Strong neutron pairing in core+4n nuclei

TL;DR

This study reveals strong neutron pairing in neutron-rich nuclei, showing direct two-neutron emission dominated decays and highlighting the role of core structure and valence neutron correlations.

Contribution

It provides the first detailed analysis of neutron pair emission in core+4n nuclei, demonstrating the dominance of direct pair emission and the impact of core configuration.

Findings

Largest two-neutron correlation strength observed

Direct pair emission dominates in $^{18}$C decay

Sequential decay component present in $^{20}$O decay

Abstract

The emission of neutron pairs from the neutron-rich $N\!=\!12$ isotones $^{18}$C and $^{20}$O has been studied by high-energy nucleon knockout from $^{19}$N and $^{21}$O 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 $^{19}$N$(-1p)^{18}$C$^*\!\rightarrow^{16}$C+$n$+$n$ is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a $^{14}$C 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 $^{21}$O$(-1n)^{20}$O$^*\!\rightarrow^{18}$O+$n$+$n$, attributed to its formation through the knockout of a deeply-bound neutron that breaks the $^{16}$O core and reduces the number of pairs.