The fragment mass analyzer at the ATLAS facility has been used to unambiguously identify the mass number associated with different decay modes of the nobelium isotopes produced via 204 Pb(48 Ca, xn) 252−x No reactions. Isotopically pure (>99.7%) 204 Pb targets were used to reduce background from more favored reactions on heavier lead isotopes. Two spontaneous fission half-lives (t 1/2 = 3.7 +1.1 −0.8 and 43 +22 −15 µs) were deduced from a total of 158 fission events. Both decays originate from 250 No rather than from neighboring isotopes as previously suggested. The longer activity most likely corresponds to a K isomer in this nucleus. No conclusive evidence for an α branch was observed, resulting in upper limits of 2.1% for the shorter lifetime and 3.4% for the longer activity.

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Excited states in 38,40,42 Si nuclei have been studied via in-beam γ-ray spectroscopy with multinucleon removal reactions. Intense radioactive beams of 40 S and 44 S provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled γ-γ coincidence measurements. A prominent γ line observed with an energy of 742 (8) 23.20.Lv, 27.40.+z, 29.38.Db Shell closures and collectivity are important properties that characterize the atomic nucleus. Interchange of their dominance along isotopic or isotonic chains has attracted much attention. The recent extension of the research frontier to nuclei far away from the valley of stability has revealed several new phenomena for neutronor proton-number dependent nuclear structure. For example, a weakening or even disappearance of shell closures occur in several neutron-rich nuclei at N = 8 [1][2][3] and N = 20 [4][5][6]. A well known example in the case of N = 20 is the so-called 'island of inversion ' [7] located around the neutron-rich nucleus 32 Mg. The low excitation energy of the first 2 + state E x (2 + 1 ) and large E2 transition probability [4][5][6] clearly indicate shell quenching in 32 Mg despite the fact that N = 20 is traditionally a magic number. The next magic number, N = 28, which appears due to the f 7/2 -f 5/2 spin-orbit splitting, has also been explored [8][9][10][11][12][13]. Weakening of the shell closure is seen by the decrease of the 2 With proton number Z = 14 and neutron number N = 28, the nuclear structure of 42 Si is of special interest. A simple but important question that arises is whether the weakening of the N = 28 shell closure continues, causing an enhancement of nuclear collectivity, or if shell stability is restored owing to a possible doubly magic structure. A study on 42 Si was made by a two-proton removal reaction experiment with radioactive 44 S beams at the NSCL [15]. The small two-proton removal cross sec-

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