Light, stable nuclei typically contain approximately the same number of protons and neutrons. Although isotopes can be generated that stray far from that balance, there is a limit, known as the drip line, to the number of excess neutrons a nucleus can hold. Beyond that point, the nuclear potential is not strong enough to keep another neutron bound to the atom, even in a short-lived metastable state; it drips off.
With 12 protons and 28 neutrons, magnesium-40, which was discovered in 2007, is precariously close to the drip line. In fact, 39Mg is already unbound: Its outermost neutron is not confined to the nucleus. However, neutron pairing makes 40Mg a bound state. Now a collaboration between researchers at Lawrence Berkeley National Laboratory, the RIKEN Nishina Center for Accelerator-Based Science in Japan, and the University of Tokyo has made the first gamma-ray spectroscopic measurements on 40Mg and revealed an unexpected nuclear structure.
Previous studies of neutron-rich Mg isotopes up to 38Mg have painted a consistent picture of a prolate spheroid (egg-shaped) nuclear structure. The first two excited-state energies observed for those isotopes (circles in the graph) agree with model predictions (indicated by lines), and the cross-section observed when 40Mg is generated from the removal of two protons from silicon-42 indicates that 40Mg should have the same shape.
In contrast, the energies for the first two nuclear transitions in 40Mg (stars in the graph) are much lower than those predicted by the models. At about 500 keV, the energy of the lower energy transition, which is tentatively ascribed to the relaxation from the first excited state to the ground state, is about 20% below all current predictions. Even more baffling is the transition from the second to the first excited state: At roughly 670 keV, its energy is only about half that predicted by various models.
Although they cannot rule out other causes, the researchers are leaning toward weak binding effects to explain the unexpected transitions. Because it is so close to the drip line, 40Mg may resemble a 38Mg nucleus surrounded by a halo of two weakly bound neutrons. More detailed theoretical calculations are needed to see if such a structure could produce the observed transition energies. (H. Crawford et al., Phys. Rev. Lett. 122, 052501, 2019.)