nEXO: Xenon-136 based 0νββ-decay detector
The Search for Neutrinoless Double Beta Decay
Countless experiments have detected decays of all types. It’s one of the key ways that scientists can explore the world of the very small. Beta decay has been well documented since it’s first detection in 1899 by Ernest Rutherford. A more similar reaction, double beta decay, has been hypothesized since Maria Goeppert-Mayer in 1935, though it would take 52 years for it to be first found experimentally by Michael Moe. One reason this finding took so long is that fact that all known half-lives of double beta decay are very long, around of years. An even more rare reaction, the neutrinoless double beta decay, has yet to be observed though a tens of experiments (either operational or in the works) are working to explore this frontier.
The current generation of 0νββ-decay detectors are based around 8 different isotopes which all theoretically have channels for this decay. These detectors, such as CANDLES, KamLAND-Zen and SNO+, have yet to yield a confirmed observation of the decay, though the search has greatly improved the data set on 2νββ-decays. Below is a table compiled by Werner Maneschg in March of 2015 of the established experiments in this field.
Of all the isotopes listed above, the EXO-200 detector uses due to three characteristics which make it preferential experimentally. Xenon is liquid in most standard pressures and temperatures which makes it easy to purify though centrifuges and similar technologies, large quantities of Xenon are well suited to self-shielding from outside sources, and has a high Q-value for the 0νββ-decay which makes it easier to isolate, though a low noise environment is still necessary.
Where we’re at: EXO-200
EXO-200 uses scintillation channels, and two planes of wires on both plane sides its cylindrical chamber to produce a time projection chamber(TPC). The use of both technologies allows for the recreation of the paths of energetic charged particles through the 140 kg liquid xenon volume. The cylinder is divided in the middle by a large cathode which induces electron drift currents towards the annodes at either side, placed on the far side of the wire planes. The inner walls of the chamber are highly reflective to ensure that the scintillation signal traverses the chamber to the two sets of Large Area Avalanche Photodiodes(LAAPDs), located by the annodes. This TPC is well suited for measurements of events like what is expected for 0νββ-decay. The initial decay produces two energetic electrons and gamma radiation, the energetic electrons quickly collide with other xenon atoms nearby exciting and releasing more electrons and photons. The timing of these photons are collected by the LAAPDS to help recreate when events happened. Once the electrons have multiplied a substantial amount from exciting other electrons, their kinetic energy will drop off to the point where they are non-interacting with the xenon atoms and simply drift to the wire planes creating the traceable ionization path. One of the key components in the success of this experiment is noise and cleanliness. All the materials used in designing these innermost elements of the detector have been specifically chosen to minimize any and all radioactive, and all parts were specially handled and treated to minimize surface contaminants. When the half-life of the reaction is years in , there can be almost no erroneous signals in the data or a positive observation will never be seen.
EXO-200 began collecting data September 2011, and has already collected two years worth of data, due to an accident which happened at the underground facility WIPP where the experiment is being housed. As of yet, there is no observation of 0νββ-decay, however it has measured the half-life of the 2νββ-decay as being years. It has also set and lower limit of years on 0νββ-decay for which it is designed. This gives an upper limit on the mass of the Majorana neutrino, 190-450 meV . Listed below are several papers about the EXO-200 detector and results.
Whats next: nEXO
The next step, after EXO-200 is to build a detector bigger and better. Taking much of what was learned from the construction and operation of EXO-200, and further R&D performed by groups attached to the EXO collaboration, nEXO aims to have a sensitivity orders of magnitude greater. One of the most straightforward ways to increase the sensitivity is to increase to magnitude of the experiment. Whereas EXO-200 had 200 kg of liquid xenon, the nEXO detector will have on the order of 5000 kg of liquid xenon. The design of the detector is still ongoing, but some key characteristics have been decided upon. First, the detector won’t be split into two drift field regions, but one continuous field with the cathode at the top of the detector, and a collection plane and anode at the bottom of the detector, with a field shaping cage encompassing the volume between. The photodiodes are under development, but will be made as planks which will create a 24 sided approximation of a cylinder which lie as close to the TPC vessel as possible to minimize the waste of liquid xenon. One key development which is expected to be included several years after the initial turn on of the nEXO detector is a barium tagging mechanism. This mechanism is expected to increase the sensitivity of the nEXO detector to the point of being able to rule out the inverted mass hierarchy.
Bibliography and References
 Maneschg, W. Review of neutrinoless double beta decay experiments: Present status and near future. Nuclear and Particle Physics Proceedings, 260, 188-193, 2015.
 Search for Majorana neutrinos with the first two … (2013, September 27). from http://arxiv.org/abs/1402.6956
 The EXO-200 detector, part I: Detector design and construction (2012, May 23). from http://http://arxiv.org/abs/1202.2192