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 204 Pb(48 Ca, xn) 252−x No reactions. Isotopically pure (>99.7%) 204 Pb targets were used to reduce background from more favored reactions on heavier lead isotopes. Two spontaneous fission half-lives (t 1/2 = 3.7 +1.1 −0.8 and 43 +22 −15 µs) were deduced from a total of 158 fission events. Both decays originate from 250 No 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.

No abstract

We have identified two isomers in 254No, built on two- and four-quasiparticle excitations, with quantum numbers K pi = 8- and (14+), as well as a low-energy 2-quasiparticle Kpi = 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.

No abstract

Isomers have been populated in 246 Cm and 252 No with quantum numbers K π = 8 − , which decay through K π = 2 − rotational bands built on octupole vibrational states. For N = 150 isotones with (even) atomic number Z = 94-102, the K π = 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 K π = 8 − energies are described with single-particle energies given by the Woods-Saxon potential and the K π = 2 − vibrational energies by quasiparticle random-phase approximation calculations. Ramifications for self-consistent mean-field theory are discussed.

The collective wobbling mode, the strongest signature for the rotation of a triaxial nucleus, has previously been seen only in a few Lu isotopes in spite of extensive searches in nearby isotopes. A sequence of transitions in the N = 94 167 Ta nucleus exhibiting features similar to those attributed to the wobbling bands in the Lu nuclei has now been found. This band feeds into the πi 13/2 band at a relative energy similar to that seen in the established wobbling bands and its dynamic moment of inertia and alignment properties are nearly identical to the i 13/2 structure over a significant frequency range. Given these characteristics, it is likely that the wobbling mode has been observed for the first time in a nucleus other than Lu, making this collective motion a more general phenomenon. PACS number(s): 21.10. Re, 23.20.Lv, 27.70.+q Our understanding of the wobbling mode in nuclei (and the associated stable triaxial deformation) has evolved quickly over the past decade. Bohr and Mottelson [1] first proposed that the rotation of a stable triaxially deformed nucleus would result in the presence of wobbling excitations. These excitations occur because the rotational angular momentum is not aligned with any of the body-fixed axes; rather it precesses and wobbles around one of these axes in a manner similar to that of an asymmetric top.In 1995, Schnack-Petersen et al.[2] first suggested that rotational bands based on proton i 13/2 excitations in 163,165 Lu are associated with a triaxial strongly deformed (TSD) potential well. The large deformation is mainly due to the occupation of the intruder i 13/2 orbital, and the triaxial deformation (γ = 20 • ) results from an N = 94 shell gap that develops with enhanced quadrupole deformation ( 2 ≈ 0.37). No direct experimental evidence for triaxiality was observed until the wobbling mode was confirmed in 163 Lu by Ødegård et al.[3]. This seminal work established the existence of a band feeding into the πi 13/2 structure where the two sequences have nearly identical moments of inertia and alignments over a large frequency range. The similarities of the moments of * Present address: inertia and alignments are a predicted feature for a wobbling band as the intrinsic structure for both bands should be the same; the only difference between the two is the degree to which the rotational angular momentum vector lies off axis. The collective wobbling behavior can thus be described within a phonon model, where the energy of each band is equal to E =¯h 2 2J I (I + 1) +hω w (n w + 1/2), wherehω w = hω rot (J x − J y )(J x − J z )/(J y J z ) [1]. The n w = 0 phonon number is assigned to the energetically lowest band in the family, as its angular momentum vector lies closest to a body axis, and in the case of the Lu isotopes, this is associated with the πi 13/2 band. Wobbling excitations with n w = 1, 2, 3, etc. then follow, each lying successively higher in energy as the rotational angular momentum vector progressively lies farther from the body axis with increasing n w . Indeed, Jense...

Rotation-aligned isomeric states and associated oblate collective sequences are established in even Pt isotopes. Reduced E2 transition probabilities for the deexcitation of the 12 + isomers indicate an abrupt and unexpected quenching of oblate collectivity around neutron number N = 120. Structure and shape evolution at high spin in the heaviest stable isotopes is found to be markedly different from observations in the lighter ones.

No abstract