[en] The tail of the equilibrium distribution of compound-nucleus shapes is shown to extend out to very deformed shapes even for a moderate excitation energy of 100 MeV. The standard Hauser-Feshbach formalism is extended to predict the decay of a deformed compound nucleus as a function of spin, spin projection, and excitation energy. The inclusion of the equilibrium distributions of shapes is found to have little effect on the predicted neutron and proton kinetic energy spectra, but for α particles, the low-energy ''sub-barrier'' region of the spectrum is enhanced, in agreement with experimental data. Langevin simulations, using dissipation given by the wall formula, predict that compound nuclei start evaporating before the shape distribution equilibrates for excitation energies above ∼100 MeV. (c) 2000 The American Physical Society

[en] Using the nuclear proximity approach and the two center nuclear shape parametrization, the interaction potential between two deformed and pole-to-pole oriented nuclei forming a necked configuration in the overlap region is calculated and its role is studied for the cluster decay half-lives. The barrier is found to move to a larger relative separation, with its proximity minimum lying in the neighborhood of the Q value of decay and its height and width reduced considerably. For cluster decay calculations in the preformed cluster model of Malik and Gupta, due to deformations and orientations of nuclei, the (empirical) preformation factor is found to get reduced considerably and agrees nicely with other model calculations known to be successful for their predictions of cluster decay half-lives. Comparison with the earlier case of nuclei treated as spheres suggests that the effects of both deformations and neck formation get compensated by choosing the position of cluster preformation and the inner classical turning point for penetrability calculations at the touching configuration of spherical nuclei. copyright 1997 The American Physical Society

[en] In terms of the interacting boson model (IBM), shape invariants for the ground state, formed by quadrupole moments up to sixth order, are studied in the dynamical symmetry limits and over the whole structural range of the IBM-1. The results are related to the effective deformation parameters and their fluctuations in the geometrical model. New signatures that can distinguish vibrator and γ-soft rotor structures, and one that is related to shape coexistence, are identified. (c) 2000 The American Physical Society

[en] Nuclei with large neutron-to-proton ratios have neutron skins, which manifest themselves in an excess of neutrons at distances greater than the radius of the proton distribution. In addition, some drip-line nuclei develop very extended halo structures. The neutron halo is a threshold effect; it appears when the valence neutrons occupy weakly bound orbits. In this study, nuclear skins and halos are analyzed within the self-consistent Skyrme-Hartree-Fock-Bogoliubov and relativistic Hartree-Bogoliubov theories for spherical shapes. It is demonstrated that skins, halos, and surface thickness can be analyzed in a model-independent way in terms of nucleonic density form factors. Such an analysis allows for defining a quantitative measure of the halo size. The systematic behavior of skins, halos, and surface thickness in even-even nuclei is discussed. (c) 2000 The American Physical Society

[en] A γ -ray spectroscopic measurement of the N=Z nucleus ⁶⁸Se has been made following the ¹²C( ⁵⁸Ni, 2 n) reaction at 185 and 220 MeV using Gammasphere and the Argonne Fragment Mass Analyzer. Despite a very low production cross section of 200(50) μb , two distinct rotational bands were found; the ground state band consistent with oblate collective rotation, and an excited band consistent with prolate rotation. These observations support long-standing predictions that nuclear ground states with substantial oblate (β₂∼-0.3) deformation should exist in this region. (c) 2000 The American Physical Society

[en] It is shown that data on E2 transition rates in ¹⁵²Sm can be reproduced in the geometric collective model and that the calculations support the phase/shape coexistence interpretation of this pivotal nucleus. The coexistence is further supported by microscopic calculations, as is the concept of a sudden onset of deformation in this region. (c) 1999 The American Physical Society

[en] Two new rotational bands have been observed in the ¹⁵²Dy nucleus following experiments performed using the EUROGAM γ-ray spectrometer. Several transitions have been observed that link these new bands into the known single-particle states. Using the Doppler shift attenuation method, the quadrupole moment of the previously-known rotational sequence has been measured. The value of Q_band=3.85±0.06 e b confirms the deformed nature of this cascade. The structures of the three bands are discussed in terms of the occupation of both paired and unpaired orbitals, and are found to be in good agreement with theoretical calculations. The observation of two new bands further extends the concept of shape coexistence in the ¹⁵²Dy nucleus. (c) 2000 The American Physical Society

[en] The yrast sequence in ¹⁸³Tl has been studied for the first time in recoil-mass selected γ-ray spectroscopic measurements. A rotational-like cascade of seven transitions is established down to the band head with probable spin and parity (13/2⁺). Unlike in the adjacent odd-mass Tl nuclei, prompt γ decay from the yrast band to a lower lying weakly deformed (oblate) structure is not observed. These features are consistent with the predicted drop of the prolate band head in ¹⁸³Tl compared to ¹⁸⁵Tl. The implications for the prolate energy minimum in odd-mass Tl nuclei at the neutron i_13/2 midshell (N=103) are discussed. (c) 2000 The American Physical Society

[en] The systematics of smoothly terminating rotational bands based on proton 2p-2h excitations in the A=110 mass region are presented. Terminating bands (or nearly so) based on this proton excitation have been found in nuclei ranging from ¹⁰⁷In, up to ¹¹⁴Te, and possibly extending to ₅₄Xe nuclei. The impressive agreement between experimental data and theoretical calculations is also presented. However, recently discovered structures based on proton 1p-1h excitations begin to show disagreement with theoretical calculations. These new bands are also discussed. The current and future directions of research into smooth band termination will be presented. (c) 1999 American Institute of Physics

[en] Superdeformation has been established for over a decade in the mass 150 region and nearly as long in the A=190 region. The first measurements directed at nuclei in these regions concentrated on mapping out the superdeformed (SD) islands by identifying SD rotational bands in γ-ray coincidence data. These early studies provided new insights into the physics of superdeformation, but also raised unexpected issues. The new gamma-ray arrays (Gammasphere, Eurogam/Euroball and Gasp) have provided a wealth of new data on properties of SD states in these two mass regions. This paper highlights some of the more recent results from the large arrays which have addressed the outstanding issues in the field, namely, ΔI=4 staggering, identical bands, SD vibrational bands, and questions about the feeding into and the decay out of the SD well. (c) 1999 American Institute of Physics