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 neutrons1,2,3. 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 studies6,7 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.
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Acknowledgements
We thank T. L. Khoo, S. K. Tandel, P. D. Stevenson and P. M. Walker for discussions. This work was supported by the EU Fifth Framework Programme ‘Improving Human Potential—Access to Research Infrastructure’ and the EU Sixth Framework Programme ‘Integrating Infrastructure Initiative—Transnational Access’ (EURONS). I.G.D., A.S. and M.V. acknowledge a European Community Marie Curie Fellowship. Support by the Academy of Finland under the Finnish Centre of Excellence Programme 2000–2005, and by the UK EPSRC, is gratefully acknowledged. This work was supported in part by the NSF and the US Department of Energy, Office of Nuclear Physics.
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Herzberg, RD., Greenlees, P., Butler, P. et al. Nuclear isomers in superheavy elements as stepping stones towards the island of stability. Nature 442, 896–899 (2006). https://doi.org/10.1038/nature05069
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DOI: https://doi.org/10.1038/nature05069
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