Structure of Fe isotopes at the limits of the pf shell

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Hoteling, Nathan James
Walters, William B
An investigation of the structure of Fe isotopes up to neutron number 40 has been carried out at Argonne National Laboratory with the application of a thick-target deep-inelastic reaction experiment. A beam of 450 MeV <sup>64</sup>Ni was incident on a 55 mg/cm<sup>2</sup> target of enriched <sup>238</sup>U located at the center of the Gammasphere spectrometer. Triple coincidence data obtained from the experiment were sorted into four time-correlated cubes and used to identify new levels in <sup>61,62,64</sup>Fe. Angular correlations were used to both confirm previously-assigned spin and parity assignments and to assign values to excited states established in this work for the first time. The effect of <em>g</em><sub>9/2</sub> neutron excitations on the structure of low-lying yrast states in these isotopes was studied with a comparison of experimental levels with the results from shell model calculations within both <em>pf</em> and truncated <em>pfg</em> configuration spaces. The effective interactions used in this work were derived from the N3LO nucleon-nucleon potential. These calculations indicate a strong influence from the <em>g</em><sub>9/2</sub> orbital, beginning at moderate energy and spin in <sup>62</sup>Fe and extending to the low-lying states of <sup>64</sup>Fe. New levels identified above a 239-ns, 9/2<sup>+</sup> isomer in <sup>61</sup>Fe appear to be consistent within a rotation-aligned coupling scheme, with prolate deformation beta ~ 0.24, a value supported with both the shell model and Particle-triaxial rotor model. The data from this work mark a significant achievement in terms of understanding the evolution of nuclear structure in this region and the possible onset of deformation near <em>N</em> = 40. Still, more theoretical work is needed in order to better characterize experimentally observed features of this region. In addition to the Fe experiments described in the body of this thesis, another measurement was carried out in which the structure of <sup>128</sup>Cd was investigated. This nucleus, like <sup>64</sup>Fe, can be viewed as two proton and two neutron holes in a double-magic system. The identification of isomeric decay, and a confirmation of 2<sup>+</sup> and 4<sup>+</sup> level energies is described.