Strong neutron pairing in core+4n nuclei

Abstract

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.