Nuclear structure of the stable even-A calcium isotopes based on new experiments and shell model calculations

In view of the striking implications of the recently measured g factors for the 2₁⁺ states of ⁴²Ca and ⁴⁴Ca we have supplemented this study with similar measurements on ⁴⁶Ca. For all three isotopes we utilized the powerful technique of projectile Coulomb excitation in inverse kinematics combined with transient magnetic fields. In contrast to the positive g(2₁⁺) factor values for both ^42,44Ca, implying large admixtures of particle-hole configurations from ⁴⁰Ca core excitations, the g factor of ⁴⁶Ca(2 ₁⁺), g=-0.19(12), is negative. This result is in very definite disagreement with the large positive value predicted by the large scale shell model (LSSM) calculations which included sd shell core excitations into the fp shell and accounted well for corresponding ^42,44Ca results. The B(E2;0₁⁺→2₁⁺) deduced from the remeasured ⁴⁶Ca(2₁⁺) lifetime is indeed substantially smaller than the corresponding value for ⁴²Ca, again contrary to LSSM predictions. Both g(2₁ ⁺) and B(E2) in ⁴⁶Ca can be explained by full fp shell model calculations using the FPD6 interaction without invoking core excitations.