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.

Excited states in 138Ce have been studied via the 12C(138Ce, 138Ce*) Coulomb excitation reaction at 480 MeV. Relative cross sections have been determined from the gamma-ray yields observed with Gammasphere. The E2 and M1 strength distributions between the lowest six 2+ states up to 2.7 MeV enables us to identify the 2(4)+ state in 138Ce as the dominant fragment of the one-phonon 2(1,ms)+ mixed-symmetry state. Mixing between this level and a nearby isoscalar state is observed and is more than 4 times larger than in the neighboring isotone 136Ba. This is direct evidence that the stability of mixed-symmetry states strongly depends on the underlying subshell structure.

The two-proton knockout reaction 9 Be( 54 Ti, 52 Ca+γ) has been studied at 72 MeV/nucleon. Besides the strong feeding of the 52 Ca ground state, the only other sizeable cross section proceeds to a 3 − level at 3.9 MeV. There is no measurable direct yield to the first excited 2 + state at 2.6 MeV. The results illustrate the potential of such direct reactions for exploring cross-shell proton excitations in neutron-rich nuclei and confirms the doubly-magic nature of 52 Ca.For decades, the cornerstone of nuclear structure has been the concept of single-particle motion in a welldefined potential leading to shell structure and magic numbers governed by the strength of the mean-field spin-orbit interaction [1]. Recent observations in exotic, neutron-rich nuclei have demonstrated that the sequence and energy spacing of single-particle orbits is not as immutable as once thought: some of the familiar magic numbers disappear and new shell gaps develop [2]. Crossshell excitations, arising from the promotion of nucleons across shell gaps, probe changes in shell structure. They are, however, not always readily identifiable in nuclear spectra. This letter demonstrates that two-proton knockout reactions can examine, selectively, cross-shell proton excitations in neutron-rich systems.Single-nucleon knockout reactions with fast radioactive beams are established tools to investigate the properties of halo nuclei [3] and to study beyond meanfield correlations, indicated by the quenching of spectroscopic strengths [4]. Eikonal theory [5] provides a suitable framework for the extraction of quantitative nuclear structure information from such reactions. In contrast, the potential of two-nucleon knockout as a spectroscopic tool has been recognized only recently. Bazin et al. [6] have shown that two-proton removal reactions from beams of neutron-rich species at intermediate energies proceed as direct reactions and that partial cross sections to different final states of the residue provide structure information. More recently, such a reaction was used to infer the magicity of the very neutron-rich 42 Si nucleus [7].In the current experiment, sizable cross sections for the 9 Be( 54 Ti, 52 Ca+γ)X reaction were found to feed only the 52 Ca ground state and a 3 − level with an excitation energy near 4 MeV, bypassing completely the first 2 + level at 2.6 MeV. These observations can be reproduced qualitatively by calculations which assign the 3 − level to the promotion of protons across the Z = 20 shell gap. In addition, the data confirm the presence of a neutron sub-shell closure at N = 32, the subject of much recent attention [8,9,10,11,12,13].The 54 Ti secondary ions were produced by fragmentation of a 130 MeV/nucleon 76 Ge beam, delivered by the Coupled Cyclotron Facility of the National Superconducting Cyclotron Laboratory, onto a 9 Be fragmentation target. The ions were selected in the A1900 largeacceptance fragment separator [14], which was operated with two settings during different phases of the experiment; 1% momentum acceptance and...

A gamma-ray spectroscopy study of ;{26g}Al+p resonant states in 27Si is presented. Excitation energies were measured with improved precision and first spin-parity assignments made for excited states in 27Si above the proton threshold. The results indicate the presence of low-lying resonances with l_{p}=0 and l_{p}=2 captures that could strongly influence the ;{26g}Al(p,gamma)27Si reaction rate at low stellar temperatures, found in low-mass asymptotic giant branch (AGB), intermediate-mass AGB, super AGB, and Wolf-Rayet stars.

We have identified two isomers in 254No, built on two- and four-quasiparticle excitations, with quantum numbers K pi = 8- and (14+), as well as a low-energy 2-quasiparticle Kpi = 3+ state. The occurrence of isomers establishes that K is a good quantum number and therefore that the nucleus has an axial prolate shape. The 2-quasiparticle states probe the energies of the proton levels that govern the stability of superheavy nuclei, test 2-quasiparticle energies from theory, and thereby check their predictions of magic gaps.