Strong neutron pairing in core+4n nuclei

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${}^{\ast }{\to }^{16}$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${}^{\ast }{\to }^{18}$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.

6 pages, 4 figures, accepted for publication in Phys. Rev. Lett