Neutron skin thickness and its volume and surface contributions in berkelium isotopes

Abstract

Accurate determination of the neutron skin thickness ($\Delta R_{\rm np}$) in finite nuclei is essential for constraining the density dependence of the nuclear symmetry energy. This work presents a systematic investigation of $\Delta R_{\rm np}$ for the transuranium berkelium (Bk) isotopes within the framework of the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc). The results indicate an overall increase in neutron skin thickness with $N$, which exhibits antikinks at the shell closures $N = 184, 258$ due to the shell effects. A decomposition of $\Delta R_{\rm np}$ into volume and surface terms, based on two-parameter Fermi (2pF) fits to angle-averaged DRHBc densities, demonstrates that the volume term dominates as much as $60\%$--$70\%$ in most nuclei, consistent with the $65\%$ found in $^{208}$Pb, thereby validating the volume-surface decomposition for deformed nuclei and confirming its correlation with the symmetry energy slope $L$. The surface term prevails only near the proton drip line, where the volume fraction drops below $50\%$ due to the reduced neutron-to-proton ratio. Deformation is found to slightly reduce the central radius $R_c$ but markedly enhance the surface diffuseness $a$, leading to a notable increase in $\Delta R_{\rm np}$, primarily driven by the surface term. Furthermore, we extend the decomposition to a directional analysis by extracting 2pF parameters along the symmetry axis ($\theta=0^\circ$) and perpendicular to it ($\theta=90^\circ$). In prolate deformed nuclei, a strong directional dependence is observed: although the nucleus is elongated along the symmetry axis, $\Delta R_{\rm np}$ is significantly larger in the perpendicular direction. This anisotropy is weak for oblate nuclei around the shell closures.