Triplet of kindred prompt-2p emitters in mass-8 proton-rich nuclei

Abstract

A triplet of kindred prompt-2p emitters in A=8 nuclei has been demonstrated. Two of these are the ground state of 8C and its isobaric analog state in 8B, both of which are analogs of the halo or thick-skinned nucleus 8He. The third member is the recently found fourth 1+ state in 8B. This new 8B state at E*=8.4 MeV was observed to decay to the ground state of 6Li by 2p emission. Momentum correlations between the decay products indicate that it is not a sequential 2p decay through a 7Be intermediate state, but indicative of prompt 2p emission with correlations similar to those of the other members of the triplet. Ab initio calculations with the symmetry-adapted nocore shell model indicate that these three states have very similar spatial wavefunctions, but with nucleons coupling to different spins, isospins, and isospin projections. The triplet of 2p emitters all have oblate shapes and by stripping two protons equatorially, decay to states which have prolate shapes.

Figures (5)

• Figure 1: An $A$=8 isobar diagram showing the states of interest in this work. The triplet of 2$p$ emitters are indicated by the thick red lines, and schematically shown by their dominant shape (oblate) and the shape of the decay product (prolate). The analog of the 1$^+_4$ state in $^8$Be is unknown, but indicated by the green line at the expected location.
• Figure 2: Data points show invariant-mass measurements associated with 2$p$ emission in $A$=8 nuclei. (a) The spectrum of 4$p$ decay energy for $^8$C states produced following the neutron knockout from a $^9$C beam Charity:2010. (b) $^8$B excitation-energy distribution of 2$p$+$^6$Li events following proton knockout from this $^9$C beam CharityLongArxiv. The solid red curves are fits to these data with the dashed blue curves showing the mix-event background Charity:2023CharityLongArxiv and the fitted peaks are shown as the dotted green curves. The inset in (b) shows the Doppler-corrected $\gamma$-ray energy distribution for events in the narrow 0$^+_2$ peak where the full-absorption and first-escape peaks (arrows) associated with the $\gamma$ decay of the 3.536-MeV 0$^+$ isobaric analog state in $^6$Li is observed.
• Figure 3: Decay diagram for the 2$p$-emitting states in (a) $^8$C and (b) $^8$B. The levels indicted by the red lines have high isospin and the green levels $\alpha$ decay.
• Figure 4: (a) Data points show the background-subtracted distribution of the $p$-core relative energy in the 2$p$ decay of the 8.4-MeV state. The magenta curve shows the expectation for sequential 2$p$ decay through the $^7$Be(5/2$^-_2$) intermediate state. (b,c) Decay correlations for this 8.4-MeV state are compared to those for $^8$C(g.s.) and its isobaric analog in $^8$B. To help differentiating these data, the green data points are connected by the green lines.
• Figure 5: Probability amplitudes of the strongest configurations predicted by the SA-NCSM for states of interest in this work, given by (a) the distribution of deformation, specified by the $\lambda$ and $\mu$ SU(3) quantum numbers of the SA basis; specifically, $(0 \, 0)$, $\lambda>\mu$, and $\lambda < \mu$, describe spherical, prolate, and oblate deformations, respectively, and (b) the distribution of $L$. Also shown in (a) are the one-body density profiles in the intrinsic frame ($z$ vs $r_{xy}=\sqrt{x^2+y^2}$, where $z$ is the symmetry axis) for the most dominant configurations.