Study of the Hyperfine Structure of the Low-Lying Ca II and Ra I-II Levels: Applying the MCDHF Models Developed for Ba I-II

Building on our previous Multiconfiguration-Dirac-Hartree-Fock (MCDHF) computational strategies taylored for the hyperfine structure (HFS) of low-lying levels in one-valence electron Sr II and Ba II ions, and in two-valence electrons Ba I atom [Nezosi et al, Atoms 14, 17 (2026). DOI: 10.3390/atoms14030017], we successfully extend these methodologies along the alkaline-earth elements to the lighter Ca II and the heavier Ra I-II ions. MCDHF recommended HFS constants, along with their uncertainty estimates, are reported for the first time and critically discussed. Where applicable, the Bohr-Weisskopf correction is applied to the HFS constants, and its effects are analyzed. This is particularly valuable for cases where no measurement is available such as for the [Rn]\(6d ^2D_{3/2,5/2}\) levels in ^223,225Ra II, for the [Rn]\(7s6d~^3D_{1,2,3}\) and ¹D₂ levels in ²²³Ra I, for the [Rn]\(7s6d ^3D_{2,3}\) and ¹D₂ levels in ²²³Ra I, and for the [Rn]\(7s7p~^3P^o_2\) level in ²²⁵Ra I. In all cases, our MCDHF values agree with the ones found in the literature within our error bars. For the ²D_5/2 levels in Ca II and Ra II, the discrepancies with experiment observed in Sr II and Ba II for the HFS \(A\) constant are not seen.