A long-standing prediction of nuclear models is the emergence of a region of long-lived, or even stable, superheavy elements beyond the actinides. These nuclei owe their enhanced stability to closed shells in the structure of both protons and neutrons. However, theoretical approaches to date do not yield consistent predictions of the precise limits of the 'island of stability'; experimental studies are therefore crucial. The bulk of experimental effort so far has been focused on the direct creation of superheavy elements in heavy ion fusion reactions, leading to the production of elements up to proton number Z = 118 (refs 4, 5). Recently, it has become possible to make detailed spectroscopic studies of nuclei beyond fermium (Z = 100), with the aim of understanding the underlying single-particle structure of superheavy elements. Here we report such a study of the nobelium isotope 254No, with 102 protons and 152 neutrons--the heaviest nucleus studied in this manner to date. We find three excited structures, two of which are isomeric (metastable). One of these structures is firmly assigned to a two-proton excitation. These states are highly significant as their location is sensitive to single-particle levels above the gap in shell energies predicted at Z = 114, and thus provide a microscopic benchmark for nuclear models of the superheavy elements.

Using a method whereby molecular and atomic ions are independently selected, an isobarically pure beam of 70 Se ions was postaccelerated to an energy of 206 MeV using REX-ISOLDE. Coulomb-excitation yields for states in the beam and target nuclei were deduced by recording deexcitation rays in the highly segmented MINIBALL -ray spectrometer in coincidence with scattered particles in a silicon detector. At these energies, the Coulomb-excitation yield for the first 2 state is expected to be strongly sensitive to the sign of the spectroscopic quadrupole moment through the nuclear reorientation effect. Experimental evidence is presented here for a prolate shape for the first 2 state in 70 Se, reopening the question over whether there are, as reported earlier, deformed oblate shapes near to the ground state in the light selenium isotopes. DOI: 10.1103/PhysRevLett.98.072501 PACS numbers: 21.10.Ky, 25.70.De, 27.50.+e A remarkable property of the nucleus is its ability to assume different configurations giving energy minima corresponding to different shapes of the mean field. It is often claimed that the presence of low-lying excited 0 states in even-even nuclei is strong evidence for such shape coexistence, since it is difficult to account for the presence of such states otherwise [1,2]. Arguably the most striking example of this is the case of 186 Pb [3], where both the first and second excited states have a spin parity of 0 . These states were interpreted as the bandheads of oblate and prolate rotational configurations, in close competition with the spherical ground state expected for this singly magic (Z 82) nuclear system. The region close to the N Z line from 56 Ni to 80 Zr is believed to be one of rapidly changing nuclear shape. At the upper end of this region, strongly prolate deformed shapes are found: 80 Zr is suggested to have 2 0:4 on the basis of the high moment of inertia [4], and a strongly prolate ground state, consistent with 2 0:4, has recently been established from the Gamow-Teller strength distribution in 76 Sr [5]. For the midshell nuclei near N Z 34 and 36, large shell gaps exist for protons and neutrons at both prolate and oblate shape, which would favor shape coexistence near the ground state. In the light krypton isotopes, 72-78 Kr, a low-lying excited 0 state is observed. There is good experimental evidence that this state has an oblate intruder configuration, and it most likely becomes the ground state in 72 Kr [6,7]. An analysis of the mixing of this state with the ground state, through consideration of their perturbed energies and the excited state lifetime, suggests an oblate ground state for 72 Kr [6], while the recently measured BE2; 0 ! 2 value in this nucleus, PRL 98,

The rotational band structure of 255Lr has been investigated using advanced in-beam gamma-ray spectroscopic techniques. To date, 255Lr is the heaviest nucleus to be studied in this manner. One rotational band has been unambiguously observed and strong evidence for a second rotational structure was found. The structures are tentatively assigned to be based on the 1/2-[521] and 7/2-[514] Nilsson states, consistent with assignments from recently obtained alpha decay data. The experimental rotational band dynamic moment of inertia is used to test self-consistent mean-field calculations using the Skyrme SLy4 interaction and a density-dependent pairing force.

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