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Metastable states with T$_{1/2}$ = 8(2) ms in $^{205}$Bi and T$_{1/2}$ = 0.22(2) ms in $^{204}$Pb, with $\approx $ 8 MeV excitation energy and angular momentum $\ge $ 22 $\hbar $, have been established. These represent, by up to two orders of magnitude, the longest-lived nuclear states above an excitation energy of 7 MeV, ever identified in the nuclear chart. Additionally, the half-life of the 10.17 MeV state in $^{206}$Bi has been determined to be 0.027(2) ms, the next highest value in this highly excited regime. These observations indicate the emergence of an island of extreme nuclear isomerism arising from core-excited configurations at high excitation in the vicinity of the doubly closed-shell nucleus $^{208}$Pb. These results are expected to provide discriminating tests of the effective interactions used in current large-scale shell-model calculations., 17 pages, 5 figures

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.

Using the on-line mass separator OSTIS for studies of short-lived fission products, we found two modes for the β-decay of 98Rb with respective half-lives of 96 and 114 ms. The latter decay populates a 0+ state in 98Sr at the unusually low energy of 215.5 keV, which is by far the lowest 0+ excited state observed in even-even nuclei. The half-lives of the 0+ and the 21+ states in 98Sr were found to be respectively 25 ns and 4 ns. The transition probabilities, the reduced E0 matrix element and the observed level structure suggest shape coexistence for 98Sr: the spheroidal 01+ ground state shows a rotational band with the 2+, 4+, (6+) levels, whereas the presumably spherical 02+ excited state may be connected with the 2+ level of a vibrational band. A simple model is proposed.

Experimental data on excitation energies and B(E2) values in the even-even Ru and Pd, and odd-A Rh and Ag isotopes suggest the existence of a Spin(3) spinor symmetry and a U ( 6 2 ) supersymmetry in these nuclei, wherein the even-even isotopes are described by the SU(5) limit of the interacting boson model and the odd proton of the odd-A isotopes is in a j = 1 2 orbit.

The decays of a 16 + four-quasiparticle state and an 8 − two-quasiparticle state in 178 Hf have been studied with emphasis on K -forbidden decay modes. An M4 branch in the decay of the 16 + state was identified, locating the isomer at an excitation energy of 2446.0 keV. Upper limits were found for the intensity of the 8 − to 6 + transition and for other possible decay modes of the 16 + state. Hindrance factors for the K -forbidden transitions are compared with those for similar transitions in neighbouring nuclides.

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.