After more than a decade of construction, the Jiangmen Underground Neutrino Observatory — better known as JUNO — turned on in 2025. As of May 2026 the detector is in its first full year of operation, an interval that, in any new neutrino experiment, is dominated less by headline physics results than by the patient, painstaking work of getting an instrument of that scale to behave the way the design said it would. This article describes what that first year actually looks like, what the detector is collecting, and which results the field is waiting for.
What JUNO is
JUNO sits under Dashi Hill in Guangdong province, southern China, roughly 700 metres underground. Its core is a transparent acrylic sphere about 35 metres across, filled with about 20,000 tonnes of organic liquid scintillator. Around the sphere, tens of thousands of photomultiplier tubes — a mix of large 20-inch and smaller 3-inch devices — cover the inside of a stainless-steel support structure to catch the scintillation flashes when neutrinos interact in the liquid.
The detector is positioned so that it sits about 53 kilometres from two large nuclear power complexes, Yangjiang and Taishan. That baseline is not accidental: it is the distance at which the oscillation pattern of reactor antineutrinos is most sensitive to the neutrino mass-state spectrum, which is exactly what JUNO is built to read out. The instrument was built around an unprecedented design goal — an energy resolution of about 3% at 1 MeV — because measuring the mass ordering from reactor data requires resolving wiggles in the antineutrino spectrum at almost that level of precision.
The fuller background lives in our dedicated explainers on JUNO and the neutrino mass ordering and our companion piece on reactor antineutrinos.
Year one: commissioning, not headlines
Big neutrino detectors don’t drop out of the box with publishable physics. Their first year is mostly spent verifying that every part of the apparatus does what the design said it would. For JUNO this means several overlapping campaigns.
Photomultiplier characterisation. Each of JUNO’s PMTs has a gain, a timing response, and an after-pulsing behaviour that has to be characterised in situ. The collaboration deploys radioactive and laser calibration sources at known positions inside the detector, then uses the responses to map the detector’s reach across its volume.
Energy scale. Because JUNO’s physics goals depend on reading the antineutrino energy spectrum at fractions of a percent, establishing the absolute and relative energy scale across the sphere is the single most important commissioning task. Calibration sources of known energy — gamma emitters at multiple energies, plus naturally occurring lines from radioactive contaminants in the surrounding rock — are used to anchor the scale.
Background characterisation. Cosmic muons surviving the 700-metre rock overburden, radioactive decays from trace elements in detector materials, and neutrons from rock and water all leave traces that have to be measured and modelled before the reactor-antineutrino sample can be cleanly extracted. This is grunt work, but it is the difference between a published result and a noisy one.
Reactor-antineutrino event accumulation. Meanwhile, the actual physics signal is already arriving. With Yangjiang and Taishan running, JUNO sees a continuous stream of inverse-beta-decay candidates — the same kind of event Clyde Cowan and Frederick Reines pioneered in 1956 — at a rate substantially higher than predecessor experiments like KamLAND and Daya Bay, thanks to the combination of detector mass and reactor proximity.
What the headline measurement actually is
JUNO’s defining target is the neutrino mass ordering: the question of whether the three known neutrino mass states are arranged in what physicists call the normal pattern (two close-lying lighter states plus a heavier one) or the inverted pattern (a close-lying heavier pair plus a lighter one). This is one of the last unmeasured fundamentals of neutrino mixing, and it ripples outwards into several open questions in the field.
The way JUNO reads the ordering is subtle. The energy spectrum of reactor antineutrinos arriving 53 km away has a small wiggle imprinted on it by the interference of two oscillation frequencies. The direction of that wiggle, in a precise sense, is different in the two possible orderings. JUNO doesn’t need to be lucky to see it — but it does need accumulated statistics, and that accumulation takes years. The original design sensitivity was a ≥3σ determination after roughly six years of operation.
Alongside that headline goal, JUNO will pin down the mixing parameters θ12 and Δm²21 to sub-percent precision, sharper than any previous measurement, and will simultaneously double as a target for solar neutrinos, atmospheric neutrinos, geo-neutrinos, supernova bursts, and searches for sterile neutrinos and exotic effects. Many of those analyses can produce useful results from data taken during the first year and a half.
