Seminar Details

Sub-shell closures near N = 50 and N = 82

Farheen Naqvi, Yale University
Monday, August 19, 11:00 AM - Special Seminar
NSCL Lecture Hall

In a recent study of mixed-symmetry states in N = 80 isotones, namely 134Xe [1], 136Ba [2] and 138Ce [3], a large effect of the single-particle structure on the evolution of these excitations was observed. The M1 transition strength between the mixed-symmetric, (2+1,ms) state and the nearby lower-lying fully-symmetric, (2+1,fs) state in 138Ce was found fragmented while in 134Xe and 136Ba the strength remains concentrated in one transition. The reason for the observed instability of (2+1,ms) state in 138Ce was attributed to the presence of a πg7/2 sub-shell closure at Z = 58 [3]. To investigate the predictions made on the basis of calculations done by Quasiparticle-Phonon Model (QPM) and to probe the configurations of the low-lying excited states, a measurement of the g factor of 2+1 in 138Ce was performed. The low-lying excited states in 138Ce were populated via inverse Coulomb excitation on a 1 mg/cm2-thick 24Mg target at ATLAS, ANL. To measure the g factor, the Time-Dependent Recoil Into Vacuum technique (TDRIV) was employed and attenuation of the angular distribution of emitted 2+1 ͢ 0+ ϒ transitions was measured. The experimental setup included Yale plunger device and Gammasphere. Principle of the TDRIV technique and the implications of the extracted g factor on the proposed proton sub-shell closure will be discussed. In a second experiment, possibility of the emergence of a sub-shell closure at N = 58 [4] was investigated via beta-decay studies of 84−86Ga and isomer spectroscopy of 85−88Ge. The excited states in neutron-rich Ga and Ge isotopes were populated in the in-flight fission of 238U beam on a 9Be target. The experiment was performed at the Radioactive Ion Beam Facility (RIBF) at RIKEN, Japan. The BigRIPS and zero-degree spectrometer was used to identify and separate the reaction residues and the ions of interest were implanted in a segmented Silicon detector array called WAS3ABI. Gamma rays emitted after the beta decay of the ions reaching the final focal plane were identified by the EURICA array, consisting of 12 HPGe cluster detectors. Results of the ongoing analysis will be presented. [1] T. Ahn et al., Phys. Lett. B 679, 19 (2009) [2] N. Pietralla et al., Phys. Rev. C 58, 796 (1998) [3] G. Rainovski et al., Rev. Lett. 96, 122501 (2006) [4] J. A. Winger et al., Phys. Rev. C 81, 044303 (2010)