Kamioka Observatory

Water is an ideal candidate because it is inexpensive, easy to purify, stable, and can detect relativistic charged particles through their production of Čerenkov radiation.

The muon rate in the KamiokaNDE experiment was about 0.4 events per second, roughly five orders of magnitude smaller than what it would have been if the detector had been located at the surface.

[4] The distinct pattern produced by Čerenkov radiation allows for particle identification, an important tool for both understanding the potential proton decay signal and for rejecting backgrounds.

Minimum ionizing muons, in contrast, produce very sharp rings as their heavier mass allows them to propagate directly.

The detector was a cylindrical tank which contained 3,000 tons of pure water and had about 1,000 50 cm diameter photomultiplier tubes (PMTs) attached to the inner surface.

In the 1930s, Hans Bethe and Carl Friedrich von Weizsäcker had hypothesized that the source of the Sun's energy was fusion reactions in its core.

Ray Davis's Homestake Experiment was the first to detect solar neutrinos – strong evidence that the nuclear theory of the Sun was correct.

Over a period of decades, the Davis experiment consistently observed only about 1/3 the number of neutrinos predicted by the Standard Solar Models of his colleague and close friend John Bahcall.

Because of the great technical difficulty of the experiment and its reliance on radiochemical techniques rather than real time direct detection, many physicists were suspicious of his result.

First, the enormous volume possible in a water Čerenkov detector can overcome the problem of the very small cross section of the 5-15 MeV solar neutrinos.

This meant that individual neutrino-electron interaction candidate events could be studied on an event-by-event basis, starkly different from the month-to-month observation required in radiochemical experiments.

Fourth, neutrino-electron scattering is an elastic process, so the energy distribution of the neutrinos can be studied, further testing the solar model.

Finally, since a water Čerenkov experiment would use a different target, interaction process, detector technology, and location it would be a very complementary test of Davis's results.

The signals produced by proton decay and atmospheric neutrino interactions are considerably larger than this, so the original KamiokaNDE detector had not needed to be particularly aggressive about its energy threshold or resolution.

Once 450 days of data had been accumulated, the experiment was able to see a clear enhancement in the number of events which pointed away from the Sun over random directions.

The Kamiokande-II experiment happened to be running at a particularly fortuitous time, as a supernova took place while the detector was online and taking data.

For his work directing the Kamioka experiments, and in particular for the first-ever detection of astrophysical neutrinos Masatoshi Koshiba was awarded the Nobel Prize in Physics in 2002.

The detector was partially restored by redistributing the photomultiplier tubes which did not implode, and by adding protective acrylic shells that it was hoped would prevent another chain reaction from recurring.

In September 2008, the detector finished its latest major upgrade with state-of-the-art electronics and improvements to water system dynamics, calibration and analysis techniques.

This enabled SK to acquire its largest dataset yet (SuperKamiokande-IV), which continued until June 2018, when a new detector refurbishment involving a full water drain from the tank and replacement of electronics, PMTs, internal structures and other parts will take place.

It is a laser interferometer with two arms, each 3 km long, and when complete around 2018, will have a planned sensitivity to detect coalescing binary neutron stars at hundreds of Mpc distance.