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The ground state rotational bands of the N = Z nuclei (72)Kr, (76)Sr, and (80)Zr have been extended into the angular momentum region where rotation alignment of particles is normally expected. By measuring the moments of inertia of these bands we have observed a consistent increase in the rotational frequency required to start pair breaking, when compared to neighboring nuclei. (72)Kr shows the most marked effect. It has been widely suggested that these "delayed alignments" arise from np-pairing correlations. However, alignment frequencies are very sensitive to shape degrees of freedom and normal pairing, so the new experimental observations are still open to interpretation.

The Eurogam-2 array has been used to study γ rays emitted following the spontaneous fission of a 248 Cm source. Analysis of Doppler-broadened lineshapes corresponding to transitions from excited rotational states in neutron-rich Sr, Zr, Mo, Ru and Pd fission fragments has enabled measurements of state lifetimes in these nuclei at nuclear spins of I ∼ 10. Extracted transition quadrupole moments are compared with various recent theoretical approaches, as well as with measurements at lower spin, and an interpretation of the data is made within the context of prolate-oblate shape competition in this region.

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HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. The ν9/2[404] orbital and the deformation in the A ∼ 100 region