Information about Super-Kamiokande.


See also pictures of Super-Kamiokande.

Contents


Overview.

Compact description (with jargon)
Super-Kamiokande is a 50,000 ton ring-imaging water Cerenkov detector located at a depth of 2700 meters water equivalent in the Kamioka Mozumi mine in Japan. It is used mostly for the search for proton decay (nucleon decay in general), observation of neutrinos (solar, atmospheric, from super-nova, ...) and cosmic rays (mostly muons: downward going muons created by cosmic ray particles in the atmosphere, and upward going muons created by neutrino interaction in the Earth beneath the detector).

Background
Some of the things that the detector can see, but we don't want it to see (a so called background to interesting signals) is natural radioactivity in the surrounding rocks and water (mostly radon gas), and to some extent cosmic muons. The detector is located deep underground in order to shield it from cosmic ray muons by the rock above it.

General layout
Super-Kamiokande is a big tank of very clean water. It is a cylinder of roughly 40 m diameter and 40 m height. On all the walls (side, top and bottom) there are many (about 13000) photomultiplier tubes (PMT's). They are very sensitive light detectors and they can detect single photons. They are 'looking' toward the inner volume of water.

What is Cerenkov light?

Any electricaly charged particle (with sufficiently large energy) traveling in water produces Cerenkov light. More precisely - the light is generated when a particle moves with a speed greater than the speed of light in water. Of course it is always less than the speed of light in vacuum. In such a case the particle generates something analogous to the shock wave generated by supersonic aircraft in the air. The 'optical shock wave' - light - is emitted in a cone.

When the cone of the light reaches the wall of the detector it forms on the wall a pattern shaped like a ring. The photomultipliers can detect the light when it arrives at the wall. They can measure how much light arrived and the time of arrival.

How does a photomultiplier work?

A photon enters the photomultiplier through the glass surface and hits the photo-cathode, which is placed on the inner surface of the glass. The photo-cathode hit by the photon emits an electron. A photo-cathode is just a special substance which is very efficient at doing this. Inside the photomultiplier there is a vacuum.

The electron is attracted and accelerated to the first dynode, which is charged positively by a high voltage. As it hits it with great energy, the dynode emits several electrons, which are then attracted to a second dynode, which has even higher positive electric potential. This process repeats many times.

At the last dynode we have a really huge number of electrons. This way the signal of a single electron was enormously amplified. This is why photomultipliers are such sensitive light detectors.

The path of electronic signals.

The signal created by the swarm of electrons enters a cable and travels from the photomultiplier to the electronics hut located at the top of the tank. There it enters amplifiers, analog-to-digital converters, and other electronics. The signal from all photomultipliers is converted to digital form, analyzed and passed to computers which further analyze it.

At many stages decisions have to be made (by electronics or computers) whether there is an interesting physical signal seen by the photomultipliers, or whether it is just noise. One very important part of the electronics which makes this decision is called a trigger, as it triggers other stages of data processing. This way a lot of noise is rejected.

At the last computer all data is stored on magnetic tapes. It is to be analyzed later, off-line, by physicists.

Why do we do all of this?

What are they (we) looking for? For many things.

Proton decay.

One of them is proton decay.

Proton decays? Nonsense!
Everybody knows that protons are stable particles i.e. they don't decay into any other lighter particles (positron i.e. anti-electron for example). Otherwise - all the matter around us would decay - rocks, water, and us.

This is possible if...
Well, that's true, but how about if the protons, even if they decay, would have a long lifetime i.e. the decay probability for a given proton at the given time would be sufficiently low? I mean really long - much longer than the lifetime of the entire Universe?

How to see it?
Then, you would say, you cannot see proton decay at all, so you cannot say can if it decays or not, and there is no point in trying to see it. Hm... not exactly. You see - there are many protons around - millions of millions of... Even if the decay probability of a single proton is low, if you have many of them in one place and let them stay for a long time, you may be able to see a few of them to decay. This is what we are looking for. This is also why the detector is so large.

The decaying proton would create energetic particles, which would generate light in the water, which in turn can be seen by our detector.

Why would they decay?
You may say: but why would one expect protons to decay? Good question. There are theories which predict proton decay. [...write more about extensions of the standard model...] They are not some crazy, odd theories - they are very serious. They allow us to unify different types of elementary interactions, namely: strong, weak and electromagnetic interactions. Proton decay is one of several predicted consequences of the theories. [...more about theory to be added...]

