The Jiangmen Underground Neutrino Observatory (JUNO) has marked a monumental milestone in the landscape of particle physics by successfully completing the filling of its colossal 20,000-ton liquid scintillator detector and commencing data acquisition as of August 26. This achievement culminates over a decade of meticulous design, development, and construction efforts, positioning JUNO as the pioneering facility of a new generation of neutrino experiments with unprecedented scale and precision. Early operational data affirm that crucial performance metrics not only meet but exceed the stringent specifications, setting the stage for JUNO to confront one of the most profound and challenging enigmas of contemporary physics: the determination of the neutrino mass ordering.
The ordering, or “mass hierarchy,” of neutrinos—specifically, whether the third neutrino mass eigenstate (ν₃) is heavier or lighter than the second (ν₂)—has been an elusive goal due to the subtle nature of neutrino oscillations and the complex interplay of matter effects. JUNO’s design circumvents these complications by implementing an independent methodology that is largely unaffected by Earth’s matter influences and free from parameter degeneracies that traditionally impede other approaches. The success of this method will narrow down the true mass arrangement of neutrinos, a revelation that has significant implications for the Standard Model of particle physics and our understanding of the universe’s fundamental structure.
JUNO’s unique location, nestled 700 meters underground in proximity to Jiangmen city within Guangdong Province, harnesses the intense flux of electron antineutrinos emitted by the nearby Taishan and Yangjiang nuclear power reactors, approximately 53 kilometers distant. Utilizing these abundant man-made neutrino sources, JUNO meticulously detects and reconstructs the energy spectra of incoming antineutrinos with an unprecedented precision unparalleled by existing detectors. This spectral precision enables it to observe subtle oscillation features that encode information about neutrino mass differences and mixing angles, thereby propelling the field into a new era of neutrino spectroscopy.
The heart of the experiment is its central detector, a vast acrylic sphere with a diameter of 35.4 meters, containing 20,000 tons of ultra-pure liquid scintillator. Surrounding the sphere is a sophisticated array of photomultiplier tubes—namely 20,000 20-inch PMTs and an additional 25,600 3-inch PMTs—embedded within a 41.1-meter-diameter stainless steel truss. These PMTs operate synergistically to convert the faint scintillation light produced when neutrinos interact with the liquid scintillator into electronic signals. This dual-PMT system significantly enhances light collection efficiency, timing resolution, and spatial reconstruction capabilities, culminating in an unprecedented level of sensitivity and precision for neutrino detection.
The assembly and filling process of JUNO’s massive detector was an extraordinary feat of engineering. Initiated in December 2021 with the installation phase and culminating in December 2024, the project involved meticulous coordination to ensure the integrity and stability of the ultra-pure liquid volumes. Prior to filling the central scintillator, the water pool surrounding the acrylic sphere was filled with 60,000 tons of ultra-pure water. Control of the liquid level differential, maintained to within centimeters, alongside a stringent flow-rate uncertainty below 0.5%, was critical to preserving the detector’s structural integrity. Subsequently, the scintillator was carefully introduced, displacing the water within the sphere while satisfying the stringent demands for purity, optical transparency, and ultra-low radioactivity.
These exacting purity standards are essential, given the extreme sensitivity of neutrino detection experiments to radioactive backgrounds and optical clarity. JUNO has implemented advanced purification methods and continuous monitoring systems to maintain these parameters, which are indispensable for achieving the precision required to extract subtle neutrino oscillation signals from background noise. The success in maintaining such ultra-high purity in a detector of this unprecedented scale represents a breakthrough in scintillator technology and sets a new benchmark for future large-scale neutrino experiments.
Beyond its primary physics goal of determining the neutrino mass ordering, JUNO is poised to revolutionize our understanding of neutrino properties through precise measurements of several oscillation parameters, such as mixing angles and mass-squared differences, with an order-of-magnitude improvement over current knowledge. Such precision measurements are vital for refining theoretical models and will influence ongoing and future searches for physics beyond the Standard Model.
JUNO’s reach extends to astrophysical neutrinos as well. The detector’s sensitivity and scale will enable detailed studies of neutrinos originating from the Sun, supernovae, Earth’s atmosphere, and geoneutrinos arising from radioactive decay within the Earth’s interior. This breadth of observation channels opens new pathways for multi-disciplinary research spanning particle physics, astrophysics, and geosciences, offering valuable insights into the inner workings of cosmic and terrestrial phenomena.
The experiment’s design intrinsically allows for exploration of exotic physics scenarios. JUNO holds the potential to search for sterile neutrinos—hypothetical particles that could extend the Standard Model—and to pursue rare processes such as proton decay, which, if observed, would provide groundbreaking evidence for grand unified theories and insights into the stability of matter.
JUNO’s inception dates back to 2008, with formal approvals secured by the Chinese Academy of Sciences and Guangdong Province in 2013. Full-scale underground construction commenced in 2015, followed by a multiyear period of intricate detector installation and commissioning. The collaborative effort unites over 700 researchers from 74 institutions spanning 17 countries and regions. This global cooperation reflects not only the international nature of particle physics but also the cumulative expertise harnessed from previous liquid scintillator experiments worldwide, driving technological innovation and setting the stage for JUNO’s ambitious scientific agenda.
Leadership from the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences has been instrumental in realizing JUNO’s construction and operational readiness. Profound ingenuity, technological advancements, and sustained commitment were necessary to meet the formidable challenges related to detector purity, mechanical stability, and operational safety. The dedication of hundreds of engineers, technicians, and scientists transformed the bold conceptual design into a fully functional instrument poised to substantially deepen humanity’s grasp of the neutrino sector.
The operational timeline envisioned for JUNO extends over 30 years, spanning multiple phases including data acquisition, analysis, and anticipated future upgrades. Notably, plans envisage augmenting JUNO with capabilities to carry out a world-leading search for neutrinoless double-beta decay, a rare nuclear transition that, if detected, would prove that neutrinos are Majorana particles—particles that are their own antiparticles. Such a discovery would revolutionize our understanding of neutrino mass generation mechanisms and could have profound consequences for particle physics, astrophysics, and cosmology.
In conclusion, JUNO’s commissioning and early operation mark a watershed moment in neutrino physics. By harnessing a massive, ultra-pure liquid scintillator detector and innovative instrumentation, JUNO is set to resolve fundamental questions about neutrino masses and mixing, while offering unprecedented opportunities to explore diverse physics domains. Its successes epitomize the power of international collaboration, cutting-edge technology, and scientific perseverance, opening a new era of discovery that is poised to unlock the deepest secrets of the subatomic universe.
Subject of Research: Neutrino Physics, Neutrino Mass Ordering, Neutrino Oscillation Parameters
Article Title: JUNO Commissioning Heralds a New Era in Neutrino Science with a 20,000-Ton Liquid Scintillator Detector
News Publication Date: August 26, 2024
Web References: https://mediasvc.eurekalert.org/Api/v1/Multimedia/1cf7a4e8-964a-4554-82c8-49e521b0cd2f/Rendition/low-res/Content/Public
Image Credits: JUNO Collaboration
Keywords
Cosmic neutrinos, neutrino mass hierarchy, liquid scintillator detector, neutrino oscillations, photomultiplier tubes, particle physics, neutrino detectors, sterile neutrinos, neutrinoless double-beta decay, astrophysical neutrinos, ultra-pure scintillator, neutrino spectroscopy
