Shape coexistence in neutron-rich nuclei near N=40

Recent data show that both the 2+ and 4+ levels in the even neutron-rich Cr and Fe isotopes decrease in excitation energy toward N=40. This observation, along with Coulomb excitation and lifetime data, strongly indicates an increase in collectivity near N=40 in contradiction with expectations based on first principles. A straightforward two-band mixing model is used to investigate the structure of these neutron-rich Cr and Fe nuclei. The approach takes advantage of the extensive data available for 60Fe to provide the parameter values with which to reproduce the experimental observations in the 58-64Cr and 60-68Fe isotopic chains. Comparisons between the model and the data suggest marked structural differences for the ground-state configurations of N=40 Cr and Fe

ATLAS with CARIBU: A Laboratory Portrait

The Argonne Tandem Linac Accelerator System (ATLAS) is the world's first superconducting accelerator for projectiles heavier than the electron. This unique system is a U.S. Department of Energy (DOE) national user research facility open to scientists from all over the world. It is located within the Physics Division at Argonne National Laboratory and is one of five large scientific user facilities located at the laboratory

The quasi-continuum of gamma rays following the decay of superdeformed bands in the Hg region

The quasi-continuum of the spectrum associated with the decay-out of the yrast superdeformed band in {sup 194}Hg has been extracted. It has for the first time been possible to compare the spin and excitation energy determined from the analysis of the quasi-continuum {gamma} rays to the exact result obtained from the on-step linking transitions

Radius of curvature of the S factor maximum in sub-barrier fusion hindrance

A maximum of the S(E) factor is evidence for an onset of sub-barrier fusion hindrance and it can be well described by a radius-of-curvature expression near the maximum. The systematics of this radius of curvature has been studied over a wide range of projectile-target combinations. It follows a tentative general trend as a function of the parameter ζ=Z1Z2μ, and is strongly affected by effects associated with the nuclear structure of the nuclei in the entrance channel. It also explains the reason why the S factor maximum is not easily recognized visually for lighter, astrophysically interesting fusion systems

Projectilelike fragments from N14 beams at 15, 25, and 35 MeV/nucleon

Momentum distributions of projectilelike fragments produced in the interaction of 15, 25, and 35 MeV/nucleon N14 beams on targets of C12, Mg24, Al27, Ti48, and Ni58 have been measured. Widths and centroids of the quasielastic component of the distributions have been extracted for fragments ranging from He4 to N13. The widths are compared with published data and several theoretical predictions. The Friedman model correctly predicts the charge and isotope dependence of the widths at the highest beam energy. At 15 MeV/nucleon some deviations from the trend of the published data suggest the possibility of a different reaction mechanism for the projectile-target combinations studied in the present work

Expectations for C12 and O16 induced fusion cross sections at energies of astrophysical interest

The extrapolations of cross sections for fusion reactions involving C12 and O16 nuclei down to energies relevant for explosive stellar burning have been reexamined. Based on a systematic study of fusion in heavier systems, it is expected that a suppression of the fusion process will also be present in these light heavy-ion systems at extreme sub-barrier energies due to the saturation properties of nuclear matter. Previous phenomenological extrapolations of the S factor for light heavy-ion fusion based on optical model calculations may therefore have overestimated the corresponding reaction rates. A new "recipe" is proposed to extrapolate S factors for light heavy-ion reactions to low energies taking the hindrance behavior into account. It is based on a fit to the logarithmic derivative of the experimental cross section which is much less sensitive to overall normalization discrepancies between different data sets than other approaches. This method, therefore, represents a significant improvement over other extrapolations. The impact on the astrophysical reaction rates is discussed

Physics of a rare isotope accelerator

Major progress in nuclear research and in observations of the cosmos has made it clear that critical issues in understanding the nucleus and astrophysical processes require abundant new sources of exotic nuclei, away from the realm of the stable ones. Recent advances in accelerator and isotope-production technology make access to these rare isotopes possible. This review examines the impact of the new reach in physics provided by a rare isotope accelerator in nuclear structure, astrophysics, and searches for physics beyond the standard model. We also touch briefly on some of the benefits of these isotopes for other important societal needs

Survey of heavy-ion fusion hindrance for lighter systems

A survey of heavy-ion fusion cross sections at extreme sub-barrier energies has been carried out for lighter systems with positive Q values. A general parametrization is proposed, which describes excitation functions for a wide range of light systems at low energies. This parametrization is then applied to a calculation of excitation functions and S factors for the system O16+O16, which has recently been investigated with various other theoretical approaches. It is suggested that this parametrization is useful for estimating sub-barrier fusion cross sections with exotic neutron-rich partners which cannot be studied in the laboratory

New results on octupole collectivity

Octupole correlations play an important role in determining the level structure of nuclei throughout the periodic chart. Microscopically, octupole correlations are the result of the long-range, octupole-octupole interaction between nucleons occupying pairs of orbitals which differ in both orbital and total angular momentum by 3 units. A review of some of the most recent findings on octupole correlations is given. Emphasis is placed on new results from the actinide region, where two distinct collective modes have long been identified: octupole vibration and octupole deformation. These new results include negative-parity structures which appear to evolve from an octupole vibration into a static octupole deformed mode. In addition, newly observed rotational structures built on an excited 0+ state have been tentatively associated with a double-octupole phonon excitation. These newly observed properties can be successively described by calculations based on the concept of rotational-aligned octupole phonon condensation

Intensity and multipolarity of low-energy components in the quasicontinuum γ-ray spectrum following α- And C12-induced reactions

Measurements of K-shell ionization yields for (α,xn) and (C12,xn) evaporation residues exclude the existence of a large M1 component in the quasicontinuum spectrum of well-deformed Dy nuclei below 500 keV. Upper intensity limits are deduced for M1, E1, and E2 components