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 consta nts of the 4d5s(2) D-2(3/2) and D-2(5/2) levels of yttrium. The wavefunctio n expansions are obtained with the active space method, where configuration -state functions of a specific parity and J value are generated by substitu tions from the reference configurations to an active set of orbitals. The a ctive set is then increased in a systematic way, allowing the convergence o f the hyperfine interaction constants to be studied. The calculated electri c quadrupole hyperfine constants are used to evaluate the nuclear electric quadrupole moment of isotope 90 of yttrium, for which the value Q=-0.125(11 ) b is found. [S1050-2947(98)06509-3].