Jeudi 7 Mai 2015 à 11h00 I.P.N.O - Bât. 100 - Salle NESTER
Following the early discovery of two-quasiparticle, high-K isomers in 250Fm (Z =100) and
254No (Z = 102) by Ghiorso et al. , a number of spectroscopic studies were carried out in
the Fermium region, which have differing sensitivities both in terms of decay branches (e.g.
α decay, γ 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 . However, more extensive calculations  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
. 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
. 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 μs) states and discovered a 4 μs isomer that depopulates via gamma
ray emissions towards intermediate structures, and subsequently to the 254Rf ground state
(T1/2 23 μs). A second gamma ray decaying isomer was also discovered and placed above
the 4 μs one. Remarkably, it was found to be much longer-lived (T1/2 300 μs) 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.
 A. Ghiorso, K. Eskola, P. Eskola and M. Nurmia, Phys. Rev. C7, 2032 (1973).
 A. Baran and Z. Lojewski, Phys. Lett. B176, 7 (1986).
 A. Baran and Z. Lojewski, Nucl. Phys. A475, 327 (1987).
 Yu.A. Lazarev, Phys. Scr. 35, 255 (1987).
 F.R. Xu, E.G. Zhao, R. Wyss, and P.M. Walker, Phys. Rev. Lett. 92, 252501 (2004).
Renseignements CSNSM : Hélène le Sueur/ Stéphanie Roccia 01 69 15 52 25/52 15