Search

A measurement on lifetimes of high-spin states in the yrast and near-yrast rotational bands in {sup 162}Yb was carried out at ATLAS in order to determine the evolution of collectivity as a function of angular momentum using the {sup 126}Te({sup 40}Ar,4n){sup 162}Yb reaction at 170 MeV. Previous lifetime measurements in the {sup 164,166,168}Yb isotopes showed a dramatic decrease in the transition quadrupole moment Q{sub t} with increasing spin. It was suggested that this decrease in Q{sub t} is brought about by the rotationally-induced deoccupation of high-j configurations, mainly i{sub 13/2} neutrons. If this interpretation is correct, the heavier isotopes should have a larger decrease in Q{sub t} than the lighter mass nuclides due to the position of the Fermi surface in the i{sub 13/2} subshell. Indeed, {sup 160}Yb does not show a clear decrease in Q{sub t} at high spin. No high spin lifetime information exists for {sup 162}Yb, thus this experiment fills the gap of measured Q{sub t}`s in the light Yb series. The data is currently being analyzed.

We have studied γ-ray multiplicity distributions for the system 16 O + 152 Sm at five beam energies in the range 60–80 MeV, near and below the Coulomb barrier. The data were obtained using a large array of γ-ray detectors and an efficient evaporation-residue detector system utilizing an electrostatic deflector. The data are in good agreement with predictions of angular momentum distributions calculated using a coupled-channels model incorporating the effects of deformation.

The influence of the reaction mechanism and of the nuclear structure at high angular momentum on the γ-ray deexcitation process is studied for Dy nuclei produced in a (40–80 MeV) and 12 C (80–110 MeV) induced reactions on 154, 155, 158, 160 Gd and 146, 150 Nd targets, respectively. The quasi-continuum (q.c.) γ-ray spectra for selected exit channels are decomposed into collective and statistical parts for which the γ-ray energies, the multiplicities and partly the multi-polarities are determined. Large differences in the constitution of the q.c. γ-ray cascades and in the population strengths of high-spin yrast states in 152 Dy and 156 Dy are attributed to nuclear structure effects at high angular momentum. This constitution and the centroids of the γ-ray entry regions in the energy versus angular-momentum plane are also evaluated as functions of beam energy and projectile. The γ-ray multiplicities and both coordinates of the entry region centroids increase initially with bombarding energy and then level off or even decrease at the highest energies. The observed increase is reproduced by a statistical-decay model, but at the higher bombarding energies strong deviations occur which are discussed in view of the reaction mechanisms, including incomplete fusion and non-equilibrium neutron emission, particularly absorptive breakup.

Evaporation residue cross sections and neutron multiplicity distributions have been measured for the 12 C + 144 Sm and 64 Ni + 92 Zr reactions leading to the same compound nucleous 156 Er. Statistical model calculations can account for the data in the 12 C-induced reaction. In contrast, the inhibition of neutron emission with respect to statistical model predictions seen in 64 Ni + 92 Zr cannot be explained even with the inclusion of the broad angular momentum distributions required to describe the fusion cross section data.

High-spin states in 167,168Hf and 170, 171W have been excited by the reactions 159 Tb ( 14 N , x n ) 173 − x Hf and 155 Gd ( 20 Ne , x n ) 175 − x W . The yrast bands have been observed up to J π = 49 2 + in 167Hf, Jπ = 28+ in 168Hf, J π = 45 2 + in 171W and Jπ = 22+ in 170W. Both even-even nuclei display a strong backbend around Jπ = 14+, whereas 167Hf shows an upbend at ω ∼- 0.33 MeV and 171W exhibits a progressive gain in aligned angular momentum above ω ∼- 0.26 MeV. At even higher rotational frequencies, bridges have been found in the central valleys of the γ-γ correlation matrices at ω ∼- 0.42 and 0.52 MeV for the Hf isotopes and ω ∼- 0.45 and 0.47 MeV for the W isotopes. Deduced moments of inertia for the Hf isotopes using the correlation data show a smooth increase up to about the rigid-sphere value at ω ∼- 0.5 MeV. The data are satisfactorily accounted for by cranked shell-model calculations. In particular, a qualitative interpretation is given for the experimentally observed systematic difference in the strength of the interaction potential for the N = 95 and N = 97 isotopes of Hf and W.

The multipolarity of continuum transitions deexciting high-spin states has been deduced from measured conversion coefficients. The investigated 146Nd(20Ne, 4n or 5n) 162,161Yb reactions were selected by gating on discrete lines. The average multipolarity gradually changes from E2 at 0.5 MeV to E1 above 1.5 MeV.