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 constant with their uncertainty estimates are reported for the first time. This is particularly valuable for cases where no measurement is available such as for the [Rn]$6d~^2D_{3/2,5/2}$ levels in $^{223,225}$Ra~II, for the [Rn]$7s6d~^3D_{1,2,3}$ and $^1D_2$ levels in $^{223}$Ra~I, for the [Rn]$7s6d~^3D_{2,3}$ and $^1D_2$ levels in $^{223}$Ra~I, and for the [Rn]$7s7p~^3P^o_2$ level in $^{225}$Ra~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 [Nezosi et al, Atoms 14, 17 (2026). DOI: 10.3390/atoms14030017] are not seen.