Multiconfiguration Dirac-Fock calculations of the hyperfine structure constants and determination of the nuclear quadrupole moment of yttrium 90

The multiconfiguration Dirac-Fock model is employed to evaluate the effects of relativity and electron correlation on the hyperfine interaction constants of the ${4d5s}^2{}^2{D}_{3/2}$ and ${}^2{D}_{5/2}$ levels of yttrium. The wave-function expansions are obtained with the active space method, where configuration-state functions of a specific parity and $J$ value are generated by substitutions from the reference configurations to an active set of orbitals. The active set is then increased in a systematic way, allowing the convergence of the hyperfine interaction constants to be studied. The calculated electric quadrupole hyperfine constants are used to evaluate the nuclear electric quadrupole moment of isotope 90 of yttrium, for which the value $Q=-0.125\left(11\right)\mathrm{}$ b is found.