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Objective
Observe atmospheric, solar, and accelerator neutrinos in a 50-kiloton water-Cherenkov detector with sufficient statistics to study oscillation patterns.
Method
A cylindrical stainless-steel tank of 50,000 tonnes of ultra-pure water, 1,000 m underground, instrumented with 11,146 inward-facing 50-cm photomultiplier tubes covering 40% of the inner surface. Charged particles above Cherenkov threshold produce light cones whose pattern, timing, and intensity reconstruct vertex, direction, energy, and flavor (e-like vs μ-like).
Key results
- June 1998: first convincing observation of neutrino oscillations, through the zenith-angle dependence of atmospheric muon neutrinos (νμ → ντ).
- Day–night asymmetry in solar ⁸B neutrinos, providing an independent MSW-effect constraint.
- Accelerator long-baseline oscillation confirmations through K2K and T2K with Super-K as the far detector.
- No proton decay found, pushing limits on p → e⁺π⁰ beyond 10³⁴ years.
- Eleven events from SN 1987A (through the predecessor Kamiokande-II).
- Since 2020: loading with gadolinium sulfate to enable neutron tagging and diffuse supernova-background detection.
Significance
Super-Kamiokande's 1998 atmospheric-oscillation result was the first direct evidence that neutrinos have mass and share the 2015 Nobel Prize in Physics (Takaaki Kajita). The detector has remained operational for nearly three decades, delivering continuing measurements across solar, atmospheric, accelerator, and astrophysical neutrino physics, and pioneering the technology now planned for Hyper-Kamiokande.
Site and construction
Super-Kamiokande is located beneath Mount Ikeno in the Mozumi mine near Hida, Gifu Prefecture. The 1,000 m of rock overburden reduces the cosmic-ray muon flux by a factor of relative to the surface. Construction of the tank, 40 m diameter by 40 m high, required extensive underground excavation and was completed in 1996.
Detector design
The 50-kiloton water volume is divided into an inner detector (fiducial mass 22.5 kt) instrumented with 11,146 20-inch photomultipliers, and an outer detector veto region instrumented with 1,885 8-inch PMTs. The water is continuously purified to maintain optical attenuation greater than 100 m at the Cherenkov wavelengths.
A subset of channels records pulse-shape information for recoil-energy reconstruction in solar neutrino analyses; the remainder record summed charge and hit time.
Operational milestones
- 1996: Data-taking begins
- 1998: First oscillation announcement at Neutrino ‘98 in Takayama
- 2001: SK-I phase ends after ~1500 PMT failure cascade during refill
- 2002–2005: SK-II operation with reduced PMT coverage
- 2006–2008: SK-III with reinforced PMT housings
- 2008–2018: SK-IV with upgraded electronics
- 2020: Gadolinium loading begins (SK-Gd)
- Present: SK-VI/VII with full gadolinium loading
Physics highlights
Beyond the 1998 oscillation discovery, Super-K has contributed:
- Precision measurement of the B solar neutrino flux and its day–night asymmetry
- Observation of solar neutrino spectral distortion from MSW matter effects
- Proton-decay lifetime limits exceeding years in leading channels
- Far-detector measurements for the K2K and T2K long-baseline programs
- Ongoing search for the diffuse supernova neutrino background, now with neutron tagging via Gd loading
Successor: Hyper-Kamiokande
Construction of Hyper-Kamiokande began in 2020 near Super-K. With a fiducial mass about eight times larger and next-generation PMTs, Hyper-K will significantly improve sensitivity to CP violation in neutrino oscillations, precision solar neutrino measurements, supernova neutrinos, and proton decay. First data is expected in 2027.
Super-K will continue operating alongside Hyper-K for cross-calibration and for solar and atmospheric measurements where its lower threshold remains advantageous.