Evidence for [formula omitted] core excitation from g factor and B(E2) measurements on the 21+ states of [formula omitted

Abstract

The g factors of the 21+ states in 42,44Ca have been measured employing Coulomb excitation of isotopically pure calcium beams in inverse kinematics combined with the transient field technique. In addition, the lifetimes were redetermined using the Doppler-shift-attenuation method. While the g factor of 44Ca(21+) has definitely a positive value, g=+0.17(3), for 42Ca a vanishingly small g factor, g(21+)=+0.04(6), was observed in a very first measurement. These are striking results since one expects for an assumed inert 40Ca core with two or four valence neutrons in the fp shell a negative g factor, g≃−0.4, in accordance with experimental data for the 7/2− ground states of the odd neighbouring isotopes. Therefore, the new experimental findings clearly require substantial admixtures of configurations from sd shell core excitations in the wave functions of the nuclear states. A “co-existence” picture, along the lines of Gerace and Green, suggests core-deformed components in the 2+1 state of 55(6)% and 66(4)% in 42Ca and 44Ca, respectively. A separate large scale shell model calculation in the (sd−fp) shell space results in good overall agreement with the experimental data.