neutrino astronomy

The attempt to detect neutrinos from cosmic sources, especially the Sun. Neutrinos are elementary particles with no electric charge and almost no mass, and they interact only very weakly with other matter. They travel essentially at the velocity of light and are produced in vast quantities by the nuclear reactions that take place in the centres of stars and in supernova explosions. Because they hardly interact with matter at all, neutrinos are very difficult to detect. The longest-running experiment to search for solar neutrinos, at Homestake Mine, South Dakota, was designed to use the fact that occasional neutrinos interact with a chlorine atom, converting it to a radioactive isotope of the gas argon. The detector consisted of a tank containing 400,000 litres of the cleaning fluid, carbon tetracholide. Such experiments need to be underground to avoid confusion from events due to cosmic rays. Theoretical considerations suggested that one interaction should be detected per day by this set-up. In practice, only one-third that number have been seen. This discrepancy is known as the neutrino problem. In another form of neutrino detector shown to work successfully, detectors in a large tank of water pick up Cerenkov radiation generated by the interaction of electrons with solar neutrinos. Detectors of this type, the Kamiokande experiment in Japan and a similar detector in Ohio, made the first observation of neutrinos from a supernova - those from SN1987A. In 1996 Kamiokande was superseded by a larger version, Super-Kamiokande. A European collaboration (GALLEX) and a Russian experiment have been designed to make use of the interaction of neutrinos with gallium. As results become available from new, more sensitive experiments, it is hoped to determine whether the current theories of solar physics are faulty or whether unknown physical processes are the cause of the "neutrino problem".