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Accueil > Séminaires > Séminaires passés > Seminaires de 2014 > Isomers and Enhanced Stability of Superheavy Nuclei

F.G. Kondev (Argonne National Laboratory)

Isomers and Enhanced Stability of Superheavy Nuclei

Jeudi 18 decembre, salle NESTER IPNO Bat. 100

Séminaire commun avec l’IPNO

Following the early discovery of two-quasiparticle, high-K isomers in 250Fm (Z =100) and 254No (Z = 102) by Ghiorso et al. [1], a number of spectroscopic studies were carried out in the Fermium region, which have differing sensitivities both in terms of decay branches (e.g. alpha decay, gamma decay, fission, etc.) and lifetime ranges. There has been continuing activity in addressing the complex question of whether such high-K states would be expected to be shorter- or longer-lived, given changes in the fission barriers, a decay and fission probabilities, and the effects of nuclear structure (such as K hindrance). One-dimensional barrier calculations that allow one to visualize a double-well structure are only part of the necessary considerations, since dynamics exploring a more complicated three-dimensional deformation space ultimately control fission probabilities. Theoretical studies predicting spontaneous-fission probabilities for the high-K, two-quasiparticle states in 250Fm and 254No using one-dimensional approach and the WKB approximation concluded that the rate of spontaneous fission could be orders of magnitude slower than that of the ground states, due to higher and wider fission barriers [2]. However, more extensive calculations [3] that examined a number of two-quasiparticle configurations in different nuclei indicated that, in general, the results depend sensitively on Z, with some examples (Z>104) having faster fission and, depending on other decay branches, shorter lifetimes than the ground states. By incorporating a dynamical treatment of pairing, the spontaneous fission of high-K states was also found to depend critically on dynamically induced superfluidity in the tunneling process [4]. Recently, the possibility that K isomers could be more stable (have longer lifetimes) in super-heavy elements has been pointed out again qualitatively in terms of the shape of the fission barriers when using configuration constraints in calculations of the potential energies and fission barriers, but without inclusion of dynamical effects or the calculation of lifetimes [5]. Theoretical predictions of spontaneous-fission probabilities (and lifetimes) for high-K, multi-quasiparticle states in super-heavy nuclei are still challenging, which is partially due to the paucity of experimental data.

In this talk, new data on the 254Rf nucleus, produced using the 50Ti+206Pb reaction and studied by means of the Fragment Mass Analyzer at ANL and the Berkeley Gas Separator at LBNL will be presented. Using a novel approach involving a pulse-shape analysis in conjunction with a digital data acquisition system, we developed sensitivity for identification of short-lived (up to 10 micro s) states and discovered a 4 micro s isomer that depopulates via gamma ray emissions towards intermediate structures, and subsequently to the 254Rf ground state (T1/2 23 micro s). A second gamma ray decaying isomer was also discovered and placed above the 4 micro s one. Remarkably, it was found to be much longer-lived (T1/2 300 micro s) than the ground state, thus providing unambiguous evidence that high-K, multi-quasiparticle states may play an important role in enhancing the stability of super-heavy nuclei.

[1] A. Ghiorso, K. Eskola, P. Eskola and M. Nurmia, Phys. Rev. C7, 2032 (1973).
[2] A. Baran and Z. Lojewski, Phys. Lett. B176, 7 (1986).
[3] A. Baran and Z. Lojewski, Nucl. Phys. A475, 327 (1987).
[4] Yu.A. Lazarev, Phys. Scr. 35, 255 (1987).
[5] F.R. Xu, E.G. Zhao, R. Wyss, and P.M. Walker, Phys. Rev. Lett. 92, 252501 (2004).

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