Nucleon alignments and collective degrees of freedom in actinide nuclei.

摘要

The actinide nuclei are an excellent laboratory for the study of collective motion because of their large quadrupole deformation. The level structure of a number of these nuclei has been considerably expanded by carrying out measurements with 209Bi beams, at bombarding energies ∼15% above the Coulomb barrier, on 232Th, 237Np, 241Am, and 248Cm targets. Large sets of gamma-ray coincidence data were collected with the Gammasphere multi-detector array.; With the new data, the debate about the nature of particle alignments under the stress of rotation in actinide nuclei could be settled. It was found that the alignment of a pair of i13/2 protons is responsible for the sudden increase in the moments of inertia at high rotational frequencies in all even-even nuclei of the region. Surprisingly, no experimental evidence was found for the anticipated alignment of a pair of j 15/2 neutrons in the same frequency range. The observations can be understood in the framework of the cranked shell model only if a sizeable reduction in proton and neutron pairing is postulated.; Several vibrational excitations have also been explored in the even-even 232Th and 248Cm nuclei. Specifically, excitations built on the octupole, beta and gamma modes of vibration have been traced to high angular momentum. The data have been compared with the results of calculations within the framework of the Random Phase Approximation (RPA) model. The evolution of the different collective structures with angular momentum was reproduced satisfactorily. For the various modes of vibration, the RPA model was also able to account properly for the properties of the measured electro-magnetic decays.