JUNO: the 20,000-ton acrylic sphere hunting the neutrino mass ordering

Seven hundred meters beneath a hill in Guangdong province, southern China, a 35-meter transparent acrylic sphere is filled with 20,000 tons of liquid scintillator. Around the sphere, 45,000 photomultiplier tubes point inward, watching for the faintest flashes of light. This is the Jiangmen Underground Neutrino Observatory — JUNO. It began full data-taking in 2025, and over the next several years it is expected to settle one of the most important open questions in neutrino physics: the mass ordering.

What “mass ordering” actually means

Three neutrino flavors are known. Each flavor is a quantum-mechanical mixture of three states with definite mass — call them ν₁, ν₂, and ν₃. Decades of oscillation experiments have measured the differences between the masses to good precision. The differences come in two distinct scales, and physicists know one is larger than the other.

What they don’t know is which state is the heaviest.

Two scenarios remain consistent with all the data:

Normal ordering: ν₁ is lightest, ν₂ slightly heavier, ν₃ much heavier. Inverted ordering: ν₃ is lightest, ν₁ slightly heavier, ν₂ much heavier.

The two scenarios have different consequences for cosmology, for the search for neutrinoless double-beta decay, and for the structure of theory beyond the Standard Model. Cosmology tentatively prefers normal ordering. Direct experiments have so far been inconclusive.

JUNO is built to settle the question by direct measurement.

How JUNO works

JUNO sits on a precisely chosen baseline. Two large nuclear power plants — Yangjiang and Taishan — collectively produce a staggering flux of electron antineutrinos at a distance of 52.5 kilometers. That distance is not accidental: it is tuned to the energy of reactor antineutrinos and the known oscillation parameters such that JUNO sees a particularly rich, structured oscillation signal.

Specifically, JUNO measures a phenomenon called the fast oscillation pattern, a kind of fine-grained ripple superimposed on the broader oscillation curve. The exact shape of those ripples depends on the mass ordering. With its enormous mass and excellent energy resolution, JUNO should be able to distinguish normal from inverted ordering at high statistical confidence within a few years of running.

The detector itself is built around three core technologies:

A 35-meter acrylic sphere, the largest of its kind ever constructed, holding the liquid scintillator. The walls are 12 cm thick acrylic, carefully engineered to be both structurally sound and optically transparent enough to let scintillation light through to the photomultipliers.

20,000 tons of high-purity liquid scintillator, a liquid scintillator chemically tuned for both high light yield and long-distance transparency. The light yield — measured in photons per MeV of deposited energy — is roughly twice that of the previous-generation KamLAND detector.

45,000 photomultiplier tubes lining the surrounding water pool. About 17,000 are 20-inch tubes optimized for high quantum efficiency; the rest are smaller tubes for energy calibration and timing. The combined optical coverage exceeds 75%, far better than any previous large scintillator detector.

Why a sphere

Most large scintillator detectors are cylindrical because cylinders are easier to construct. JUNO chose a sphere for a single, decisive reason: light collection is symmetric in every direction. There are no corners where photons can get trapped, no path length variations that complicate energy reconstruction.

The trade-off is engineering. A 35-meter transparent acrylic sphere is the largest object of its kind ever built. The acrylic panels had to be fabricated, polished, and chemically bonded with seams that maintain optical transparency. The sphere has to support its own weight (around 600 tons of acrylic) plus the buoyant force of the surrounding water. Every detail of the geometry had to be solved before the first scintillator was poured.

The work paid off. JUNO’s energy resolution — about 3% at 1 MeV — is better than any previous scintillator detector by a factor of nearly two.

What else JUNO will measure

The mass ordering is the headline goal. JUNO is also designed to provide:

Precision oscillation parameters. The dominant oscillation parameters — the mixing angle θ₁₂ and the mass-squared differences — should be measured to sub-percent precision within a few years. That would be the most precise oscillation measurement anywhere.

Supernova neutrino detection. Like Hyper-Kamiokande and IceCube, JUNO would catch thousands of neutrinos from a galactic supernova. Its excellent energy resolution would let it map the supernova’s energy spectrum more finely than any water Cherenkov detector.

Geo-neutrinos and solar neutrinos. JUNO’s low energy threshold and large mass make it the best detector for these neutrinos as well, opening new windows into the Earth’s interior and the Sun’s core processes.

Proton decay. With careful background suppression, JUNO is also sensitive to certain proton decay channels, although Hyper-Kamiokande will be more sensitive overall.

Timeline

Construction of the cavern, the acrylic sphere, and the photomultiplier array took roughly a decade. Filling the sphere with liquid scintillator began in 2024. Full data-taking started in 2025. The first major physics results — particularly on the precision oscillation parameters — are expected within a year or two of full operations. The mass ordering result is expected to reach 3σ significance after roughly six years of running, and 5σ thereafter.

That timeline places JUNO’s definitive answer in the early 2030s — likely before DUNE reaches comparable sensitivity from accelerator-based measurements. The two experiments are complementary: JUNO uses reactor antineutrinos at short distances; DUNE uses an accelerator beam over a 1,300-km baseline. If they agree on the ordering, the result is bulletproof. If they disagree, particle physics has a much more interesting problem.

Why this matters beyond particle physics

The mass ordering is one of those measurements whose value extends far beyond the experiment that makes it. A confirmed normal ordering would be consistent with the simplest see-saw models and with current cosmological data. An inverted ordering would point to richer physics — possibly including more exotic theoretical scenarios.

Either way, JUNO is doing one of the most rigorous neutrino measurements ever attempted. The 20,000 tons of scintillator inside a hand-blown acrylic sphere in southern China are listening, right now, for the structured oscillation pattern that will tell us which arrangement nature chose.

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For the broader detector landscape, see our interactive atlas. For the phenomenon JUNO is measuring, see How neutrino oscillation works.

Neutrino Times Editorial Team. (2025, July 28). JUNO: the 20,000-ton acrylic sphere hunting the neutrino mass ordering. Neutrino Times. https://neutrino-times.com/articles/juno-jiangmen-neutrino-mass-ordering/

Neutrino Times Editorial Team. "JUNO: the 20,000-ton acrylic sphere hunting the neutrino mass ordering." Neutrino Times, July 28, 2025. https://neutrino-times.com/articles/juno-jiangmen-neutrino-mass-ordering/.

Neutrino Times Editorial Team. "JUNO: the 20,000-ton acrylic sphere hunting the neutrino mass ordering." Neutrino Times, 28 Jul. 2025, https://neutrino-times.com/articles/juno-jiangmen-neutrino-mass-ordering/.

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