I took this picture, which is a photograph of the photomultiplier tubes in the Super Kamiokande neutrino observatory in Japan. This photograph was part of “The Universe and Art” exhibition in Singapore’s ArtScience Museum in April’17.
The Kamiokande was the first neutrino detector which could measure the direction of neutrinos and proved that they were indeed coming from the Sun. It started running in 1987 as a 2140 ton water-Cherenkov detector, situated in Kamioka mine, at a depth of 1000m.
Its’ measured flux of solar neutrinos confirmed the “Solar Neutrino Problem” which was first established by the Homestake Chlorine Solar Neutrino Experiment in 1968 by R. Davis et al. This problem was that the measured flux of neutrinos from the sun were half of the flux predicted by the Standard Solar Model. Later, in 1999, the SNO Solar Neutrino Experiment was the first to confirm the existence of neutrino oscillations, which was a solution to the solar neutrino problem.
In 1996, the Kamiokande upgraded to Super-Kamiokande, which was a 50 kiloton water-Cherenkov detector. A cherenkov detector detects cherekov radiation, which is electromagnetic radiation emitted when a charged particle with sufficiently large energy travels through the a medium (in this case, water) with a speed greater than the speed of light in that medium. It is somewhat like an “optical shock wave”. Cherenkov radiation is emitted in a cone, which when reaches the wall of the photomultiplier detectors, form a ringed pattern shape:
credit: http://www.slac.stanford.edu/econf/C040802/lec_notes/Casper/Casper.pdf
How a photomultiplier works: A photon enters it through the glass surface. It hits the photocathode which is placed on the inner surface of the glass. The photocathode, when hit by the photon, emits an electron. The electron is attracted and accelerated to the first dynode, which is charged positively by a high voltage. This causes the dynode to emit several electrons. These electrons are attracted to a second dynode, which has an even higher positive electric potential. This process repeats many times till the last dynode has a huge number of electrons. This is how the signal of a single electron is enormously amplified.
Neutrino research have spanned across particle physics, nuclear physics, astrophysics and cosmology. With the first discovery stage (of neutrino masses and mixing) over, the second discovery stage aims to solve the remaining issues:
What is the absolute value of neutrino masses?
To establish the character of the neutrino mass spectrum
To search for sterile neutrinos (does it exist?)
Are neutrinos with definite masses Majorana or Dirac particles?
To investigate effects of CP violation in the lepton sector and to determine the phase angle.
This is all so exciting as the existence of neutrino oscillation implies the need for an extension or a revamp of the Standard Model.
