Journal Club: The impact of nuclear shape on the emergence of the neutron dripline
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 limit1,2, 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 dripline3.
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 interaction4, 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.