Simplified Version of the Nucleon and Delta-Isobar Description

The low-lying $1/2^+$ state of $^9$Be has been interpreted either as a near-threshold resonance or as an $^8$Be+$n$ virtual state. Because both pictures can produce a pronounced threshold enhancement, its nature is difficult to determine from energy-domain observables alone. We investigate the low-lying $1/2^+$ state through its real-time decay dynamics and compare its behavior with that of the well-established $5/2^+$ resonance. A wave packet produced by electric-dipole excitation of the $^9$Be ground state is propagated in an $α+α+n$ three-body model. We analyze the survival probability, the fit-window dependence of the effective decay width, and time-dependent densities in different Jacobi-coordinate representations. The $1/2^+$ survival dynamics are further compared with an $s$-wave effective-range description motivated by the dominant $^8$Be+$n$ component found in our previous three-body studies. The $5/2^+$ channel shows approximately exponential late-time decay, with an effective width of $0.264$ MeV, close to the complex-scaled pole width of $0.285$ MeV. In contrast, the $1/2^+$ effective width decreases as the fitting interval is shifted to later times, and no stationary decay width is obtained. Its late-time survival probability is well described by the effective-range spectral distribution. The density evolution develops an $^8$Be+$n$-like asymptotic configuration while retaining a localized $p_{3/2}$ $αn$ component suggestive of a $^5$He+$α$-like configuration in the internal region. The $1/2^+$ decay differs qualitatively from resonance-like exponential decay and is consistent with near-threshold virtual-state dynamics. The distinct internal and asymptotic configurations revealed by the time-dependent densities provide complementary structural information beyond that contained in a single decay width.

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arXiv2026

Precise description of spontaneous fission half-lives of Rutherfordium isotopes within the relativistic Hartree-Bogoliubov theory

I investigate the spontaneous-fission (SF) half-lives of $^{254,256}$Rf within the multidimensionally constrained relativistic Hartree-Bogoliubov (MDC-RHB) model combined with the local quasiparticle random-phase approximation (LQRPA) method. The collective inertia is evaluated using the finite-amplitude method (FAM), which enables a fully self-consistent treatment of dynamical residual effects beyond the conventional cranking approximation. The effective collective potential is constructed by subtracting both the vibrational zero-point energy and rotational energy correction from the deformation energy surface. The SF half-lives are evaluated within the Wentzel-Kramers-Brillouin (WKB) approximation. Compared with the nonperturbative and perturbative cranking approximation, the FAM-LQRPA collective inertia is significantly enhanced, especially around the ground-state minimum, due to the dynamical residual interaction included in the QRPA. As a consequence, the predicted SF half-lives increase by several orders of magnitude and become consistent with experimental data. The calculated SF half-lives are 5.65 $μ$s for $^{254}$Rf and 0.95 ms for $^{256}$Rf, both within approximately two orders of magnitude of the corresponding experimental values. These results indicate that a self-consistent treatment of dynamical residual effects and beyond mean-field correlations is crucial for quantitative microscopic descriptions of SF within covariant density functional theory.

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arXiv2026

Energy-Resolved Spin and Parity Distributions of Fission Fragments

Fission fragments are born rotating rapidly, but the evolution of this spin with excitation energy and its impact on fission observables remain largely unknown. Combining state-of-the-art microscopic and statistical reaction theory, we simulate the full fission process - from the initial state, through scission, to fragment decay - and report the first microscopic, energy-resolved spin and parity distributions of the fragments produced in $^{239}$Pu(n,f). From thermal energy to $E_n = 10$ MeV, the spin of individual fragments evolves non-monotonically, tracking their deformation. The parity content, strongly non-statistical at thermal energy, remains correlated with spin throughout this range. For the first time, we quantify the impact of microscopic spin-parity distributions on prompt photon multiplicities, demonstrating the predictive potential of parameter-free microscopic inputs.

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