Japan is home to some of the most brilliant physicists of our age. It’s been like that for a long time now, and in today’s article, I will introduce you to one of their greatest instruments, the Super-Kamiokande Observatory. 

How It All Began?

Super-Kamiokande is actually the successor of another great observatory in Japan, the Kamioka Observatory, also a neutrino and gravitational waves laboratory. Its construction began in 1982 and it was completed in April 1983. The observatory’s purpose was to see whether proton decay exists, a fundamental question for our understanding of the Universe. All of the attempts to this day have failed.

Super-kamiokande detector

However, work at the Kamioka Observatory couldn’t be more prolific. For his work directing the observatory, and for the first-ever detection of astrophysical neutrinos, Masatoshi Koshiba was awarded the Nobel Prize in Physics in 2002. It was not going to be the last time the award was going to Kamiokande physicists, as Takaaki Kajita won it for his work in 2015. Ever since their hypothesis, neutrinos have always puzzled the physicists. They are known as nature’s ghost particles.

The Super-Kamiokande detector was designed in response to a very important problem in astrophysics, the solar neutrino problem. It was suspected that the problem had something to do with the neutrino oscillation, and so the Super-Kamiokande was made to test the oscillation hypothesis. Its instruments started functioning in 1996.

Since then, Super-Kamiokande has made enormous progress. 

Method, In Brief

Super-Kamiokande is a large water Cherenkov detector. What does that mean? It contains a cylindrical stainless steel tank holding a lot of ultrapure water. Mounted inside, there are photomultipliers, which detect Cherenkov radiation. The Cherenkov radiation is the electromagnetic radiation emitted when a charged particle passes through a dielectric medium at a speed greater than the (phase) velocity of light in that medium.

Cherenkov radiation - Super-Kamiokande detector

As the medium here is water, we have the velocity of light in water. What happens is that a cone of Cherenkov radiation is created because a neutrino interaction with the electrons of the nuclei of water can produce an electron or positron which travels faster than the velocity of light in water. And so, the directions and flavors of neutrinos can be determined. It is similar to the bow wave produced by a boat traveling faster than the speed of water waves.

Super-Kamiokande: Decoding Nature's Ghost Particles 2
The Cherenkov Radiation

What Is Super-Kamiokande Made Of?

In greater detail, the water tank is 39.3 m in diameter, and 41.4 m in height. There are around 13,000 PMTs (photomultiplier tubes) installed on the detector walls, costing around $3,000 each. The detector is filled with 50 kT of ultrapure water. 

Super-Kamiokande: Decoding Nature's Ghost Particles 3

There are two optically isolated volumes of PMTs, the ID (inner detector), and the OD (outer detector). There are 11,129 inward PMTs, out of which more than a half were destroyed in a cascade after an implosion in 2001. After 2002, in order to avoid other cascade disasters, all of the inner PMTs were covered with fiberglass and acrylic shields. 

So why do they need an outer detector (OD) too? Long story short, it helps distinguish the neutrino events from cosmic muon events. 

Also read:

The Solar Neutrino Puzzle

I feel like I left you with a lot of questions regarding the proton decay, the solar neutrino problem, or the neutrino oscillation. I know I did. I will try to explain them in brief, focusing more on the solar neutrino problem, but after all, they are all related to each other. And all of these point towards one thing: we do not understand how neutrinos work. And that is what Super-Kamiokande tries to do. 

Watch: What is string theory?

When hearing “solar neutrino problem”, we think about the fact that the quantity of neutrinos observed to be coming from the Sun is not the same as the quantity we expected to come from the Sun. This discrepancy was first observed in the 60s. The number deficit was between one half and two thirds.

The Standard Model tells us that neutrinos come in three flavors: electron neutrinos, muon neutrinos, and tau neutrinos. The electron neutrinos are the ones produced by the Sun, and the ones we can detect on Earth.

Standard model
The standard model of particle physics

In the 70s, it was believed by almost the whole scientific community that neutrinos were massless. However, Bruno Pontecorvo proposed in 1968 that neutrinos could have mass, and if they could, he came to the conclusion that they could change flavors. So, the solar neutrinos that were “missing” could be actually there, but in other flavors that we cannot observe.

Basically, in the last paragraph, I described briefly to you what neutrino oscillation is. It is the “oscillation” between flavors. Strong evidence in favor of the existence of neutrino oscillation came in 1998, from the Super-Kamiokande detector. Takaaki Kajita was awarded the Nobel Prize in Physics for his work on these experiments. 

More in particle physics:

What Will We See Next?

There are a lot of problems we still did not find the answer to, including the primary goal of the Kamioka Observatory: finding whether proton decay exists or not. Construction of the Hyper-Kamiokande detector began in 2019, and it is expected to start working this year. The beginning of data-taking is scheduled for 2027. The detector was ranked in the 28 top-priority projects of Japan.

Read all the articles of the Basics of Astrophysics series here

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