© David Parker/Science Photo Library
Raymond Davis Jr (1914–2006) in front of a prototype neutrino detector.
Raymond Davis Jr (1914–2006) in front of a prototype neutrino detector.
In 1930, the Austrian physicist Wolfgang Pauli suggested that energy which seemed to be being lost during certain particle interactions was actually being carried away by a ghostly particle, which became known as the neutrino. The hypothetical particle had to have a very tiny mass (much less than that of an electron) and no electric charge, making it very difficult to detect. It would travel not just through space but through solid matter at almost the speed of light. But if it did not exist, the law of conservation of energy, which is one of the most fundamental laws of science, would have to be abandoned.
Theory said that apart from gravity, neutrinos only interact with matter through the so-called weak nuclear force, which is very weak indeed. If a beam of neutrinos like the ones thought to be produced by the nuclear reactions going on inside the Sun were to travel through a wall of solid lead for 3,500 light years, only half of them would be stopped along the way. Pauli himself thought that he had done ‘a frightful thing’, by proposing a particle that could never be discovered. He considered it so unlikely that any experiment would ever detect neutrinos directly that he offered a case of champagne to any experimenter who successfully took up the challenge.
But the development of nuclear reactors stimulated by the Second World War (see here) offered experimental possibilities undreamed of in the early 1930s. To trap one neutrino is almost impossible. But if you have a lot of neutrinos and a large enough detector you might hope to see the effects of a few of them interacting with the atoms in your detector.
The challenge was taken up by Clyde Cowan and Frederick Reines in the 1950s. Their detector was simply a tank of water (holding about 1,000 pounds, or 400 litres, of liquid) placed next to the Savannah River nuclear reactor in the United States. It was calculated that 50 trillion (5 x 1013) neutrinos should be passing through every square centimetre of the side of the tank every second, and that, in the course of an hour, one or two of these neutrinos should be captured by a proton (a nucleus of hydrogen) in the water. This would convert the proton into a neutron and release a positron, the positively charged antiparticle counterpart to the electron. It was the positrons that Cowan and Reines set out to detect in their experiment. Each positron very quickly meets an electron, when the pair annihilate, emitting a pair of gamma rays with very distinctive properties.
Hints of the anticipated ‘neutrino signal’ came in 1953, and full confirmation that Pauli’s idea was correct came in 1956. Cowan and Reines sent Pauli a telegram telling him the news, and he paid up on his 25-year-old bet by sending them a case of champagne. In 1995 Reines received a share of the Nobel Prize for this work; but Cowan had died in 1974 so could not receive this honour.
This was not the end of the story. Because neutrinos are so reluctant to interact with anything, the neutrinos from the heart of the Sun escape into space and cross past the Earth, and through it, almost unnoticed. Tens of billions of them pass through every square centimetre of your skin every second. Astronomers realized that if some of these solar neutrinos could be detected and analysed, they would provide a direct insight into what was going on in the very centre of the Sun.
It was another ‘impossible’ experiment, but Raymond Davis, of the Brookhaven National Laboratory, decided to give it a try. His detector had to be shielded from all sources of interference, such as cosmic rays (particles from space), so it was constructed 1,500 metres below ground at the bottom of the Homestake gold mine in Lead, South Dakota. Seven-thousand tons of rock had to be removed to make room for the detector, a tank the size of an Olympic swimming pool, filled with 400,000 litres of perchlorethylene, a fluid that used to be used in ‘dry cleaning’ processes.
The key component of this fluid was chlorine. On the rare occasions that a solar neutrino interacted with a chlorine atom in one of the molecules of perchlorethylene, it would convert the chlorine atom into an atom of a radioactive form of argon, which would be released into the liquid. Every few weeks, the fluid in the tank had to be swept clean of argon, by bubbling helium through it, and the number of argon atoms counted by detecting their radioactive decays. The experiment began operating in 1968. After all that effort, each run of the experiment yielded about a dozen counts. One radioactive argon atom was being produced in the tank every few days.
Once Davis had proved that solar neutrinos could be detected, other experiments were devised, and ‘neutrino astrophysics’ is now an important branch of astronomy which has provided insights into the workings of the Sun and into the nature of the neutrinos themselves. Davis received a share of the Nobel Prize, in 2002, for ‘pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos’.