Superheavy elements – laying the grounds for the landing on the island of stability
Jeudi 10 juin, à 14h00
par- 26 mai 2010
The search for the next closed proton and neutron shells beyond 208Pb has yielded a number of exciting results in terms of the synthesis of new elements [1,2,3] at the upper end of the charts of nuclides, in a region of exotic high-Z nuclear matter. In particular, the results obtained at the Flerov Laboratory of Nuclear Reactions (FLNR) for a rich number of decay patterns for 48Ca induced reactions on actinide targets  have by now been confirmed for reactions on 238U and 244Pu at GSI [4,5], and on 242Pu at LBNL .
These superheavy elements (SHE), however, are a nuclear structure phenomenon. They owe their existence to shell effects, an energy contribution of quantum mechanical origin to the nuclear potential, without which they would not be bound. Experimental activities in this field, apart from attempts to directly synthesise new elements, have to investigate reaction mechanism studies and, in particular, they have to pursue nuclear structure investigations to study the development of single particle levels towards the expected gap for the proton and neutron shell closure in the region of the spherical SHE. Chemistry studies of SHE provide additional input on chemical properties and the Z assignment. Precision mass measurements yield an important input in terms of nuclear binding energies for theoretical models.
In recent years the development of efficient experimental set-ups, including separators and advanced particle and photon detection arrangements, allowed for more detailed nuclear structure studies for nuclei at and beyond Z=100. A review of recent achievements is given in ref. . Among the most interesting features is the observation of K-isomeric states. Experimentally about 14 cases have been identified in the region of Z>96. K-isomers or indications of their existence have been found for almost all even-Z elements in the region Z=100 to 110. We could recently establish and/or confirm such states in the even-even isotopes 252,254No . The heaviest nucleus where such a state was found is 270Ds with Z=110 as we reported in 2001 . Precision mass measurements with the SHITRAP set-up are important new source of valuable information. In particular, the successful mass measurements for 252,253,254No performed in 2009  establish together with α-decay chains the connection up to Ds isotopes. In this way we are able to lay out the grounds for a detailed understanding of these heavy and high-Z nuclear systems, and contribute at the same time valuable information to the preparation of strategies to successfully continue the hunt for the localisation of the next spherical proton and neutron shells beyond 208Pb.
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