Yrast

Abstract: Evaporation residue mass distributions measured recently for several heavy-ion induced fusion reactions are analyzed. The reactions investigated are 19F on 12C (Elab = 50, 63 and 76 MeV), 19F on 27Al (Elab = 40, 63, 76 and 92 MeV) and 16O on 27Al (Elab = 30, 40, 60 and 80 MeV). Starting from the assumption that a compound nucleus is formed which decays by multiple emission of neutrons, protons, α-particles and γ-rays with probabilities as predicted by the statistical theory the mass spectra of the stable or long-lived products are calculated. It is shown that they are in good agreement with the experimental data, if reasonable assumptions are made about the level densities and the shape of the yrast line. The possibility to use these methods for studying the properties of highly excited nuclei at high angular momentum is investigated.

Abstract: The yrast spectra of the nuclides 68 ≦ Z ≦ 70, 89 ≦ N ≦ 99 are analysed in terms of quasiparticle configurations in a deformed field rotating with the frequency ω. The excitation energy, e', in the rotating frame (Routhian) and the aligned angular momentum, i, relative to the ground state band as functions of ω are extracted from the experimental rotational bands which are classified with respect to their signature, α, and parity, π. These experimental quantities are compared with the ones calculated from the quasiparticle level diagrams. Phenomena like backbending and the occurrence of aligned bands in both even-even and odd-mass nuclei and their mutual relationship are interpreted in terms of quasiparticle configurations that may cross each other with increasing frequency.

Abstract: We have investigated the Coriolis effects at high angular momentum in a system consisting of two (or four) particles in the i 13 2 orbital coupled to a deformed core. As the angular momentum is increased in this system, we find that the particles tend to decouple from the core and add their angular momenta, ultimately 2j−1 for two particles, directly to that produced by the rotation of the core. In our calculations, enough energy is gained in this partial decoupling so that somewhere between I = 10 and I = 20 these states become lower in energy than those involving just rotation of the initial system. Such a change in the character of the lowest-lying, or yrast, level in this spin region is consistent, in so far as we can tell, with existing experimental data on rotational-level spacings and de-excitation cascades.

Abstract: A cranked modified-oscillator model (with triaxial shape coordinates ϵ and γ) is used to study the nuclear potential-energy surface (based on a Strutinsky type of shell correction method) for very high angular momenta (30 ≦ I ≦ 100). For this region of spin, pair correlation is assumed to have collapsed. The influence of rapid rotation on the shell structure has been studied in the light and heavy rare-earth region as well as the Te-Ba region. Preliminary studies have also been made in the regions of superheavy and light nuclei. The possible occurrence of yrast traps is discussed.

Abstract: Shell-model calculations in the full pf -shell are carried out for the A =50, 51 and 52 isobars. The most frequently used effective interactions for the pf -shell, KB3 and FPD6 are revisited and their behaviour at the N =28 and Z =28 closures examined. Cures to their — relatively minor — defaults are proposed, and a new mass-dependent version called KB3G is released. Energy spectra, electromagnetic transitions and moments as well as beta decay properties are computed and compared with the experiment and with the results of the earlier interactions. A high-quality description is achieved. Other miscellaneous topics are addressed: the Coulomb energy differences of the yrast states of the mirror pair 51 Mn– 51 Fe and the systematics of the magnetic moments of the N =28 isotones.