In a former railway tunnel through the Spanish Pyrenees, an unusual experiment is hunting for one of the rarest processes that physics might allow. The NEXT-100 detector — for “Neutrino Experiment with a Xenon Time Projection Chamber” — uses 100 kilograms of gaseous xenon-136 to search for neutrinoless double-beta decay.
The choice of gas phase rather than liquid is unusual. nEXO, the leading xenon-based 0νββ program, uses liquid xenon at ~170 K. KamLAND-Zen dissolves xenon in liquid scintillator. NEXT, by contrast, keeps the xenon in gas phase at about 15 bar of pressure.
The trade-off is straightforward. Gas xenon has about 1/3000 the density of liquid xenon. To get the same number of xenon atoms, you need a vastly larger volume. NEXT-100’s 100 kilograms of xenon fills about 5 cubic meters of pressure vessel, while the same isotopic mass of liquid xenon would fit in a small bathtub.
In return, gas xenon provides something liquid xenon cannot: the ability to image the actual particle tracks produced by candidate 0νββ events. This is a powerful background-rejection tool that no other 0νββ experiment can match.
Why imaging matters
A 0νββ event produces two electrons sharing the full energy of the decay — about 2,458 keV for xenon-136. In a high-pressure gas, the two electrons travel several centimeters before depositing all their energy, with each electron losing energy along its path.
The track of a 0νββ event has a distinctive shape. The energy loss rate per unit length increases as the electron slows down (the Bragg peak effect). Near the end of each track, the electron deposits more energy per unit length than near the start. The track therefore has two characteristic “blobs” at its ends, with thinner connecting segments in between.
Background events at the same total energy have different topology. A gamma ray that Compton-scatters in the gas might deposit 2,458 keV of energy through multiple separate scatter points, leaving a track with multiple short segments rather than two well-separated blobs. A single-electron event from a Standard-Model beta decay near the endpoint of the spectrum would leave only one blob.
By imaging the track topology and counting the number of blobs, NEXT can distinguish 0νββ events from backgrounds on an event-by-event basis — something no other 0νββ technology can do at all.
How the detector works
NEXT-100 is built around a high-pressure pure-xenon gas at about 15 bar — the highest pressure that maintains good imaging properties without making the gas behave more like a liquid.
The active volume is a cylinder about 1.4 meters long and 1.2 meters in diameter. Charged particles depositing energy in the gas produce:
Ionization charge. Free electrons released along the particle’s track. These electrons drift toward an anode plane under the influence of an electric field maintained across the chamber. The arrival time and position of the electrons at the anode are recorded with millimeter precision.
Primary scintillation light. A brief UV flash produced at the moment of energy deposition, providing the “t = 0” timestamp for the event.
Secondary scintillation light. As the drift electrons enter a region of stronger electric field near the anode (the “electroluminescence” region), they emit additional UV photons proportional to the deposited energy. This light is detected by an array of photomultiplier tubes opposite the anode.
The combination of three signals — the prompt scintillation flash, the spatial pattern of the drift electrons, and the electroluminescence energy measurement — provides energy resolution of about 0.5% at the 0νββ Q-value plus full 3D imaging of each event.
The Canfranc location
NEXT-100 is housed in the Laboratorio Subterráneo de Canfranc (LSC) in the Spanish Pyrenees. The lab occupies a portion of an old railway tunnel infrastructure originally built in the late 19th and early 20th centuries. After the railway service through the tunnel was discontinued, the infrastructure was repurposed in the 2000s into a low-background physics laboratory.
The lab sits at about 850 meters underground, providing roughly 2,500 meters water-equivalent of rock overburden. This is shallower than the deepest physics labs (SNOLAB at 2,070 m, Jinping at 2,400 m) but adequate for low-background work, and the lab’s infrastructure is well-developed.
Canfranc hosts several other physics experiments alongside NEXT-100, including the ANAIS dark-matter program and various smaller efforts. The lab is operated by a consortium of Spanish universities and research institutions.
What’s been measured
NEXT-100 is the third in a sequence of progressively larger NEXT-program detectors.
NEXT-DEMO (2009-2014) was the original demonstrator, with about 1 kilogram of xenon, used to develop the basic detection technique.
NEXT-WHITE (2016-2021) was the first underground operational detector, with 5 kilograms of xenon at Canfranc. It demonstrated the topological background-rejection capability and measured the two-neutrino double-beta-decay half-life of xenon-136 at high precision.
NEXT-100 is the current generation, with 100 kilograms of xenon. Construction completed in the early 2020s, and physics data-taking is now underway. First 0νββ-search results are expected in the next few years.
What comes next
The full NEXT program planning includes several future stages.
NEXT-HD (high-definition) — a planned upgrade incorporating improved sensors and a larger fiducial volume. Targeted sensitivity around 10²⁷ years half-life.
NEXT-BOLD (barium tagging) — a long-term proposal to add chemical tagging of the daughter barium ion produced in 0νββ. If a single barium atom can be detected with near-100% efficiency after each candidate event, essentially all backgrounds can be eliminated. This would allow exposures at ton scale with essentially zero background — a path to sensitivities beyond what any current technology can achieve.
The barium-tagging concept is the most ambitious feature of the long-term NEXT roadmap. Several groups internationally are working on the underlying chemistry and detection physics. If it works, it could extend the reach of 0νββ searches by several orders of magnitude beyond what conventional techniques can achieve.
The xenon-based 0νββ program
NEXT-100 is one of three major xenon-based 0νββ experiments in operation or near completion.
