The discovery of a new type of fission turns a tenet of nuclear theory on its head.
The observation of an unexpected nuclear reaction by an unstable isotope of the element mercury has thrown up a rare puzzle. The enigma is helping theorists to tackle one of the trickiest problems in physics: developing a more complete model of the atomic nucleus. Nuclear fission, the process in which a nucleus heavier than that of iron breaks into pieces, is generally observed to be symmetric, with the resulting fragments being roughly equal in size. Although instances of asymmetric fission are known, they are usually attributed to the preferential formation of 'magic' nuclei, in which shells in the nuclear structure are filled to capacity. Read more