1When considering the smallest unit of matter - the atom
2most people know of electrons, protons, and neutrons,
3but almost none know of another particle,
4even though they are constantly emitted from the sun in the trillions,
5with 100 to 200 billion of them regularly passing through your body every second.
6To repeat, that's not thousands, not millions, but billions, every second.
7You don't feel them because they are small,
8in fact, so tiny that we can barely detect their presence at all.
9These mysterious particles are called neutrinos.
10Despite such an abundance, detecting them is a huge undertaking,
11and there are many reasons for this.
12Firstly, the neutrino itself is so small that you need to eliminate absolutely all other particles around.
13To do this, you need what is called a clean room,
14one that has an extremely low level of dust, microbes, floating particles, or chemical vapours.
15You probably don't know it, but the air around you right now has almost 40 million particles per cubic meter.
16In contrast, the cleanest of clean rooms has less than 10.
17The second problem is that you also need an environment with absolutely no background radiation.
18At the surface of the Earth, such radiation is all around,
19from the sun and sky, and from TVs and communication devices.
20The only way to screen out all that is to go underground, and I mean deep underground.
21For example, the Sudbury Neutrino Observatory in Canada uses an old nickel mine, one of the deepest in the world,
22and puts the Observatory in its lowest tunnel, more than two kilometers below the surface.
23At such depths, stray radiation is sufficiently screened out to allow neutrinos only to pass by.
24The final problem is that you need an elaborate detection system,
25and this apparatus is huge,
26and its installation in this deep underground cavity presents quite a headache.
27Holding such a weighty construction safe and secure requires complex engineering work,
28such as rock-bolting and support structuring.
29This obviously requires great care, and takes a lot of effort.
30So, I've told you about the difficulty in detecting neutrinos.
31They are tiny, virtually weightless, have no electric charge,
32and hardly interact with anything at all.
33Yet we can detect them, and to see how, let's consider the Sudbury installation once again.
34The detector there consists of a spherical container filled with heavy water.
35This rests inside another vessel filled with normal water,
36which helps support the weight of the inner sphere,
37as well as providing further shielding from any stray radiation.
38At the edge of this inner sphere are about 10,000 electronic detectors.
39These are extremely sensitive, able to multiply a hundred million times any electric current which occurs.
40So, as the neutrinos pass through this sphere of water,
41there is a very very very small chance that one of them may hit a water molecule.
42To increase the likelihood of this, two strategies are used.
43One. the larger the sphere of water, the better,
44and the Sudbury tank holds not 10 tons, not 100 tons, but 1000 tons.
45Two, the water is special, consisting as it does of heavier molecules.
46So, what happens is this.
47If the neutrino hits the water molecule, the neutrino is absorbed,
48but the molecule itself splits apart, producing a tiny electric current.
49It is this which is detected, and analysed, giving key information about the neutrino.
50The final question is why do we care about these elusive particles?
51Well, just think,
52they can pass right through the core of our sun at the speed of light without being affected or losing strength.
53No other form of radiation can do that,
54meaning that the knowledge we get about neutrinos can help us to control them.
55With this ability, we can probe the centre of our Earth,
56the inner layers of our sun, and the outer limits of our solar system,
57and that makes it all worth the effort.