K. A. Walsh

Collectivity atN=50:Ge82andSe84

The neutron-rich N = 50 isotones 82 Ge and 84 Se were investigated using intermediate-energy Coulomb excitation on a 197 Au target and inelastic scattering on 9 Be. As typical for intermediate-energy Coulomb excitation with projectile energies exceeding 70 MeV/nucleon, only the first 2 + states were excited in 82 Ge and 84 Se. However, in the inelastic scattering on a 9 Be target, a strong population of the first 4 + state was observed for 84 Se, while there is no indication of a similarly strong excitation of the corresponding state in the neighboring even-even isotone 82 Ge. The results are discussed in the framework of systematics and shell-model calculations using three different effective interactions.

Inverse-kinematics one-neutron pickup with fast rare-isotope beams

Measurements and reaction model calculations are reported for single-neutron pickup reactions onto a fast 22 Mg secondary beam at 84 MeV per nucleon. Measurements made on both carbon and beryllium targets, having very different structures, were used to investigate the likely nature of the pickup reaction mechanism. The measurements involve thick reaction targets and γ-ray spectroscopy of the projectile-like reaction residue for final-state resolution, which permit experiments with low incident beam rates compared to traditional low-energy transferreactions.Frommeasuredlongitudinalmomentumdistributionsweshowthatthe 12 C( 22 Mg, 23 Mg + γ)X reaction largely proceeds as a direct two-body reaction, with the neutron transfer producing bound 11 Ct arget residues. The corresponding reaction on the 9 Be target seems to largely leave the 8 Be residual nucleus unbound at excitation energies high in the continuum. We discuss the possible use of such fast-beam one-neutron pickup reactions to track single-particle strength in exotic nuclei and also their expected sensitivity to neutron high-�(intruder) states, which are often direct indicators of shell evolution and the disappearance of magic numbers in the exotic regime.

Collectivity of Exotic Heavy Fe Isotopes

The properties of exotic neutron-rich nuclei between the proton shell closures Z = 20 and Z = 28 are of particular interest for the understanding of the shell structure for large neutron excess. Effects related to the energy gap between the neutron fp and 1g9/2 shells lead to a strong variation of collectivity for nuclei around N = 40. Whereas 68Ni was found to have doubly magic properties, this was not observed in neighbouring nuclei. Recent shell model calculations for the neutron rich iron isotopes clearly reveal the difficulty to describe nuclei in this mass region and resulted in large deviations of the predicted collectivity depending on the valence space. However, no experimental data on the transition strength existed for the very exotic nucleus 66Fe at N = 40. Here we present the newest results on absolute transition strengths of the lowest excited states in 62,64,66Fe measured model independently using the recoil distance Doppler-shift (RDDS) method. The experiments were performed at NSCL at Michigan State University with the Cologne/NSCL plunger device using Coulomb excitation in inverse kinematics at energies of 80 MeV/u. Our results yield a much higher collectivity for 64,66Fe than expected and allow tests of new calculations.

Lifetime Measurement of the 21+ state in 20C

The mean lifetime of the 21+ state in the near drip‐line nuclide 20C was measured for the first time using the Recoil Distance Method with intermediate energy radioactive beams via a knockout reaction. The measured value of τ21+ = 9.8±2.8(stat)−1.1+0.5(syst) ps gives a reduced transition probability of B(E2;21+→0g.s.+) = 7.51.7+3.0(stat) e2fm4. This value is in good agreement with shell model calculations using a p—sd shell model space and isospin‐dependent (N—Z) effective charges.

Neutron‐Rich 62,64,64Fe Show Enhanced Collectivity: The Washout of N = 40 in Terms of Experiment, Valence Proton Symmetry and Shell Model

Probing shell structure at a large neutron excess has been of particular interest in recent times. Neutron‐rich nuclei between the proton shell closures Z = 20 and Z = 28 offer an exotic testing ground for shell evolution. The development of the N = 40 gap between neutron fp and lg9/2 shells gives rise to highly interesting variations of collectivity for nuclei in this region. While 68Ni shows doubly magic properties in level energies and transition strengths, this was not observed in neighbouring nuclei. Especially neutron‐rich Fe isotopes proved particularly resistant to calculational approaches using the canonical valence space (fpg) resulting in important deviations of the predicted collectivity. Only an inclusion of the d5/2‐orbital could solve the problem [1]. Hitherto no transition strengths for 66Fe have been reported. We determined B(E2,21+→01+) values from lifetimes measured with the recoil distance Doppler‐shift method using the Cologne plunger for radioactive beams at National Superconducting ...