Francesca Bonaiti

The atomic nucleus is not just a jumble of protons and neutrons. The particles inside arrange themselves in layers, like spectators in a stadium: the lower rows are filled, and each new proton or neutron looks for a spot on a higher tier. When a row is completely filled, the nucleus becomes especially stable — physicists call this a magic number.

Imagine an hourglass where the sand grains transform into other grains as they fall: that's what unstable atomic nuclei are like. They live for an instant and then decay, ejecting an electron (this is beta decay). In September 2025, theorists and experimentalists gathered under the FRIB Theory Alliance to discuss how to use such decays to probe the structure of matter.