Why neutrons drip out of nuclei

Take an atomic species and keep adding neutrons to its nucleus. You’re going to get a number of stable isotopes, but at some point neutrons simply won’t stick anymore and will ‘drip’ out. The existence limit of neutron-rich isotopes for each atomic species form what is known as the neutron dripline. Finding experimental evidence of the position of the dripline is extremely challenging. But, it turns out, it’s even harder to describe it theoretically. Up to neutron-rich isotopes of oxygen (proton number Z = 8) our theories make correct predictions, but these are at odds with recent experiments on heavier elements. Naofumi Tsunoda and colleagues now present a mechanistic explanation for the origin of the dripline that seems able to reconcile theory and experiments. Their explanation requires going beyond a single-particle picture, as they show that from fluorine (Z = 9) up to magnesium (Z = 12) nuclei accommodate the addition of neutrons by assuming an increasingly deformed, ellipsoid form. The saturation of this effect, when the nucleus cannot be further deformed, yields the dripline as new neutrons are immediately ejected.