Structure of nuclei far from stability

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The region of N=Z nuclei between 58Ni and 100Sn became the subject of vigorous theoretical and experimental studies due to a remarkable diversity of shapes. Ground-state shapes in these nuclei are predicted to evolve from spherical to triaxial, oblate, prolate, and back to spherical as the mass increases due to the occupation of identical deformation-driving orbitals. Moreover, the excited levels can have significantly different structure than the ground state. For example, in 68Se and 72Kr the oblate-deformed ground state band is reported to coexist with a prolate-deformed excited band. Variations in the excitation energy of low-lying states in this region are often used to analyze the evolution of structure away from the doubly-magic 56Ni core. However, electromagnetic transition rates are recognized as providing a more sensitive probe of collectivity and deformation. The goal of the current proposal is to initiate a program of studies addressing electromagnetic transition rates in heavy nuclei along the N=Z line aimed at the measurements of quadrupole deformation and triaxiality, as well as the identification of signatures of shape transitions and shape coexistence. The experimental methods of choice are the Recoil Distance Method (RDM) and the Doppler Shift Attenuation Method (DSAM) for lifetimes longer and shorter then 1 picosecond, respectively. States of interest will be populated following compound nucleus reactions; the experimental setup will include the TIGRESS array, the TIGRESS integrated plunger (TIP) device working in conjunction with an array of CsI charged-particle detectors, and, in the future, the DESCANT array of neutron detectors. The first experiments will address shape coexistence in 68Se and 72Kr populated via the 40Ca(36Ar,2alpha) and 40Ca(40Ca,2alpha) reactions, respectively.