1. Department of Physics, University of Liverpool, Liverpool L69 7ZE, UK
2. Department of Physics, University of Jyväskylä, FI-40014 Jyväskylä, Finland
3. Department of Nuclear Physics and Biophysics, Comenius University, 842 48 Bratislava, Slovakia
4. Department of Physical Sciences, University of Helsinki, FI-00014 Helsinki, Finland
5. Gesellschaft für Schwerionenforschung (GSI), D-64291 Darmstadt, Germany
6. DAPNIA/SPhN, CEA Saclay, F-91191 Gif/Yvette Cedex, France
7. Department of Physics and Joint Institute for Nuclear Astrophysics, University of Notre Dame, Notre Dame, Indiana 46556-5670, USA

Correspondence to: R.-D. Herzberg¹ Correspondence and requests for materials should be addressed to R.-D.H. (Email: rdh@ns.ph.liv.ac.uk).

Abstract

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^{1, 2, 3}. 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^{6, 7} 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.

1. Department of Physics, University of Liverpool, Liverpool L69 7ZE, UK
2. Department of Physics, University of Jyväskylä, FI-40014 Jyväskylä, Finland
3. Department of Nuclear Physics and Biophysics, Comenius University, 842 48 Bratislava, Slovakia
4. Department of Physical Sciences, University of Helsinki, FI-00014 Helsinki, Finland
5. Gesellschaft für Schwerionenforschung (GSI), D-64291 Darmstadt, Germany
6. DAPNIA/SPhN, CEA Saclay, F-91191 Gif/Yvette Cedex, France
7. Department of Physics and Joint Institute for Nuclear Astrophysics, University of Notre Dame, Notre Dame, Indiana 46556-5670, USA

Correspondence to: R.-D. Herzberg¹ Correspondence and requests for materials should be addressed to R.-D.H. (Email: rdh@ns.ph.liv.ac.uk).

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