Inside an active salt mine in southeastern New Mexico, about 700 meters below the desert surface, a stainless-steel cryostat held 200 kilograms of liquid xenon at a temperature of about 170 K. The xenon — enriched to 81% in the isotope xenon-136 — was the heart of the EXO-200 experiment (the Enriched Xenon Observatory), one of the leading neutrinoless double-beta decay searches of the 2010s.
EXO-200 ran from 2011 to 2018, setting some of the most stringent limits on the 0νββ half-life of xenon-136 ever achieved. Its planned successor, nEXO, will scale the same approach to five tons of xenon and push the sensitivity by another order of magnitude. Together they represent one of the three major 0νββ programs of the next decade, alongside LEGEND using germanium and KamLAND-Zen using xenon in liquid scintillator.
Why liquid xenon
Liquid xenon has a remarkable combination of properties that make it attractive for rare-event searches. It is dense (about 3 grams per cubic centimeter), making it efficient at converting ionizing radiation to detectable signal. It is transparent to its own scintillation light, which allows photodetectors to be placed at the edges of large volumes and still see signals from the interior. It is easy to purify chemically, so trace radioactive contaminants can be removed to extraordinary levels. And it scintillates and ionizes in known proportions, allowing dual-signal readout for precise event characterization.
The same properties have made xenon the dominant target for dark matter direct detection (XENONnT, LZ, PandaX) and a leading choice for 0νββ. The technique is being developed in parallel in both communities, with substantial cross-fertilization.
For 0νββ specifically, xenon-136 has the additional advantage of a high Q-value of about 2,458 keV. The 0νββ signal would appear as a sharp spectral line at this energy, well above the bulk of natural radioactive backgrounds (most of which peak below 2 MeV). The result is a cleaner signal region than some other 0νββ isotopes provide.
How the EXO-200 detector worked
EXO-200 used a single-phase liquid xenon time projection chamber (TPC) — meaning the entire detector volume was filled with liquid xenon, with no gaseous phase above it.
When a charged particle deposits energy in the xenon (from a 0νββ event, an environmental gamma ray, or other source), it produces both:
Prompt scintillation light in the ultraviolet, recorded by an array of large-area avalanche photodiodes (LAAPDs) placed at the ends of the cylindrical detector volume.
Ionization charge, which drifts toward wire grids under the influence of a uniform electric field. The drift takes microseconds; the resulting signal on the wire grids encodes the event’s energy and three-dimensional position.
The combination of the two signals gave EXO-200 several useful capabilities:
Energy resolution. The combined scintillation-plus-ionization energy measurement achieved a resolution of about 1.4% at the 0νββ region of interest — competitive with other techniques.
Position reconstruction. The arrival time of ionization on the wire grids gave the z-coordinate; the x-y coordinate came from the pattern of charge collection on the grids. Total position resolution was on the order of a centimeter.
Single-site versus multi-site discrimination. A 0νββ event deposits its energy in a single small region (the two emitted electrons travel only a few millimeters). A background gamma ray, by contrast, typically deposits energy in multiple separated locations as it Compton-scatters through the detector. The pattern of charge collection distinguishes the two cases, providing a powerful background rejection tool.
What EXO-200 measured
Across approximately seven years of physics running, EXO-200 accumulated about 100 kilogram-years of fiducial exposure. The 0νββ analysis found no significant signal above expected backgrounds.
The published lower limit on the 0νββ half-life of xenon-136 was:
T₁/₂(0νββ) > 3.5 × 10²⁵ years (at 90% confidence)
This translates to an effective Majorana neutrino mass upper limit of roughly 93-286 meV, depending on which nuclear matrix element calculation is used.
In addition to the headline 0νββ result, EXO-200 made several other notable measurements.
Two-neutrino double-beta decay. The much-longer-lived two-neutrino double-beta decay of xenon-136 (a Standard Model-allowed process that EXO-200 routinely detected as a “calibration” background) was measured with a half-life of 2.2 × 10²¹ years — a precision measurement that has fed back into nuclear-physics calculations relevant to the 0νββ predictions.
