JUNO begins its watch from Jiangmen

Seven hundred meters beneath a hill in Guangdong province, a transparent acrylic sphere thirty-five meters across now holds twenty thousand tonnes of liquid scintillator so clear that light travels more than twenty meters through it before being absorbed. Tens of thousands of photomultiplier tubes stare inward from its outer skin, waiting for the millisecond-spaced double flash that marks a reactor antineutrino. The Jiangmen Underground Neutrino Observatory — JUNO — finished filling in the second half of 2025 and is now running through its commissioning phase. If it performs as designed, it will resolve one of the last open questions in the basic structure of the neutrino sector: which of the three mass states is the lightest.

A pair of reactor complexes, 52 kilometers away

JUNO sits at a deliberately chosen distance from two large nuclear power complexes — Taishan and Yangjiang — both roughly 52.5 kilometers from the experimental hall. That number is not arbitrary. It is close to the baseline where the oscillation pattern from the smaller mass splitting (Δm²₂₁) reaches its first deep minimum, while the faster wiggles driven by the larger splittings (Δm²₃₁ and Δm²₃₂) ride on top. The two patterns interfere, and the precise shape of that interference encodes information about which splitting is positive and which is negative — that is, about the mass ordering.

Combined, the two reactor complexes will eventually deliver roughly 26 gigawatts of thermal power. That translates into something like sixty inverse-beta-decay events per day inside JUNO’s fiducial volume, give or take a reactor duty cycle. It is not a lot of statistics by collider standards. But for a measurement that depends on the shape of a spectrum rather than a counting rate, it is enough — provided the detector can resolve the wiggles at all.

Energy resolution as the entire design problem

Almost every choice in JUNO’s construction was driven by one number: 3 percent energy resolution at 1 MeV. That is roughly twice as good as KamLAND, the previous generation’s reference detector, and pushing it required almost everything to be scaled up at once. Photocathode coverage was driven to about 75 percent — far beyond what most past scintillator detectors achieved — using 17,612 large 20-inch photomultiplier tubes from Hamamatsu and a Chinese consortium called NNVT, plus another 25,600 smaller 3-inch tubes interleaved between them to cross-check the calibration. The scintillator itself, a blend of linear alkylbenzene with PPO and bis-MSB wavelength shifters, had to be purified until its attenuation length stayed above twenty meters at 430 nanometers, the wavelength where the dyes emit most strongly.

The acrylic sphere holding all of this is the largest such structure ever built, for any purpose. It was assembled in 263 curved panels glued on-site and floats in turn inside an outer stainless-steel framework, the whole arrangement immersed in 35 kilotons of ultrapure water. The water is not just shielding. It is also instrumented with its own photomultipliers as a cosmic-ray veto, and the muon flux at JUNO’s modest depth — overburden of about 1,800 meters of water equivalent — makes that veto non-negotiable.

A detector with more than one job

The mass ordering is JUNO’s signature measurement, but it is far from the only one on the program. The same detector will collect solar neutrinos, particularly from the ⁸B and ⁷Be branches, where the energy threshold and target mass allow precision tests of matter-effect oscillation in the Sun. It will see atmospheric neutrinos with enough statistics to extract a second, partially independent constraint on the mass ordering. It is sensitive to geo-neutrinos from uranium and thorium decay chains in the Earth’s crust and mantle, helping to refine the planet’s radiogenic heat budget. And if a galactic supernova goes off during JUNO’s lifetime, the detector will record one of the highest-statistics burst signals ever — possibly tens of thousands of events in a handful of seconds, across all flavors. Such a burst would tell us things about supernova dynamics that no other detector currently online can.

What to expect next

First physics results are expected in 2027, with simple oscillation parameters — the mixing angles θ₁₂ and θ₁₃, and the splittings Δm²₂₁ and the larger Δm² — improved to a precision that will set the global benchmark. The mass-ordering result itself, depending on reactor duty cycles and how well the energy scale holds, is more likely to arrive around 2030 at the three-to-four sigma level. By then, JUNO will not be alone: DUNE in the United States and Hyper-Kamiokande in Japan will be taking long-baseline data, and IceCube’s atmospheric-neutrino program will contribute its own answer. The hope, and the design intent, is that the multiple approaches converge.

We have known there are three neutrinos since 1962. Sixty-four years later, we still do not know which of their three masses is the smallest. A glass sphere in southern China is finally in a position to find out.

Frequently asked

What is JUNO designed to measure?

Primarily, the neutrino mass ordering — whether the third mass state is the heaviest (normal ordering) or the lightest (inverted ordering). JUNO sits at a 52.5 km baseline from two reactor complexes, where the interference between the small mass splitting (Δm²₂₁) and the large splittings (Δm²₃₁ and Δm²₃₂) imprints a characteristic shape on the antineutrino energy spectrum. JUNO will also produce the world's most precise measurements of θ₁₂, Δm²₂₁, and |Δm²₃₂|.

How big is JUNO?

The central detector is a 35.4-meter-diameter acrylic sphere holding 20 kilotons of liquid scintillator — the largest such sphere ever built. Around it, 17,612 large 20-inch and 25,600 small 3-inch photomultiplier tubes provide 75% photocathode coverage on the inner surface, immersed in an outer 35-kiloton water buffer. The detector sits 700 meters underground in Guangdong province, China.

When did JUNO start taking physics data?

Filling and commissioning were completed in 2025. Full physics data-taking began in late 2024-early 2025. First mass-ordering results at 3σ significance are expected by the early 2030s, with definitive sensitivity following from the combined long-baseline programs at DUNE and Hyper-K.

Why does energy resolution matter so much for JUNO?

The mass-ordering signature in JUNO's reactor antineutrino spectrum is a small, fast-oscillating wiggle riding on top of the dominant Δm²₂₁ oscillation envelope. Resolving that wiggle requires extraordinary energy resolution — about 3% at 1 MeV, roughly twice as good as the previous generation's KamLAND. Almost every design choice in JUNO is driven by hitting that 3% number.

Cite this article 5 formats

APA

Neutrino Times Editorial Team. (2025, May 30). JUNO begins its watch from Jiangmen. Neutrino Times. https://neutrino-times.com/articles/juno-watches-from-jiangmen/

Chicago

Neutrino Times Editorial Team. "JUNO begins its watch from Jiangmen." Neutrino Times, May 30, 2025. https://neutrino-times.com/articles/juno-watches-from-jiangmen/.

MLA

Neutrino Times Editorial Team. "JUNO begins its watch from Jiangmen." Neutrino Times, 30 May. 2025, https://neutrino-times.com/articles/juno-watches-from-jiangmen/.

BibTeX

@misc{neutrino-times-juno-watches-from-jiangmen,
  author       = {Neutrino Times Editorial Team},
  title        = {JUNO begins its watch from Jiangmen},
  howpublished = {Neutrino Times},
  year         = {2025},
  month        = {may},
  url          = {https://neutrino-times.com/articles/juno-watches-from-jiangmen/},
  note         = {Accessed: 2025-05-30}
}

RIS

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