Discovering how heavy elements were made by measuring masses of short-lived, nuclear-rich nuclei

Nuclear physicists at RIKEN have measured the mass of a neutron-rich nucleus of the element palladium to a greater accuracy than ever before¹. When this mass is fed into models of merging neutron stars, it reproduces well the abundance of heavy nuclei found in the Solar System.

The Big Bang is thought to have only produced the three lightest elements: hydrogen, helium and lithium. The other 80 or so stable elements were formed through later processes such as supernovae, the death of low-mass stars, and nuclear fission induced by cosmic rays.

The mergers of neutron stars may have created a large proportion of heavy nuclei that contain 41 protons or more. They are conjectured to create new elements through nuclei capturing loose neutrons, followed by a neutron decaying into a proton and an electron through beta decay.

A long-term aspiration of nuclear astrophysicists is to be able to reproduce the abundances of the elements that we see in the Universe today. But that requires precise knowledge of nuclear parameters including nuclear masses, which is challenging because many neutron-rich nuclei are highly unstable and have only fleeting existences.

“A big problem is that the abundances are very difficult to calculate because the nuclei involved in this process are unstable and so we don’t know much about them,” explains Sarah Naimi at the RIKEN Nishina Center for Accelerator-Based Science.