Strong Neutron Pairing in $\text{core}+4n$ Nuclei

The emission of neutron pairs from the neutron-rich $N=12$ isotones ${}^{18}\mathrm{C}$ and ${}^{20}\mathrm{O}$ has been studied by high-energy nucleon knockout from ${}^{19}\mathrm{N}$ and ${}^{21}\mathrm{O}$ secondary beams, populating unbound states of the two isotones up to 15 MeV above their two-neutron emission thresholds. The analysis of triple fragment-$n\text{−}n$ correlations shows that the decay ${}^{19}\mathrm{N}(-1p){{}^{18}\mathrm{C}}^{*}\to {}^{16}\mathrm{C}+n+n$ is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a ${}^{14}\mathrm{C}$ 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 ${}^{21}\mathrm{O}(-1n){{}^{20}\mathrm{O}}^{*}\to {}^{18}\mathrm{O}+n+n$, attributed to its formation through the knockout of a deeply bound neutron that breaks the ${}^{16}\mathrm{O}$ core and reduces the number of pairs.

Figure 1

Experimental decay energy spectra of ${}^{16}\mathrm{C}+n+n$ and ${}^{18}\mathrm{O}+n+n$ measured in the proton and neutron knockout reactions from ${}^{19}\mathrm{N}$ and ${}^{21}\mathrm{O}$, respectively (blue histograms represent events in coincidence with known $\gamma$ rays in ${}^{16}\mathrm{C}$ and ${}^{18}\mathrm{O}$, corrected by ${ϵ}_{\gamma }$). The corresponding locations of the $2n$ and $4n$ thresholds are noted. The right panels illustrate the shell model configuration of the 12 neutrons in each isotone.

Figure 2

Dalitz plots of $\text{fragment}+n+n$ decays (fragment-$n$ vs $n\text{−}n$ normalized squared invariant masses). The three left panels correspond to ${}^{16}\mathrm{C}+n+n$; the right panels to ${}^{18}\mathrm{O}+n+n$. The four upper panels represent simulations of (a) phase space, (b) sequential decay through a fragment-$n$ resonance, (c) direct decay with $n\text{−}n$ FSI, and (d) a combination of the latter two. The lower panels (e,f) correspond to the experimental data for the decay energies noted.

Figure 3

Projection of the Dalitz plots defined in Fig. 2 onto the axes for the data of ${}^{18}{\mathrm{C}}^{*}$ (left) and ${}^{20}{\mathrm{O}}^{*}$ (right) decays. The rows correspond to the four $E_d$ bins defined in the text, from lower (top) to higher (bottom). The yellow histograms represent phase space, normalized to the data at $m_{nn}^2>0.6$. The red histograms are the projections of the best two-dimensional fit of the plots, with their direct (green) and sequential (purple, with percentage noted) decay components.

Figure 4

(a) Two-neutron correlation functions from the three higher-energy bins of ${}^{18}{\mathrm{C}}^{*}$ (blue) and ${}^{20}{\mathrm{O}}^{*}$ (red) $2n$ decays. The solid lines are traced to guide the eye, while the dashed lines correspond to the fits of the experimental data from the breakup of ${}^{14}\mathrm{Be}$ (green) [30] and the neutron evaporation from ${}^{44}\mathrm{Ca}$ (black) [47]. (b) Numerator (measured relative momentum distribution, blue points) and denominator (phase space, yellow) of $C_{nn}$ for the ${}^{18}{\mathrm{C}}^{*}$ case.