The Impact of Nuclear Shape on the Emergence of the Neutron Dripline

Atomic nuclei are composed of a certain number of protons Z and neutrons N . A natural question is how large Z and N can be. The study of superheavy elements explores the large Z limit 1 , 2 , and we are still looking for a comprehensive theoretical explanation of the largest possible N for a given Z —the existence limit for the neutron-rich isotopes of a given atomic species, known as the neutron dripline 3 . The neutron dripline of oxygen ( Z = 8) can be understood theoretically as the result of single nucleons filling single-particle orbits confined by a mean potential, and experiments confirm this interpretation. However, recent experiments on heavier elements are at odds with this description. Here we show that the neutron dripline from fluorine ( Z = 9) to magnesium ( Z = 12) can be predicted using a mechanism that goes beyond the single-particle picture: as the number of neutrons increases, the nuclear shape assumes an increasingly ellipsoidal deformation, leading to a higher binding energy. The saturation of this effect (when the nucleus cannot be further deformed) yields the neutron dripline: beyond this maximum N , the isotope is unbound and further neutrons ‘drip’ out when added. Our calculations are based on a recently developed effective nucleon–nucleon interaction 4 , for which large-scale eigenvalue problems are solved using configuration-interaction simulations. The results obtained show good agreement with experiments, even for excitation energies of low-lying states, up to the nucleus of magnesium-40 (which has 28 neutrons). The proposed mechanism for the formation of the neutron dripline has the potential to stimulate further thinking in the field towards explaining nucleosynthesis with neutron-rich nuclei.