Structure of 33Mg sheds new light on the N=20 island of inversion

R. Kanungo, C. Nociforo, A. Prochazka, Y. Utsuno, T. Aumann, D. Boutin, D. Cortina-Gil, B. Davids, M. Diakaki, F. Farinon, H. Geissel, R. Gernhäuser (+23 others)
2010 Physics Letters B  
The first reaction spectroscopy on the ground state structure of 33 Mg through the measurement of the longitudinal momentum distribution from the one-neutron removal reaction using a C target at 898 A MeV is reported. The experiment was performed at the FRS, GSI. The distribution has a relatively narrow width (150 ± 3 MeV/c (FWHM)) and the one-neutron removal cross-section is 74 ± 4 mb. An increased contribution from the 2p 3/2 orbital is required to explain the observation showing its lowering
more » ... compared to existing model predictions. This provides new information regarding the configuration of 33 Mg and the island of inversion. Exploring the changes in shell structure of neutron-rich nuclei has been one of the major foci of nuclear physics in recent times. The so-called island of inversion [1] around neutron-rich N = 20 nuclei continues to be a region of great interest. The increased separation energy of the neutron-rich Na isotopes was the first signature for the breakdown of the conventional N = 20 shell gap [2]. The observations of a low-lying first 2 + excited state in 32 Mg [3-5] indicated the presence of deformation. The lowering of the first 2 + (R. Kanungo). excited state is found to continue progressively for 34 Mg [6] and 36 Mg [7]. Large BE(2) values were also observed for lighter N = 20 isotones 31 Na [8] and 30 Ne [9]. The low-lying (2 + ) excited state of 32 Ne [10] was a signature of extension of the island of inversion to N = 22 for the Ne isotopes. The cause for the quenching of the N = 20 shell gap has been theoretically studied in the shell model framework [11, 12, 14, 13] where f p shell contributions are taken into account. It has been discussed that in this region due to the reduced strength of the T = 0 attractive monopole interaction, the 1d 3/2 and 1 f 7/2 orbitals come closer to each other leading to deformation through np-nh neutron excitations [12, 13] . The deformation increases the binding of the neutron-rich N = 20 isotones. 0370-2693
doi:10.1016/j.physletb.2010.02.008 fatcat:4srzzct26bapfg556wds37427y