EXO-200 is searching for the neutrinoless double beta decay of 136Xe. This is an extremely rare nuclear decay that can only occur if neutrinos are Majorana particles — i.e. if there is no fundamental difference between a neutrino and antineutrino. Observing neutrinoless double beta decay would provide direct evidence for new physics beyond the Standard Model of particle physics. It would also have implications for theories attempting to describe the nature of neutrinos, how neutrinos acquire their masses, and how the small excess of matter over antimatter was generated in the early universe.
If this decay exists, it is extremely rare. Current experiments imply that the half life must be longer than 1025 years! To see such a rare decay requires very large detectors, which contain 100s of kg of an isotope that can undergo this decay, such as 136Xe. In EXO-200, ~150 kg of liquid 136Xe is contained in a radiopure time projection chamber (TPC).
The detector is installed underground at the Waste Isolation Pilot Plant (WIPP) in Carlsbad, NM. By operating the experiment in a mine, the detector is shielded from interactions from cosmic rays that would completely overwhelm the signal from double beta decay if the detector were on the surface of the earth. In addition, the detector is made from radiopure materials, to eliminate any interactions from natural radioactivity.
Finding such low radioactivity materials is extremely challenging. Due to trace amounts of potassium, uranium, and thorium, most everyday materials are highly radioactive by these standards.
For example, a typical 0.1 kg banana produces ~15 radioactive decays a second. In EXO-200, we’re attempting to identify only a few decays from neutrinoless double beta decay in 100 kg of liquid Xe in a year!
EXO-200 took data at WIPP between 2011-2018. See the publications link below for EXO-200 results.