| When considering the smallest unit of matter - the atom |
| most people know of electrons, protons, and neutrons, |
| but almost none know of another particle, |
| even though they are constantly emitted from the sun in the trillions, |
| with 100 to 200 billion of them regularly passing through your body every second. |
| To repeat, that's not thousands, not millions, but billions, every second. |
| You don't feel them because they are small, |
| in fact, so tiny that we can barely detect their presence at all. |
| These mysterious particles are called neutrinos. |
| Despite such an abundance, detecting them is a huge undertaking, |
| and there are many reasons for this. |
| Firstly, the neutrino itself is so small that you need to eliminate absolutely all other particles around. |
| To do this, you need what is called a clean room, |
| one that has an extremely low level of dust, microbes, floating particles, or chemical vapours. |
| You probably don't know it, but the air around you right now has almost 40 million particles per cubic meter. |
| In contrast, the cleanest of clean rooms has less than 10. |
| The second problem is that you also need an environment with absolutely no background radiation. |
| At the surface of the Earth, such radiation is all around, |
| from the sun and sky, and from TVs and communication devices. |
| The only way to screen out all that is to go underground, and I mean deep underground. |
| For example, the Sudbury Neutrino Observatory in Canada uses an old nickel mine, one of the deepest in the world, |
| and puts the Observatory in its lowest tunnel, more than two kilometers below the surface. |
| At such depths, stray radiation is sufficiently screened out to allow neutrinos only to pass by. |
| The final problem is that you need an elaborate detection system, |
| and this apparatus is huge, |
| and its installation in this deep underground cavity presents quite a headache. |
| Holding such a weighty construction safe and secure requires complex engineering work, |
| such as rock-bolting and support structuring. |
| This obviously requires great care, and takes a lot of effort. |
| So, I've told you about the difficulty in detecting neutrinos. |
| They are tiny, virtually weightless, have no electric charge, |
| and hardly interact with anything at all. |
| Yet we can detect them, and to see how, let's consider the Sudbury installation once again. |
| The detector there consists of a spherical container filled with heavy water. |
| This rests inside another vessel filled with normal water, |
| which helps support the weight of the inner sphere, |
| as well as providing further shielding from any stray radiation. |
| At the edge of this inner sphere are about 10,000 electronic detectors. |
| These are extremely sensitive, able to multiply a hundred million times any electric current which occurs. |
| So, as the neutrinos pass through this sphere of water, |
| there is a very very very small chance that one of them may hit a water molecule. |
| To increase the likelihood of this, two strategies are used. |
| One. the larger the sphere of water, the better, |
| and the Sudbury tank holds not 10 tons, not 100 tons, but 1000 tons. |
| Two, the water is special, consisting as it does of heavier molecules. |
| So, what happens is this. |
| If the neutrino hits the water molecule, the neutrino is absorbed, |
| but the molecule itself splits apart, producing a tiny electric current. |
| It is this which is detected, and analysed, giving key information about the neutrino. |
| The final question is why do we care about these elusive particles? |
| Well, just think, |
| they can pass right through the core of our sun at the speed of light without being affected or losing strength. |
| No other form of radiation can do that, |
| meaning that the knowledge we get about neutrinos can help us to control them. |
| With this ability, we can probe the centre of our Earth, |
| the inner layers of our sun, and the outer limits of our solar system, |
| and that makes it all worth the effort. |