When an extra neutron is added to an atomic nucleus, it starts behaving like an overfilled cup: one more drop and it spills over. Physicists at the French facility GANIL orchestrated such a 'pass' of a neutron from a deuteron (the nucleus of heavy hydrogen) to an oxygen-19 nucleus. The result was an oxygen-20 nucleus — with one extra neutron.
This nucleus is unstable: some of its states live for less than a millionth of a second and immediately break apart, emitting a neutron. For the first time, scientists observed several of these short-lived states with energies from 7.6 to 9.8 MeV, like capturing freeze-frames of a falling cup. To spot them, they had to simultaneously detect the ejected proton and the gamma radiation from the decay.
Such neutron-rich nuclei occur in nature, for example, inside neutron stars. By understanding how they live and decay, we get a better picture of how matter behaves in the most extreme corners of the universe.
The oxygen-20 nucleus lives for less than one trillionth of a second, yet it still manages to tell us about the forces that hold atoms together.
Science fiction often features unstable elements that explode at the slightest touch. Here we have a similar idea: an extra neutron makes the nucleus so fragile that it decays almost instantly.