Tests of physics beyond the Standard Model. The data was analyzed for evidence of exotic 0νββ modes (Majoron emission, right-handed currents, etc.). No signals were found, providing competitive constraints on a range of beyond-Standard-Model scenarios.
Lorentz invariance tests. The detector’s stability over years of running allowed sensitive tests of certain Lorentz-violating effects in double-beta decay kinematics.
Why the WIPP site
The Waste Isolation Pilot Plant in New Mexico is, unusually for an underground physics laboratory, a working radioactive-waste repository. The salt formations 700 meters below the surface are used by the US Department of Energy for the disposal of transuranic waste from defense activities.
The unusual choice of host for a 0νββ experiment was driven by practical considerations. The salt overburden provides excellent radiation shielding. The DOE-operated facility has substantial infrastructure for working underground. And the salt itself is geologically stable, making for a clean laboratory environment.
The location also had drawbacks. In February 2014, a small radiological release accident at WIPP unrelated to EXO-200 forced the closure of the underground facility for over a year. EXO-200 had to suspend operations during this period and lost approximately a year of physics running.
EXO-200 eventually completed its data-taking program in 2018, and the experiment was decommissioned. The enriched xenon was recovered for use in the successor experiment.
What nEXO will do
The next-generation experiment, nEXO, scales the EXO-200 design by a factor of about 25 in mass — from 200 kilograms to about 5 tons of enriched xenon-136. The basic approach is the same: single-phase liquid xenon TPC with dual scintillation-and-ionization readout.
Several design improvements push the sensitivity well beyond what scale-up alone would provide.
Larger fiducial volume. A larger TPC has a much larger central region where external gamma-ray backgrounds are negligible. The signal-to-background ratio in the inner volume improves dramatically.
Improved photodetectors. Silicon photomultipliers replace the avalanche photodiodes used in EXO-200, providing better light collection and improved energy resolution. The target resolution is about 0.8%, roughly halving EXO-200’s.
Better materials. A decade of materials science development has produced ultra-low-radioactivity components — cables, connectors, structural materials — that further suppress backgrounds in the signal region.
SNOLAB location. nEXO will be deployed at SNOLAB in Sudbury, Canada — a 2-kilometer-deep facility with very low cosmic-ray background and substantial existing infrastructure for low-background physics. The same underground laboratory hosts the SNO+ experiment (the successor to SNO) and several other low-background efforts.
The target sensitivity is roughly T₁/₂(0νββ) > 10²⁸ years — about 30 times better than EXO-200 and into the regime where, if the mass ordering is inverted and neutrinos are Majorana, a discovery is essentially guaranteed.
How nEXO fits into the broader program
Three major 0νββ experiments are now in advanced planning:
- nEXO (xenon-136 TPC at SNOLAB)
- LEGEND-1000 (germanium-76 detectors at Gran Sasso and SNOLAB)
- CUPID (molybdenum-100 bolometers at Gran Sasso, succeeding CUORE)
Each uses a different isotope with different nuclear physics, different experimental techniques, and different systematic uncertainties. The three experiments are sometimes called the “ton-scale” 0νββ program because each targets approximately a ton of enriched isotope.
If any one of them sees a positive 0νββ signal, the others would be essential cross-checks. A signal in xenon but not germanium would be deeply suspicious; a signal in all three at consistent rates would essentially settle the question.
If none sees a signal by the early 2030s, the inverted-ordering Majorana scenario will be excluded by a strong combined limit. The implications for theory would be substantial: either the mass ordering is normal, or neutrinos are Dirac rather than Majorana, or the see-saw picture is wrong, or some combination of these. Each possibility points in different theoretical directions.
A long arc to a single sentence
EXO-200 and nEXO illustrate, in many ways, what modern rare-event physics looks like. A decade-long experimental program produces a single sentence — an upper limit on a half-life — that constrains a fundamental property of the neutrino. The successor experiment runs for another decade and pushes the limit by an order of magnitude. With luck, eventually, an experiment in this lineage will see a real signal and produce a different kind of sentence: not an upper limit but a measurement of an actual decay rate.