What is a neutrino?

Other thing we are looking for are solar neutrinos. What is a neutrino? The neutrino is a light (some say massless), neutral (no electrical charge) particle virtually non-interacting with matter. Millions of millions of them are crossing the Earth at each second, but only very few of them would interact with the Earth. In practice you can say - they are invisible. So how can we detect them? Well - you can guess the answer by now - by building a very large detector and waiting long enough.

The neutrino was detected for the first time about 40 years ago in 1956. The first observation of a neutrino was made by Frederick Reines, professor at UCI, who received the 1995 Nobel Prize for this work.

The solar neutrino puzzle.

The Sun is a neutrino source.
One source of neutrinos are nuclear reactions. Inside our Sun nuclear reactions are occurring on a gigantic scale. Lots of neutrinos are produced. There is enough of them, that when they reach the Earth they can still be detected.

The puzzle.
And people see them. And other people can calculate how many of them should be seen. But there is a big problem: we see too few solar neutrinos! Roughly 2 times too few. This is so called the solar neutrino problem.

Possible solutions.
There can be several solutions to the puzzle.

1. We don't understand the Sun well enough.
We may be predicting the wrong number of neutrinos produced inside the Sun. This is ruled out easily - we know exactly how much energy is produced by the Sun, and we know how many neutrinos we should expect per unit energy. But a modified version of this explanation is more difficult to reject: we may be predicting the wrong shape of the neutrino energy spectrum. It turns out that the efficiency of our detectors drops down very quickly as we go down in neutrino energy. So measuring the shape of the neutrino energy spectrum is important.

2. Neutrino oscillations.
Another explanation is that maybe there is something about neutrinos that we don't know about. The thing, which many people suspect may solve the problem, is neutrino oscillations.

There are 3 types of neutrinos in nature: electron-, muon-, and tau-neutrinos. Inside the Sun electron-neutrinos are produced. We know that all the neutrinos are light particles, possibly massless. But if they have some (small) mass, there is a possibility of 'mixing' between them if the so called 'mixing angle' is non-zero.

Mixing (and oscillations) of particles is nothing new in physics, it has been observed a long time ago in the neutral kaon system. The idea is that an electron-neutrino on its way from the Sun to the Earth can transform itself into e.g. a muon-neutrino and which will escape detection.

Atmospheric neutrinos.

Another source of neutrinos is... Earth's atmosphere!

Showers.
They are produced by energetic cosmic ray particles. The particle after entering the atmosphere interacts with air atoms and produces several other particles, which in subsequent interactions with air produce even more particles, and so on. It is called a cosmic ray shower.

Some of the produced particles are unstable. They are mostly pions which decay into muons, which then decay to electrons, which are stable of course and don't decay any further. At each of these decays neutrinos are produced. They are electron- and muon-neutrinos (and anti-neutrinos).

Discrepancy.
We can calculate how many neutrinos of each type are produced. We can detect them in our detectors and compare the measurement with the prediction. And they disagree! The measured ratio of electron-neutrinos to muon-neutrinos is different then the predicted ratio. This could be explained by neutrino oscillations.

Maybe we don't know neutrino interactions well enough.
Another explanation is that maybe we don't understand interactions of neutrinos with our detectors. The trouble is that neutrino interactions are made complicated by the fact that most of the neutrons and protons are bound together in oxygen nuclei in water.

Super-nova.

There are even more exotic sources of neutrinos, like super-novas. Super-novas are exploding stars. Not every star explodes as a super-nova. This is how the explosion can happen.

The story of one star...

Gas cloud.
At the beginning, somewhere in the empty space of the Universe there is a cloud of gas. It is giantic - as are most things in the cosmic scale.

Shrinking (collapsing).
We know (as Isaac Newton discovered when an apple fell from a tree on him) that all the bodies in the universe attract each other by means of the force of gravity. So as time goes on, particles of the gas come closer and closer to each other, and the gas cloud shrinks. As it shrinks - its density grows. And as its density grows - its temperature increases as well - it becomes hotter and hotter.

Microscopic view.
Let's have a look at single atoms. They are mostly hydrogen atoms, because this is the most abundant element in the Universe. The hydrogen atom consists of a heavy positively charged nucleus (proton) and a light electron orbiting around it.

