Strong Neutron Pairing in core+4n Nuclei

The emission of neutron pairs from the neutron-rich N=12 isotones 18C and 20O has been studied by high-energy nucleon knockout from 19N and 21O secondary beams, populating unbound states of the two isotones up to 15 MeV above their two-neutron emission thresholds. The analysis of triple fragment-n−n correlations shows that the decay 19N(−1p)18C∗→16C+n+n is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a 14C core surrounded by four valence neutrons arranged in strongly correlated pairs. On the other hand, a significant competition of a sequential branch is found in the decay 21O(−1n)20O∗→18O+n+n, attributed to its formation through the knockout of a deeply bound neutron that breaks the 16O core and reduces the number of pairs.Received 8 January 2018Revised 17 February 2018DOI:https://doi.org/10.1103/PhysRevLett.120.152504© 2018 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDirect reactionsNuclear forcesNucleon-nucleon interactionsNuclear Physics