The EXO collaboration is a low background particle physics experiment that searches for the neutrinoless double beta decay of 136Xe. We are currently operating the EXO-200 prototype detector, designing the ton-scale nEXO detector, and performing R&D to develop the capability to identify the daughter 136Ba ion in coincidence with a 136Xe double beta decay. Experimental results from the EXO-200 detector include:
The discovery of the 2νββ decay mode of 136Xe
Constraints on the effective neutrino Majorana masses of less than 140-380 meV.
The most precise measurement of the 2νββ of any isotope.
Double Beta Decay
The decay of certain even-even nuclei via two-neutrino double beta decay is a special case of beta decay, the more familiar one being single beta decay. The process, first proposed by Maria Goeppert-Mayer, is allowed within the standard model of particle physics, occuring when the single-beta decay of the isotope is energetically or spin forbidden. Like with 136Xe...
In two neutrino double beta decay, two electrons and two antineutrinos are emitted from the parent nucleus. The very long half-lives of these decays (~1020 for many) make them incredibly difficult to measure, having been directly observed for only 9 isotopes.
Neutrinoless double beta decay is an even more special, and hypothetical, form of the decay. In this decay, no neutrinos escape the nucleus; a phenomenon which is only possible if neutrinos are also their own anti-particles. Such a neutrino is called a majorana particle, after Ettore Majorana who first posited the possibility. If neutrinoless double beta decay were observed it would be a discovery of the violation of lepton number, one of the last surviving exact global conservation laws in particle physics. It would also indicate the discovery of the 2-component Majorana fermion, and set the neutrino mass scale.
Here's a great article from physicsworld.com.
We search for neutrinoless double beta decay by building large radiation detectors that serve as both the source of the decay (136Xe) and the medium with which the emitted betas interact. EXO-200 is an extraordinarily low background detector that contains 200 kgs of liquid xenon enriched to 81% in 136Xe. Radioactive backgrounds are controlled by using screened materials that are known to have low intrinsic radioativity (copper, Teflon, phosphorbronze, acrylic, etc.) and assembling the parts in a class 100 clean room (see below).
The clean rooms are located 2,150 ft underground in the WIPP (the Waste Isolation Pilot Plant) salt mine, outside of Carlsbad, NM, to help shield the experiment from cosmic-induced radiation.
The liquid is instrumented so that both the ionization electrons and scintillation photons produced by the interaction are recorded. The detector is a set of two time projection chambers (TPCs), where the time difference between the measurement of the scintillation energy and the arrival time of the charge that has been drifted to the edge of the volume provides the event location and topology of an interaction. Below is one of the TPCs. Many more pictures can be found here.
In EXO-200, the energy of an event is primarily split between the scintillation and ionization channels. The energy resolution of the detector is improved by measuring both channels, which are anti-correlated in liquid xenon detectors. This provides a more precise measurement of the event energy, which in turn improves sensitivity of the experiment.
Over the next few years, we will be continiuing to take low-background data with EXO-200. We are improving some characteristics of the detector that might help with identifying the topology of events, or reducing certain dominant backgrounds in order to improve our experimental reach beyond what was originally planned...so stay tuned!
In addition to continuing work on EXO-200, we are also looking ahead to the next step. What is the next step? We call it nEXO, a ton scale experiment that builds off of the many successes of the EXO-200 prototype detector. The idea is to begin the nEXO experiment initially as a scaled-up version of EXO-200, but to build it with the capability to tag the 136Ba daughter ion in coincidence with the 136Xe double beta decay.
Ba-tagging opens up an extraordinary, paradigm shifting possibility. That is, the capability to require a coincidence between any radiation event occuring in the detector and the identification of the decay daughter. The experiment would be able to perform a search for neutrinless double beta decay that was essentially free of any background. Extensive R&D is ongoing within the collaboration towards the development of nEXO and towards a Ba-tagging capability.