Plasma.
As the temperature grows - the atoms are hitting each other more and more violently, and eventually they kick off an electron from its orbit around a proton, which we call ionization. Then we have a mixture of protons and electrons, not bound together in atoms, but floating freely and moving very fast. This is called plasma.

Nuclear fusion.
As the gas cloud shrinks more (it looks like a giant fire-ball right now), protons collide with each other more strongly. When they hit each other strongly enough - nuclear reactions can occur. Eventually they can form a helium nucleus - 2 protons and 2 neutrons bound together. In the process of the formation of the helium nucleus - 2 protons are transformed into neutrons, and neutrinos are produced. Each time the transformation between a proton and a neutron occurs - neutrinos are produced.

Energy, burning, equilibrium.
In the process of the helium nucleus formation energy is released. This is very important. The energy heats the gas cloud from inside and increases the pressure. This pressure holds the gas cloud from further collapsing. The process of the formation of helium nuclei from protons (hydrogen nuclei) is sometimes called burning of hydrogen into helium. Of course it has nothing to do with the burning we know everyday (a candle for example) which is a chemical reaction, not a nuclear reaction. The cloud no longer collapses but keeps constant size and density. There is equilibrium between gravitational forces pulling it to collapse and pressure generated by hydrogen burning.

Star.
At this stage we call the cloud - a star. This is how our Sun looks today. This period of star life is very stable and long. Hydrogen burning is very efficient energy generation, and there is plenty of hydrogen inside the star.

Helium burning.
But finally all the fuel (hydrogen) is burnt - then what? Then not enough energy is generated to hold the pressure and keep the star from collapsing - the star starts shrinking. As it shrinks - its temperature grows and helium nuclei collide more strongly. At some point another nuclear reaction can occur - helium nuclei can join together to form even heavier elements, and release energy. This makes more pressure which stops the collapse again. The star enters another period of its life - helium burning.

Stages of burning.
This scenario repeats several times - the star keeps burning heavier elements in sequences of collapses and stable periods. But it can't do it forever.

No more fuel.
There is an element, namely iron, which can't burn anymore. When you try to form from iron nuclei a heavier nucleus, it turns out that you don't release any energy, you even must put in some energy in order to make this process occur. When the star reaches the stage when its core is mainly iron, it can't burn.

Super-nova.
The balance of gravitational and internal pressures is lost. Suddenly the star collapses, and there is nothing (for the moment) that can stop it. In a few seconds (!) the interior of the star collapses. This collapse produces a giant shock wave, which travels outwards and pushes outer layers of the star away, heating them. At this moment the star becomes as bright as millions of stars, comparable to the brightness of the whole galaxy. This is a super-nova.

Neutron star
But what is going on with the core of the star? The electrons and protons get squeezed together so much, that they join together to form neutrons.

Neutrinos
And as you remember: each time there is a transformation between protons and neutrons - neutrinos are released. At that moment a huge number of neutrinos is produced. They can be detected on Earth.

SN1987A
And it really happened, quite recently. In 1987 a super-nova (called SN1987A) explosion occurred in a nearby galaxy (a small irregular galaxy which is a satellite of our Galaxy) the Large Magellanic Cloud, at about 170000 light years away. And the neutrino burst was observed by IMB and Kamiokande detectors. A few neutrino events in a short period of a few seconds were seen exactly at the same moment in both detectors. You can see the picture of the supernova after the explosion.

Neutron star
But let's return to our star. At this moment we have a bizarre object - a neutron star. This is like a huge nucleus, all made from neutrons. This object is incredibly dense. The so-called Fermi-Dirac gas pressure can stop the star from further collapsing.

Black hole
But if the mass of the star is large enough, then the gravitational force is strong enough to overcome even this pressure and the star continues collapsing eventually forming a black hole. This is an exotic object. The gravity in it is so strong, that even light cannot escape it. Time-space in the vicinity of it is badly distorted. Anything that goes beyond so-called 'horizon' can't leave the black hole.

Can we see it again?
How often do super-nova explosions occur close enough that the neutrinos from there can be observed on Earth? This means in practice that the explosion should occur in our Galaxy. Some estimates say that this occurs several times in each century. So if we are lucky, we can see it.


Acknowledgments

I would like to thank Prof. Dennis Silverman and Dr. Danuta Kielczewska for the time they took to read this document and make corrections (especially English) and suggestions.