. Since the discovery that neutrinoes have a (very small) mass, it is thought that they are Majorona particles, which means they are their own antiparticle. Many particles have an antiparticle partner - the electron has the positron, for instance. The neutral pion is its own antiparticle.

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. EX0-200 may not see neutrinoless double beta decay, but it should be able to improve the limit on the half life.

EXO-200
EXO-200 is a prototype to develop techniques of working with liquid xenon in a time projection chamber (TPC). One possibility for ton-scale EXO is a liquid TPC, so familiarity with EXO-200 technologies will contribute to the design of ton-scale EXO. Additionally, EXO-200 provides a testing ground for developing and procuring extremely radiopure materials and removing backgrounds. EXO-200 will provide fundamental competitive science measurements, such as a measurement of the (not yet observed) double beta decay of Xenon 136 and an improved limit on 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 an event occurs the energetic electrons interacts with the LXe to create scintillation light that we detect with Avalanche Photodiodes (APD's). The energetic electrons also ionize some of the Xenon and the ionized electrons drift to charge collection wires at the ends of the vessel in an electric field. The time between the light pulse and the electrons reaching the wires tell us how far in the event occurred since we know the drift time.

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, a cryogenic buffer fluid. The HFE is within a large copper cryostat, which is then inside another coper cryostat with a vacuum gap in between. The cryostat is shielded with lead and contained in a class-100 cleanroom. This helps prevent radioactive backgrounds. All materials contained within the lead have been extensively counted for radiopurity. Materials have been chosen to be low in radioactive isotopes and contamination. The majority of the material is ultrapure copper, teflon, phosphorbronze, and acrylic.