Evidence for a spin-aligned neutron–proton paired phase from the level structure of 92Pd

In nuclei with equal neutron (N) and proton (Z) numbers, the observed enhanced neutron–proton correlations are predicted to favour isoscalar neutron–proton pairing, an unusual interaction that is distinct from normal nuclear superfluidity. Now, in a major experiment at the GANIL heavy-ion accelerator in France, observations of excited states of the N = Z = 46 palladium nucleus 92Pd provide evidence for spin-aligned isoscalar neutron–proton pairing that is rather different to that predicted, and which has not been seen previously in nuclei. Nuclei with equal neutron (N) and proton (Z) numbers show enhanced correlations that have been predicted to favour an unusual type of pairing, distinct from normal nuclear superfluidity. Here, technically challenging observations are reported of excited states in the N = Z = 46 nucleus 92Pd, from which evidence is inferred for a type of spin-aligned structure in the ground and low-lying excited states, not established in nuclei before and differing from previous predictions. Shell structure and magic numbers in atomic nuclei were generally explained by pioneering work1 that introduced a strong spin–orbit interaction to the nuclear shell model potential. However, knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), enhanced correlations arise between neutrons and protons (two distinct types of fermions) that occupy orbitals with the same quantum numbers. Such correlations have been predicted to favour an unusual type of nuclear superfluidity, termed isoscalar neutron–proton pairing2,3,4,5,6, in addition to normal isovector pairing. Despite many experimental efforts, these predictions have not been confirmed. Here we report the experimental observation of excited states in the N = Z = 46 nucleus 92Pd. Gamma rays emitted following the 58Ni(36Ar,2n)92Pd fusion–evaporation reaction were identified using a combination of state-of-the-art high-resolution γ-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron–proton coupling scheme, different from the previous prediction2,3,4,5,6. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling7,8) in the ground and low-lying excited states of the heaviest N = Z nuclei. Such strong, isoscalar neutron–proton correlations would have a considerable impact on the nuclear level structure and possibly influence the dynamics of rapid proton capture in stellar nucleosynthesis.