What we’ll likely hear about first
If history is any guide, the public communications from JUNO during the next year or two will not be about the mass ordering. They will be about the detector’s performance — energy resolution achieved, light-yield uniformity across the sphere, background levels, reactor-event rate consistency with prediction — and about early physics confirmations: a precision measurement of θ12 and Δm²21 in agreement with the global picture; perhaps a first JUNO-flavoured solar neutrino signal; an event rate from atmospheric neutrinos that confirms the detector lives up to its spec.
These are not headline-grabbing in the way “mass ordering resolved” will eventually be. But they are the foundation on which the headline measurement is built, and they are exactly the right things to be doing in year one.
Why this matters
The deeper significance of JUNO’s first year is that it marks the start of a new era of precision reactor neutrino physics. Daya Bay and RENO opened it in 2012 by measuring θ13. KamLAND built the case that scintillator detectors at multi-kilometre baselines could deliver clean oscillation results. JUNO inherits both and pushes the field to a new precision floor — one at which previously unobservable features of the neutrino spectrum become routine.
For the wider context, see our overview of JUNO and our explainer on the mass ordering problem.
For related coverage, see Daya Bay and RENO: measuring θ13, KamLAND: the kiloton scintillator that watched a continent’s reactors, and Open questions, part 2: mass ordering.
Frequently asked
When did JUNO start taking data?
JUNO began full operations in 2025 after more than a decade of construction. The first year of data taking is largely devoted to commissioning, photomultiplier calibration, energy-scale verification, and accumulating an initial sample of reactor-antineutrino interactions. Physics-grade analyses build on this baseline.
What is JUNO designed to measure?
Its flagship goal is the neutrino mass ordering — whether the three neutrino mass states are arranged in the so-called normal or inverted pattern. JUNO will also make sub-percent measurements of the mixing parameters θ12 and Δm²21, observe solar and atmospheric neutrinos, watch for a galactic supernova burst, and search for the diffuse supernova neutrino background and exotic effects.
Why does the mass ordering matter?
It is one of the last unmeasured fundamentals of neutrino mixing. The ordering affects how laboratory experiments interpret CP-violation searches, how cosmology bounds the absolute neutrino mass, and how nuclear-physics experiments approach neutrinoless double beta decay. Determining it is a milestone on the path to the full Standard-Model-plus-neutrinos picture.
How does JUNO see reactor antineutrinos?
Antineutrinos from two nearby nuclear power plants about 53 kilometres away occasionally interact with protons inside JUNO's 20,000 tonnes of liquid scintillator, producing a positron and a neutron. The positron annihilation gives a prompt flash; the neutron capture a few hundred microseconds later gives a delayed flash. This 'inverse beta decay' signature is reconstructed by tens of thousands of photomultiplier tubes lining the detector sphere.
When will JUNO publish its first physics results?
First physics results from commissioning data are expected during the late 2020s, with the headline mass-ordering determination taking longer to accumulate the necessary statistics — the design sensitivity for a ≥3σ result corresponds to roughly six years of data. Calibration-quality and event-rate results, however, can be reported much earlier.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, May 21). JUNO's first year: what the world's newest reactor neutrino observatory is doing right now. Neutrino Times. https://neutrino-times.com/articles/juno-first-year-commissioning-2026/
Chicago
Neutrino Times Editorial Team. "JUNO's first year: what the world's newest reactor neutrino observatory is doing right now." Neutrino Times, May 21, 2026. https://neutrino-times.com/articles/juno-first-year-commissioning-2026/.
MLA
Neutrino Times Editorial Team. "JUNO's first year: what the world's newest reactor neutrino observatory is doing right now." Neutrino Times, 21 May. 2026, https://neutrino-times.com/articles/juno-first-year-commissioning-2026/.
BibTeX
@misc{neutrino-times-juno-first-year-commissioning-2026,
author = {Neutrino Times Editorial Team},
title = {JUNO's first year: what the world's newest reactor neutrino observatory is doing right now},
howpublished = {Neutrino Times},
year = {2026},
month = {may},
url = {https://neutrino-times.com/articles/juno-first-year-commissioning-2026/},
note = {Accessed: 2026-05-21}
} RIS
TY - GEN TI - JUNO's first year: what the world's newest reactor neutrino observatory is doing right now AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-05-21 PB - Neutrino Times UR - https://neutrino-times.com/articles/juno-first-year-commissioning-2026/ ER -