KamLAND-Zen at Kamioka in Japan uses xenon dissolved in liquid scintillator, with 750 kilograms of enriched xenon as of 2026 and the world’s tightest current 0νββ limit.
nEXO at SNOLAB in Canada will use 5 tons of liquid xenon TPC — the largest of the xenon-based programs by isotopic mass.
NEXT-100 at Canfranc uses 100 kilograms of gas xenon — much smaller in mass but with the unique topological background-rejection capability.
The three programs are complementary. Each can confirm or rule out signals seen in the others. Each has different systematic uncertainties. Together they form one of the strongest cross-checks any rare-event search has ever had.
A different bet
NEXT is, in many ways, the most experimentally distinctive of the major 0νββ programs. The gas-phase approach trades raw target mass for unique imaging capability. The bet is that, at the sensitivities the field is approaching, the imaging advantage will matter more than the mass disadvantage.
The bet has not yet been resolved. NEXT-100 is still accumulating data. The first competitive 0νββ-search results are expected in the next few years. If the topological background rejection performs as designed, NEXT-100 may produce limits comparable to or better than the much larger xenon-based programs — a result that would justify the gas-phase approach and motivate the next stages of the NEXT program.
If the topological rejection underperforms, NEXT-100 will still produce a useful 0νββ limit at the 10²⁶-year level, contributing to the broader xenon-based program’s cross-checks. The bet pays off in either case, just with different magnitudes.
A 100-kilogram cylinder of gas, deep in a former railway tunnel through the Pyrenees, may turn out to be one of the more decisive experiments in the field. Or it may turn out to be one piece of the broader xenon program. Either outcome will substantially shape what comes next.
For the broader 0νββ context, see The hunt for neutrinoless double-beta decay. For other xenon-based programs, see EXO-200/nEXO and KamLAND-Zen. For other isotope approaches, see CUORE and The Majorana Demonstrator. For the underground laboratory hosting NEXT, see Deep underground laboratories.
Frequently asked
What is NEXT-100?
NEXT-100 is a 0νββ experiment located in the Laboratorio Subterráneo de Canfranc (LSC) in the Spanish Pyrenees. The detector uses about 100 kilograms of xenon-136 in gas phase, instrumented as a time projection chamber. Construction was completed in the 2020s, and the experiment is now taking physics-quality data.
Why gas-phase xenon?
Because gas-phase xenon allows the experiment to image the actual particle tracks produced by 0νββ candidate events, providing a uniquely powerful background-rejection capability. The two electrons produced in a 0νββ event leave distinctive 'two-blob' tracks in gas xenon that cannot be confused with single-electron tracks from gamma-ray backgrounds. Liquid xenon, by contrast, allows no track imaging — only total deposited energy.
How does NEXT-100 compare to nEXO?
Both use xenon-136 as the target isotope, but with completely different detection approaches. nEXO uses 5 tons of liquid xenon for a high-statistics measurement with limited background discrimination. NEXT-100 uses 100 kg of gas xenon with extraordinary background-rejection capability. The two approaches are complementary: nEXO accumulates more data per unit time but with more background, while NEXT-100 has tighter individual-event analysis but fewer events.
What is the planned sensitivity?
NEXT-100 targets a 0νββ half-life sensitivity around 10²⁶ years after several years of running. The follow-up experiment NEXT-HD aims for 10²⁷ years, comparable to the leading next-generation programs. The topological background-rejection capability of the gas approach may eventually allow the design to reach ton-scale exposures with extremely low backgrounds.
Where is Canfranc?
The Laboratorio Subterráneo de Canfranc occupies a former railway tunnel infrastructure in the Spanish Pyrenees, near the French border. The lab sits at about 850 meters underground, with about 2,500 meters water-equivalent depth of rock overburden. While shallower than some other underground physics labs, the depth is sufficient for low-background work, and Canfranc has been developing its underground physics infrastructure since the 2000s.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, December 27). NEXT-100: how a gas-phase xenon TPC chases neutrinoless double-beta decay in the Spanish Pyrenees. Neutrino Times. https://neutrino-times.com/articles/next-100-xenon-gas-tpc-canfranc/
Chicago
Neutrino Times Editorial Team. "NEXT-100: how a gas-phase xenon TPC chases neutrinoless double-beta decay in the Spanish Pyrenees." Neutrino Times, December 27, 2025. https://neutrino-times.com/articles/next-100-xenon-gas-tpc-canfranc/.
MLA
Neutrino Times Editorial Team. "NEXT-100: how a gas-phase xenon TPC chases neutrinoless double-beta decay in the Spanish Pyrenees." Neutrino Times, 27 Dec. 2025, https://neutrino-times.com/articles/next-100-xenon-gas-tpc-canfranc/.
BibTeX
@misc{neutrino-times-next-100-xenon-gas-tpc-canfranc,
author = {Neutrino Times Editorial Team},
title = {NEXT-100: how a gas-phase xenon TPC chases neutrinoless double-beta decay in the Spanish Pyrenees},
howpublished = {Neutrino Times},
year = {2025},
month = {dec},
url = {https://neutrino-times.com/articles/next-100-xenon-gas-tpc-canfranc/},
note = {Accessed: 2025-12-27}
} RIS
TY - GEN TI - NEXT-100: how a gas-phase xenon TPC chases neutrinoless double-beta decay in the Spanish Pyrenees AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-12-27 PB - Neutrino Times UR - https://neutrino-times.com/articles/next-100-xenon-gas-tpc-canfranc/ ER -