H. Puchta

Evolution of the decay modes of 158Yb with spin

Abstract The decay of the entry states in 158 Yb populated in the reactions of 149 MeV 20 Ne with 144,146 Nd has been investigated with the Spin Spectrometer gated by a Ge counter. The average excitation energy, the γ -ray spectra and the angular distributions as a function of multiplicity suggest a region of non-collective behavior at low spin. At higher spin collective characteristics are apparent, but between I ≈40 and 49 a strong stretched dipole component is seen. Above I ≈49 no additional dipole transitions are observed. These characteristics are discussed in terms of recent calculations.

High Spin Phenomena in the Mass 100-200 Region Seen Through the Crystal Ball

The average properties of the gamma ray entry region and the decay from it are studied systematically, for 49 nuclear systems, in the spin spectrometer. Preliminary results are given for the mass the neutron number dependence of the gamma ray fold distribution and of unresolved γ spectra. The possibility of gating simultaneously on narrow regions of fold and excitation energy is exploited.

Evolution of nuclear shapes in 157–161yb as a function of spin and neutron number

Abstract The population distributions of the entry states, the entry lines 〈E∗> versus multiplicity, the energy spectra and the angular distributions of the continuum γ-rays as a function of multiplicity in 157–161yb from the reactions of 136 MeV and 149 MeV 20Ne with 144Nd and 146Nd have been investigated with a 4π multidetector system gated with a Ge detector. The observed entry lines and γ-decay modes indicate changes in the nuclear structure with increasing spin. This systematic change for N = 87– 91 Yb isotopes suggests an evolution of nuclear shapes from prolate at low spins to particle aligned oblate structure for 157yb and 158yb followed by the onset of high-K bands built on largely deformed oblate states at increasingly higher spin between 38 and 50 kh with increasing neutron number for 157–161yb. Furthermore evidence for an evolution to triaxial shape at I ⋍ 50 for 158yb was found.

Heavy-Ion Reaction Mechanisms Studied with the Spin Spectrometer

Experimental data and statistical-model calculations for xn and αxn products of the reaction 20Ne + 146Nd at 136 MeV are shown to be in generally good agreement, indicating that equilibrium processes are dominant. Preliminary results on the heavy-ion ejectiles from 19F + 159Tb are presented.

Research highlights with the spin spectrometer

Abstract The excitation energy and angular momentum dependence of the entry states in fusion reactions measured with the spin spectrometer is discussed. A new decay mode involving the onset of localized stretched dipole radiation at half the accompanying stretched E2 collective radiation is found in 157–161 Yb. The appearance of this mode correlates smoothly with neutron number and spin. Possible interpretations are presented in terms of the evolution of the nuclear shapes from prolate to aligned-quasiparticle oblate to collective oblate and then to triaxial. Evidence for nuclear deformation that increases with spin at very high excitation is presented based on α-particle angular distributions measured relative to the spin direction, using a new method for deriving the spin alignment.