Pushing Towards the Neutron Dripline: The Nuclear Structure of 40Mg and 64Cr
- Dr. Heather Crawford, Lawrence Berkeley National Laboratory
Monday, July 9, 11:00 AM - Special Seminar
NSCL Lecture Hall
Moving away from the valley of beta-stability and towards the driplines, it is understood that the position and robustness of the ‘magic numbers’ is modified by the residual nucleon-nucleon interaction. Such modifications can result in dramatic changes in the structure of the most exotic nuclei. In N=40 nuclei below Z=28, for example, the tensor monopole component of the effective nucleon-nucleon interaction results in a weakened sub-shell gap and the onset of quadrupole collectivity in neutron-rich Cr and Fe isotopes. First evidence for a weakening of the N=40 shell gap, and onset of quadrupole collectivity has been observed in the Cr and Fe isotopes as a steady decrease of the 2+ energies through N=40. Excited state lifetime measurements have confirmed collectivity in the Fe isotopes up to N=40, and added to mounting evidence for a new “island of inversion” centered near ¬64Cr. At lighter masses, the weakening of the N=28 shell closure in neutron-rich nuclei also leads to significant changes, inducing changes to the nuclear shape. The collapse of this shell closure leads to a large prolate deformation in 44S, and has suggested a large oblate deformation in 42Si at Z=14 and N=28. The isotonic nucleus 40Mg may be expected to have “mid-shell” character, and a similarly large deformation.
We have recently studied the evolution of nuclear shell structure in the regions around 64Cr and 40Mg. The collectivity of 66,68Fe isotopes and 64Cr has been studied via intermediate-energy Coulomb excitation at NSCL using the S800 spectrograph and the scintillator array CAESAR. Preliminary results for the B(E2) of 66, 68Fe and 64Cr will be presented, and discussed in terms of the evolution of collectivity near Z=24 and N=40. In the neutron-rich N=28 isotones, the inclusive two-proton knockout reaction cross-section for 42Si into 40Mg has been measured in an experiment performed at the RI Beam Factory, at RIKEN Nishina Center. New cross-section results, and the systematic evolution of nuclear structure along the N=28 isotonic chain will be presented and discussed.