Neutrino: is the sun still shining? – Frank Close at the Royal Institution

NeutrinoFrank Close is Professor of Physics at Oxford University and a Fellow of Exeter College. He was formerly vice president of the British Association for Advancement of Science, Head of the Theoretical Physics Division at the Rutherford Appleton Laboratory, and Head of Communications and Public Education at CERN. And he’s a very interesting speaker. I heard him speak last year at the British Science Festival about his book Antimatter, the antidote to Angels and Demons.

This talk (the audio for which is already on the website) at the Royal Institution was to promote his new book, Neutrino, which is as much about the story of the discovery of the particle as about the particle itself. Frank Close won a writing award for his obituary of Ray Davies, the scientist whose work proved the existence of the neutrino, and who won the Nobel Prize in 2002.

Neutrinos are very hard to detect as they have a tendency not to interact with anything – they were described as being like a bullet passing through a bank of fog. And they are very common, 40,000,000 neutrinos pass through each of your eyeballs every second; the fusion in the sun produces 2×1038/second, of which 400 billion per second hit every square metre of the earth; and a person produces about 400 per second.

The discovery of the neutron and beta decay in 1932 showed an unexplained energy loss, which Pauli postulated was cause by the loss of a particle which he called the neutrino, but which would be nigh-on impossible to detect, but they have been detected, and people are now doing neutrino astronomy. If you’ve heard about tanks of cleaning fluid in mines, then they’re looking for neutrinos. If a neutron hits a chlorine atom it knocks out an electron and produces argon.

They are very hard to detect because it is hard to get them to do anything, so the answer is to have a very large ‘net’ to catch them in. Hence 100,000 gallons of cleaning fluid, in a mine to keep out extraneous ‘noise’. Over forty years they detected solar neutrinos – about 40 a year, which was approximately one third of the predicted number, and this came to be known as the Solar Neutrino Problem. The answer to the problem is that neutrinos come in different ‘flavours’ – muon, electron, and tau – and it is now known that neutrinos have a mass (an extremely small one) and therefore, quantum theory tells that they can oscillate between types. As there are three types, one type was detected.

Nuclear reactors produce antineutrinos, whereas the sun produces neutrinos; the electromagnetic radiation produced by the sun was actually produced 100,000 years before it reaches the earth, as it has been bouncing round inside the sun at the speed of light trying to get out; neutrinos arrive at the earth 8 minutes after they are produced.

Theory suggests that 99% of a supernova is produced in neutrinos; serendipitously this was verified when two years after a neutrino detector was made, a supernova was observed, and the neutrinos which had travelled 187000 light years were detected.

An interesting thing is that neutrinos travel through water faster than light does (light travels at about 75% of c through water, whereas neutrinos just carry on at almost c); this causes an electromagnetic shockwave (Cherenkov radiation) which can be detected.

There’s a large water-based detector in Japan, and CERN are producing neutrinos which are fired at a detector in Gran Sasso in Italy; on May 31 this year they measured an event which is the first tau particle candidate.

In the future it is hoped that neutrino astronomy will enable us to observe the centre of the galaxy, and also the echo of the Big Bang. To this end a detector is being built, using the ice on Antarctica as the detector, with hundreds of photodetectors over hundreds of square kilometres.

And neutrinos may be candidates for Dark Matter; they may also hold the answer to the large scale asymmetry of the universe.

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