T. Baugher

Collectivity at N=50: Ge-82 and Se-84

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.

Mirror Energy Differences at Large Isospin Studied through Direct Two-Nucleon Knockout

The first spectroscopy of excited states in 52Ni (T(z)=-2) and 51Co (T(z)=-3/2) has been obtained using the highly selective two-neutron knockout reaction. Mirror energy differences between isobaric analogue states in these nuclei and their mirror partners are interpreted in terms of isospin nonconserving effects. A comparison between large-scale shell-model calculations and data provides the most compelling evidence to date that both electromagnetic and an additional isospin nonconserving interactions for J=2 couplings, of unknown origin, are required to obtain good agreement.

Isospin Symmetry at High Spin Studied via Nucleon Knockout from Isomeric States

One-neutron knockout reactions have been performed on a beam of radioactive ^{53}Co in a high-spin isomeric state. The analysis is shown to yield a highly selective population of high-spin states in an exotic nucleus with a significant cross section, and hence represents a technique that is applicable to the planned new generation of fragmentation-based radioactive beam facilities. Additionally, the relative cross sections among the excited states can be predicted to a high level of accuracy when reliable shell-model input is available. The work has resulted in a new level scheme, up to the 11^{+} band-termination state, of the proton-rich nucleus ^{52}Co (Z=27, N=25). This has in turn enabled a study of mirror energy differences in the A=52 odd-odd mirror nuclei, interpreted in terms of isospin-nonconserving (INC) forces in nuclei. The analysis demonstrates the importance of using a full set of J-dependent INC terms to explain the experimental observations.

Electron capture in core-collapse supernovae investigated through configuration mixing in neutron-rich nuclei

Electron capture on neutron-rich medium-mass nuclei is a key process where the electrons that impede the collapse of the core of massive stars are captured, thereby producing very neutron-rich nuclei. As the core collapses, the supernova is then initiated. For the electron capture to proceed, however, the allowed Gamow-Teller (GT) transition must be unblocked either by thermal excitations or by mixing of proton configurations from a higher-lying shell into the ground-state configuration of the nucleus. The present paper presents an experiment performed at the National Superconducting Cyclotron Laboratory at Michigan State University, in which we study the configuration mixing in the neutron-rich76Zn isotope. The experiment utilised single-proton and single-neutron knockout with detection of reaction-residue γ rays and measurement of the parallel momentum of the residue. Through this we investigate the proton components of the 76Zn ground state, with a particular interest in π-g9/2, which may unblock the GT electron capture even at low temperatures and thereby open a new pathway for the initiation of the collapse of the pre-supernova stellar core.

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.