By the mid-1930s, physicists had good reason to believe that their proposed neutrino was real. But nobody would manage to observe it for another two decades – it’s tough to detect a particle that barely interacts with other matter and has little or no mass.
Neutrinos finally revealed themselves in 1956 in an experiment that took advantage of inverse beta decay (not to be confused with electron capture), in which an antineutrino and a proton interact to become a neutron and a positron.
When that positron hits a nearby electron, the two annihilate one another and produce two gamma-ray photons with energies of 0.5 MeV each. When the neutron is captured by an atomic nucleus, that interaction releases a single gamma ray with an energy of 2.2 MeV. Detecting these gamma rays together indicates that the interactions are taking place as predicted – and therefore that neutrinos must exist.
At Los Alamos National Laboratory in the US, Clyde L Cowan and Frederick Raines set about trying to make the discovery using a nuclear reactor expected to produce a huge number of antineutrinos, two nearby tanks of a total of 200 litres of water to provide protons for those antineutrinos to interact with, and 40 kg of cadmium chloride dissolved in the water to absorb the resultant neutrons.
Between the water tanks were other containers filled with a liquid that would create a flash of light when hit by a gamma ray, with the flash then being picked up by an array of photomultiplier tubes. Cowan and Raines calculated that the single gamma ray produced by neutron capture should be detected around five microseconds after the pair produced by electron–positron annihilation, creating an unmistakable signature showing the presence of neutrinos.
It worked. Months of data-collection at Los Alamos had the two physicists detecting an average of three neutrinos an hour. To be certain of what they were seeing, the pair turned off the reactor to see the detection rate plummet. The discovery of neutrinos was confirmed.
Cowan died in 1974, but Raines was awarded the Nobel Prize for Physics in 1995 for their joint efforts. It would prove just one of many occasions on which the award was neutrino-related.
Over the years, Cowan and Raines’ methods were adapted and improved upon, and completely new detection techniques developed. There are now various types of neutrino detectors operating all over the world – and several varieties of neutrinos to detect.
- The 27th International Conference on Neutrino Physics and Astrophysics is taking place at Imperial College London from 4–9 July. The keynote speakers are Nobel Prize winners Professor Takaaki Kajita of the University of Tokyo and Professor Arthur B McDonald of Queen’s University, Canada. Follow the action from the conference on Twitter.