Super-Kamiokande

From Kamiokande to Super-Kamiokande

The Kamiokande detector, led by Masatoshi Koshiba, had observed neutrinos from the supernova SN 1987A, for which Koshiba received the 2002 Nobel Prize in Physics. Its successor, Super-Kamiokande, was built in the Mozumi Mine in Kamioka, under about 1,000 metres of rock that shields it from cosmic rays, and began observing in April 1996. Yoji Totsuka led the construction, and the Institute for Cosmic Ray Research of the University of Tokyo operates it.

The detector is a cylinder of 39 metres diameter and 42 metres height, filled with ultrapure water. When a neutrino interacts with a nucleus in the water, it produces a charged particle that emits a cone of Cherenkov light, recorded by the photomultiplier tubes that line the walls.

Neutrinos change

At the Neutrino '98 conference in Takayama the collaboration presented evidence that muon neutrinos made by cosmic rays in the atmosphere disappear on the way through the Earth, a sign that they oscillate into another type, which requires them to have a small mass. Takaaki Kajita shared the 2015 Nobel Prize in Physics for this discovery.

The result changed the Standard Model of particle physics, which had assumed massless neutrinos.

Accident and upgrade

In November 2001, as the tank was being refilled after maintenance, one tube imploded and the shock wave triggered a chain reaction that destroyed about 6,600 of the tubes; the detector was partly restored in 2002 with fewer tubes, protected in acrylic cases. In 2020 gadolinium sulfate was added to the water to make neutrons visible, an upgrade called SK-Gd, aiming to detect the diffuse background of supernova neutrinos. A larger detector, Hyper-Kamiokande, is under construction nearby.

The slow science of rare events

A neutrino detector records only a few events a day, so a result needs years of data and careful understanding of backgrounds. The 1998 result came after two years of data, and many cross-checks. The team had to show that the number of upward-going neutrinos was lower than the number of downward-going ones, and that no instrument effect explained it. Such caution is the reason that the result was accepted quickly: the collaboration had already answered the objections. In an observatory, the culture of checking for artefacts is as important as the hardware.

Instruments and technologies

• Water Cherenkov tank A cylindrical tank holding 50,000 tonnes of ultrapure water, with an inner detector viewed by 11,000 photomultiplier tubes and an outer veto detector.
• Photomultiplier tubes Fifty-centimetre vacuum tubes that detect single photons, the world's largest, specially made for the detector.
• Water purification A system that circulates and cleans the water continuously to keep it transparent over tens of metres.

The people

• Takaaki Kajita Physicist who led the atmospheric neutrino analysis; Nobel Prize 2015.
• Masatoshi Koshiba Founder of the Kamiokande experiments; Nobel Prize 2002.
• Yoji Totsuka Leader of Super-Kamiokande's construction.

Timeline

• 1996 Observation begins in April.
• 1998 Evidence of atmospheric neutrino oscillation presented at Neutrino '98, Takayama.
• 2001 About 6,600 photomultiplier tubes destroyed in a chain-reaction implosion; partial restoration in 2002.
• 2015 Takaaki Kajita shares the Nobel Prize in Physics for the discovery of neutrino oscillations.
• 2020 Gadolinium sulfate added to the water (SK-Gd).

Key lessons for observation today

• Go underground, away from the noise Rock shields the detector from cosmic rays. Shielding is an environmental design requirement.
• Failure modes need engineering A single implosion destroyed thousands of tubes. Large detectors must be designed against chain-reaction failures.
• Upgrade the medium, not just the sensors Adding gadolinium to the water created a new capability. Sometimes the cheapest upgrade is a change in the medium.

Sources

• Nobel Prize in Physics 2015, summary
• Royal Swedish Academy of Sciences, The Nobel Prize in Physics 2015
• Super-Kamiokande Collaboration (1998), Evidence for oscillation of atmospheric neutrinos, arXiv:hep-ex/9807003
• Wikipedia, Super-Kamiokande (secondary)