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REX-ISOLDE [1] is a pilot experiment at ISOLDE/CERN with the aim to study the nuclear structure of neutron-rich nuclei using radioactive ion beams. Radioactive ions with the charge state 1+, which are delivered by the on-line mass separator ISOLDE, are post-accelerated. A new concept is being employed, where the ions are first accumulated in a Penning trap, then converted to highly charged ions in an EBIS ion source and finally accelerated in a short linear accelerator to an energy of 2.2 MeV/u.

The two very neutron-deficient isotopes of lead A = 190, 192 have been produced and studied through the reaction 156, 154Gd(40Ar, 4nγ)192, 190Pb. The identification of those nuclei was also checked by cross bombardment using the two additional reactions 182W(16O, 6γ)192Pb and 156Gd(40Ar, 6nγ)190Pb. The pulsed 40Ar beam available at ALICE (Orsay) enabled us to record singles γ-spectra as well as four-dimensional (energy-time) spectra corresponding to the correlation of two Ge(Li) detectors for different time windows. Tentative level schemes are proposed. No significant deviation from the systematic behaviour previously reported for the heavier isotopes is found. The data are consistently reproduced within a two-quasiparticle formalism using a surface δ-interaction. However, a slow decrease of the 21+ level energy with decreasing A is observed, which could be an indication of a weakly increasing collective trend when the number of neutron holes increases. These experimental conclusions are in fair agreement with the theoretical results obtained by Dickman and Dietrich who predict a spherical shape for the ground states of the lead isotopes with A ≧ 184 .

The g -factor of the 12 + isomeric state in lead isotopes with A = 194, 196, 198 was measured using the time-differential perturbed angular distribution method (TDPAD). The values obtained are respectively g (194) = −0.158(6); g (196) = −0.157(7); g (198) = −0.144(11). A more precise determination of the 12 + level half-life is also made. The g -factprs of these nuclear states, which are described with v (i 13 2 ) −2 as the main configuration, are surprisingly constant over a large mass range (between A = 206 and 194). A core polarization analysis explains this trend: the polarization induced on neutrons in i 13 2 orbit decreases with the mass number A (blocking effect), but a compensation is provided by the other spin-orbit partners f 7 2 -f 5 2 and P 3 2 -P 1 2 .

The half-line of the 2323 keV state (6 + ) in 144 Sm is determined to be 880 ± 25 ns.

Un état isomérique 12+, interprété comme un état à deux quasi-particules (νi 13/2)-2, a été mis en évidence dans les isotopes de plomb déficients en neutrons. Il possède une période de quelques centaines de ns, qui se prête à la mesure du moment magnétique par la méthode des distributions angulaires différentielles perturbées (TDPAD). Ces mesures ont été réalisées, afin de confirmer la configuration de l'état et d'étudier l'effet du nombre de nucléons sur les phénomènes de polarisation du coeur. Le tableau indique les valeurs expérimentales du facteur g et de la période du niveau 12+ des isotopes de masse A = 198, 196, 194. Une analyse multidimensionnelle des informations (Eγ, t) permet d'étudier simultanément ces trois noyaux, formés par la réaction naturelle W(16O, 4n)Pb. Les facteurs g sont reportés sur la figure, ainsi que les valeurs obtenues par ailleurs pour l'état 12+ du 206Pb [1], 200Pb [2] et l'état 13+/2 du 205Pb [3]. [MATH]. La constance de g en fonction de A suggère que les effets observés dans le 206Pb se retrouvent dans les isotopes plus légers, et notamment que la configuration (νi 13/2)-2 demeure largement dominante. L'écart notable à la valeur de Schmidt (g = - 0,294) peut s'expliquer par un important effet de polarisation du coeur. Un calcul, utilisant le formalisme de Arima et Horie, et une interaction résiduelle delta de surface (SDI) sont effectués pour rendre compte de ces résultats.