That signal, if it comes, will tell us that neutrinos are their own antiparticles. It will reshape particle physics. And it will validate the patience and engineering of three or four generations of physicists who built the detectors and ran them long enough for the answer to emerge.
The waiting continues. The tank is being filled.
For the broader 0νββ context, see The hunt for neutrinoless double-beta decay. For other approaches with different isotopes, see CUORE. For why this matters fundamentally, see Majorana or Dirac?.
Frequently asked
What is EXO-200?
EXO-200 was a neutrinoless double-beta decay experiment that operated in the Waste Isolation Pilot Plant (WIPP) underground laboratory in New Mexico from 2011 to 2018. It used about 200 kilograms of liquid xenon enriched to 81% in xenon-136 as both the target and the detector medium. The detector was a single-phase time projection chamber instrumented to record both ionization charge and scintillation light from each event.
Why xenon-136?
Xenon-136 is one of the leading candidate isotopes for 0νββ searches. It has a relatively high natural abundance (about 9% of natural xenon, easily enrichable), a high Q-value of 2,458 keV (above many common backgrounds), and well-understood nuclear physics. Liquid xenon also has excellent properties as a detector medium — it scintillates, ionizes, and is dense, allowing the same material to serve as both target and detector.
What did EXO-200 find?
EXO-200 set a lower limit on the 0νββ half-life of xenon-136 of about 3.5 × 10²⁵ years. The result was competitive with other 0νββ experiments at the time and contributed to the global constraints on Majorana neutrino mass. The experiment also published one of the first measurements of the much-longer-lived two-neutrino double-beta decay of xenon-136 at high precision.
What is nEXO?
nEXO is the proposed successor to EXO-200. It will use about 5 tons of enriched xenon-136 in a much larger single-phase TPC, with target sensitivity to half-lives around 10²⁸ years — about 30 times better than EXO-200. nEXO is currently planned for deployment in SNOLAB in Canada, with construction expected to begin in the late 2020s.
How does nEXO compare to other 0νββ experiments?
Three major 0νββ programs are now in advanced planning: nEXO (xenon TPC), LEGEND-1000 (germanium semiconductor), and KamLAND2-Zen (xenon-loaded liquid scintillator). All three aim for sensitivity to half-lives in the 10²⁷-10²⁸ year range — well into the regime needed to fully test the inverted-ordering Majorana scenario. Each uses a different detection technology, providing independent cross-checks if a discovery is made.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, October 9). EXO-200 and nEXO: hunting double-beta decay in a tank of liquid xenon. Neutrino Times. https://neutrino-times.com/articles/exo-200-nexo-xenon-tpc-double-beta-decay/
Chicago
Neutrino Times Editorial Team. "EXO-200 and nEXO: hunting double-beta decay in a tank of liquid xenon." Neutrino Times, October 9, 2025. https://neutrino-times.com/articles/exo-200-nexo-xenon-tpc-double-beta-decay/.
MLA
Neutrino Times Editorial Team. "EXO-200 and nEXO: hunting double-beta decay in a tank of liquid xenon." Neutrino Times, 9 Oct. 2025, https://neutrino-times.com/articles/exo-200-nexo-xenon-tpc-double-beta-decay/.
BibTeX
@misc{neutrino-times-exo-200-nexo-xenon-tpc-double-beta-decay,
author = {Neutrino Times Editorial Team},
title = {EXO-200 and nEXO: hunting double-beta decay in a tank of liquid xenon},
howpublished = {Neutrino Times},
year = {2025},
month = {oct},
url = {https://neutrino-times.com/articles/exo-200-nexo-xenon-tpc-double-beta-decay/},
note = {Accessed: 2025-10-09}
} RIS
TY - GEN TI - EXO-200 and nEXO: hunting double-beta decay in a tank of liquid xenon AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-10-09 PB - Neutrino Times UR - https://neutrino-times.com/articles/exo-200-nexo-xenon-tpc-double-beta-decay/ ER -