ISOLDE Workshop and Users meeting 2012

Shape transition and coexistence in neutron-deficient rare earth nuclei: Coulomb excitation of 140Sm

19 Dec 2012, 15:40

20m

503/1-001 - Council Chamber (CERN)

Submitted Medium nuclei II

Speaker

Andreas Goergen (University of Oslo)

Description

The open-shell nuclei with Z>50 and N<82 are predicted to have the largest ground-state deformation in the entire nuclear chart. The shapes are predicted to be prolate except for a small region of nuclei with Z>60 and N≈78, which are predicted to be oblate. Similar to the situation in Hg and Pb isotopes at neutron mid-shell, prolate and oblate shapes can be expected to lie close in energy for N=78 isotones at proton mid-shell. Calculations beyond the mean-field approach with the generator coordinate method predict shape coexistence and a transition from prolate to oblate ground-state shapes between 140Sm and 142Gd. We have performed a Coulomb excitation measurement with a 140Sm beam scattered on a 94Mo target in order to measure spectroscopic quadrupole moments for excited states and transition strengths between them. The ISOLDE facility has provided a quasi-pure beam of 140Sm with an average intensity of 2e5 particles per second in June/July 2012. At least three excited states in 140Sm were populated during the experiment: the 2+ and 4+ states of the ground-state band and a state at an excitation energy of 990 keV which is tentatively assigned as a second 0+ state. Such a low-lying excited 0+ state would support the predicted scenario of shape coexistence. The statistics collected during the experiment will allow analyzing differential Coulomb excitation cross sections as a function of scattering angle. Experimental details and first results from the experiment will be discussed.