Nuclear rotation in the continuum

TL;DR

This paper investigates how collective rotational states in the continuum, specifically in the halo nucleus $^{11}$Be, are stabilized by decay channel closure and angular momentum alignment, revealing conditions for long-lived states at high excitation energies.

Contribution

It introduces a non-adiabatic coupled-channel approach with the Berggren ensemble to analyze continuum effects on nuclear rotation in $^{11}$Be, highlighting mechanisms that stabilize collective motion.

Findings

Ground band rotation is stabilized by closed decay channels and angular momentum alignment.

High-spin states show increased high-$ ext{l}$ components, reducing decay widths.

Long-lived collective states can exist at high excitation energies in weakly bound nuclei.

Abstract

${\textbf{Background:}}$ Atomic nuclei often exhibit collective rotational-like behavior in highly excited states, well above the particle emission threshold. What determines the existence of collective motion in the continuum region, is not fully understood. ${\textbf{Purpose:}}$ In this work, by studying the collective rotation of the positive-parity deformed configurations of the one-neutron halo nucleus $^{11}$Be, we assess different mechanisms that stabilize collective behavior beyond the limits of particle stability. ${\textbf{Method:}}$ To solve a particle-plus-core problem, we employ a non-adiabatic coupled-channel formalism and the Berggren single-particle ensemble, which explicitly contains bound states, narrow resonances, and the scattering continuum. We study the valence-neutron density in the intrinsic rotor frame to assess the validity of the adiabatic approach as the excitation energy increases. ${\textbf{Results:}}$ We demonstrate that collective rotation of the ground band of $^{11}$Be is stabilized by (i) the fact that the $\ell=0$ one-neutron decay channel is closed, and (ii) the angular momentum alignment, which increases the parentage of high-$\ell$ components at high spins; both effects act in concert to decrease decay widths of ground-state band members. This is not the case for higher-lying states of $^{11}$Be, where the $\ell=0$ neutron-decay channel is open and often dominates. ${\textbf{Conclusion:}}$ We demonstrate that long-lived collective states can exist at high excitation energy in weakly bound neutron drip-line nuclei such as $^{11}$Be.