Collective oblate rotation at high spins in neutron-rich Hf180

We report on experimental evidence for collective oblate rotation becoming favored at high spins in a rigid, well-deformed, axially symmetric nucleus. Excited states established up to spin 20 in Hf180 are consistent with predictions that nucleon alignments would favor oblate over prolate shapes at high spins in neutron-rich Hf isotopes. The results highlight the influence of valence orbitals on the interplay between nucleon alignments and nuclear shapes and provide a rare example of independent particle dynamics in competing potential wells

Search for strongly deformed structures and observation of multiple nucleon alignments in W174

Highly excited states, up to spin 39, have been established in W174, using the Gammasphere array. Ultimate cranker calculations predict the appearance of triaxial, strongly deformed structures above spin 30 in W174. A new approach was developed for a comprehensive search of the data for such structures, similar to those observed in the Lu and Hf isotopes. No evidence was found for strongly deformed bands in the W isotopes populated in this experiment. Existing rotational structures have been considerably extended, allowing for the observation of both neutron and proton alignments in a number of bands. There is evidence for the i13/2 neutron and possibly both the h9/2 and h11/2 proton crossings. The observed neutron and proton crossing frequencies are in good agreement with predictions of Woods-Saxon cranking calculations using an empirical pair-gap energy, and they lead to an improved understanding of the underlying structure of the bands

Rotational bands in odd-A Cm and Cf isotopes: Exploring the highest neutron orbitals

Rotational bands have been identified up to high spins (≈28-h) in the odd-A nuclei 247,249Cm and 249Cf through inelastic excitation and transfer reactions around the Z = 100 region where stability results from shell effects. The [620]1/2 Nilsson configuration in 249Cm is the highest-lying neutron orbital, from above the N = 164 spherical subshell gap, for which high-spin rotational behavior has been established. The data allow for an unambiguous experimental assignment of configurations to the observed bands, unusual for odd-A nuclei near Z = 100. The high-spin properties are described in terms of Woods-Saxon cranking calculations

Decay modes of No250

The fragment mass analyzer at the ATLAS facility has been used to unambiguously identify the mass number associated with different decay modes of the nobelium isotopes produced via Pb204(Ca48,xn)No252-x reactions. Isotopically pure (>99.7%) Pb204 targets were used to reduce background from more favored reactions on heavier lead isotopes. Two spontaneous fission half-lives (t1/2=3.7-0.8+1.1 and 43-15+22 μs) were deduced from a total of 158 fission events. Both decays originate from No250 rather than from neighboring isotopes as previously suggested. The longer activity most likely corresponds to a K isomer in this nucleus. No conclusive evidence for an α branch was observed, resulting in upper limits of 2.1% for the shorter lifetime and 3.4% for the longer activity

K isomers in No254: Probing single-particle energies and pairing strengths in the heaviest nuclei

We have identified two isomers in No254, built on two- and four-quasiparticle excitations, with quantum numbers Kπ=8- and (14+), as well as a low-energy 2-quasiparticle Kπ=3+ state. The occurrence of isomers establishes that K is a good quantum number and therefore that the nucleus has an axial prolate shape. The 2-quasiparticle states probe the energies of the proton levels that govern the stability of superheavy nuclei, test 2-quasiparticle energies from theory, and thereby check their predictions of magic gaps

Multi-quasiparticle structures up to spin ~44ħ in the odd-odd nucleus Ta168

High-spin states in the odd-odd nucleus Ta168 have been populated in the Sn120(V51,3n) reaction. Two multi-quasiparticle structures have been extended significantly from spin ~20 to above 40. As a result, the first rotational alignment has been fully delineated and a second band crossing has been observed for the first time in this nucleus. Configurations for these strongly coupled rotational bands are proposed based on signature splitting, B(M1)/B(E2) ratio information, and observed rotation-alignment behavior. Properties of the observed bands in Ta168 are compared to related structures in the neighboring odd-Z, odd-N, and odd-odd nuclei and are discussed within the framework of the cranked shell model

Kπ=8- isomers and Kπ=2- octupole vibrations in N=150 shell-stabilized isotones

Isomers have been populated in Cm246 and No252 with quantum numbers Kπ=8-, which decay through Kπ=2- rotational bands built on octupole vibrational states. For N=150 isotones with (even) atomic number Z=94-102, the Kπ=8- and 2- states have remarkably stable energies, indicating neutron excitations. An exception is a singular minimum in the 2- energy at Z=98, due to the additional role of proton configurations. The nearly constant energies, in isotones spanning an 18% increase in Coulomb energy near the Coulomb limit, provide a test for theory. The two-quasiparticle Kπ=8- energies are described with single-particle energies given by the Woods-Saxon potential and the Kπ=2- vibrational energies by quasiparticle random-phase approximation calculations. Ramifications for self-consistent mean-field theory are discussed

Bridging the nuclear structure gap between stable and super heavy nuclei

International audienceDue to recent advances in detection techniques, excited states in several trans-fermium nuclei were studied in many laboratories worldwide, shedding light on the evolution of nuclear structure between stable nuclei and the predicted island of stability centered around spherical magic numbers. In particular, studies of K-isomers around the Z=100 and N=152 deformed shell closures extended information on the energies of Nilsson orbitals at the Fermi surface. Some of these orbitals originate from spherical states, which are relevant to the magic gaps in super-heavy nuclei. The single-particle energies can be used to test various theoretical predictions and aid in extrapolations towards heavier systems. So far, the Woods-Saxon potential reproduces the data best, while self-consistent approaches miss some of the observed features, indicating a need to modify the underlying effective nucleon-nucleon interactions