Single-particle enhancement of heavy-element production

Fusion barriers are calculated in a macroscopic-microscopic model for several cold-fusion heavy-ion reactions leading to heavy and superheavy elements. The results obtained in such a picture are very different from those obtained in a purely macroscopic model. For reactions on ²⁰⁸Pb targets, shell effects in the entrance channel result in fusion-barrier energies at the touching point that are only a few MeV higher than the ground state for compound systems near Z = 110. The entrance-channel fragment-shell effects remain far inside the touching point, almost to configurations only slightly more elongated than the ground-state configuration, where the fusion barrier has risen to about 10 MeV above the ground-state energy. Calculated single-particle level diagrams show that few level crossings occur until the peak in the fusion barrier very close to the ground-state shape is reached, which indicates that dissipation is negligible until very late in the fusion process. Whereas the fission valley in a macroscopic picture is several tens of MeV lower in energy than is the fusion valley, we find in the macroscopic-microscopic picture that the fission valley is only about 5 MeV lower than the fusion valley for cold-fusion reactions leading to compound systems near Z = 110. These results show that no significant “extra-extrapush” energy is needed to bring the system inside the fission saddle point and that the typical reaction energies for maximum cross section in heavy-element synthesis correspond to only a few MeV above the maximum in the fusion barrier.     

Isomeric states in 214Th and 213Th

Isomeric states in ²¹⁴Th and ²¹³Th were identified by means of γ -rays measured in delayed coincidence with the implanted evaporation residues. These were produced in irradiations of ¹⁶⁴Dy with ⁵⁴Cr projectiles and separated in-flight by the velocity filter SHIP. An isomeric state of I ^π = 8⁺ with a half-life of (1.24±0.12) μs was identified in ²¹⁴Th . The configuration π[1h _9/2 ⊗ 2f _7/2] was assigned to this state. An isomeric state with a half-life of (1.4±0.4) μs was observed in ²¹³Th . Tentatively it was assigned to an I ^π = 13/2⁺ state.     

Inversion domain boundaries in ZnO with additions of Fe2O3 studied by high-resolution ADF imaging

Columns of metal atoms in the polytypoid compound Fe2O3(ZnO)15 could be resolved by high angle annular dark field imaging in a transmission electron microscopy (TEM)/STEM electron microscope--a result which could not be realized by high-resolution bright field imaging due to inherent strain from inversion domains and inversion domain boundaries (IDBs) in the crystals. The basal plane IDB was imaged in [11 00] yielding the spacing of the two adjacent ZnO domains, while imaging in [21 1 0] yields the position of single metal ions. The images allow the construction of the entire domain structure including the stacking sequence and positions of the oxygen ions. The IDB consists of a single layer of octahedrally co-ordinated Fe3+ ions, and the inverted ZnO domains are related by point symmetry at the iron position. The FeO6 octahedrons are compressed along the ZnO c-axis resulting in a FeO bond length of 0.208 nm which is in the range of FeO distances in iron containing oxides. The model of the basal plane boundary resembles that of the IDB in polytypoid ZnO-In2O3 compounds.     

A CEMS apparatus for in situ studies of ion beam modified metals

An apparatus for CEMS studies of ion beam modified metals is described. The spectrometer can be coupled directly to the ion implanter. During ion bombardement the sample can be cooled to liquid nitrogen temperature or heated to about 500 K. CEMS measurements can be taken directly after ion beam modification between room temperature and liquid nitrogen temperature. As a first test of the performance of the apparatus CEMS spectra of boron-ion implanted iron at room temperature are presented.     

Enhanced electron–ion recombination in ion storage rings

We review recent advances in the understanding of the enhanced electron–ion recombination observed in storage ring experiments. The measured recombination rates show a strong enhancement relative to what the standard radiative recombination rates predict. A transient motional electric field is induced in the merging region of an electron and an ion beam in the electron cooler. This induced field opens an additional pathway for free-bound transitions of electrons. The formed Rydberg states can be radiatively stabilized and contribute to the measured rate. We show that this “field induced recombination” (FIR) explains the gap previously observed between measurements and the standard radiative recombination rate.     

Molecular-Dynamics Method for the Simulation of Grain-Boundary Migration

A molecular-dynamics method for the simulation of the intrinsicmigration behavior of individual, flat grain boundaries is introducedand validated. A constant driving force for grain-boundary migrationis generated by imposing an anisotropic elastic strain on a bicrystalsuch that the elastic-energy densities in its two halves aredifferent. For the model case of a large-planar-unit-cell, high-angle(001) twist boundary in Cu we show that an elastic strain of1%–4% is sufficient to drive thecontinuous, viscous movement of the boundary at temperatures wellbelow the melting point. The driving forces thus generated (at thehigh end of the experimentally accessible range) enable aquantitative evaluation of the migration process during the timeframe of 10^-9 s typically accessible bymolecular-dynamics simulation. For this model high-angle grainboundary we demonstrate that (a) the drift velocity is, indeed,proportional to the applied driving force thus enabling us todetermine the boundary mobility, (b) the activation energy forgrain-boundary migration is distinctly lower than that forgrain-boundary self-diffusion or even self-diffusion in the melt and(c) in agreement with earlier simulations the migration mechanisminvolves the collective reshuffling during local disordering(melting) of small groups of atoms and subsequentresolidification onto the other crystal.     

Directionality of Gaussian Schell-model beams propagating in atmospheric turbulence

It is known that some partially coherent Gaussian Shell-model beams may generate, in free space, the same angular distribution of radiant intensity as a fully coherent laser beam. We show that this result also holds even if the beams propagate in atmospheric turbulence, irrespective of the particular model of turbulence used. The result is illustrated by an example.     

Effects of spatial coherence on near-field spectra

Expressions are derived for the spectrum of the field generated by a planar, homogeneous, secondary source of any spectral distribution and of any state of spatial coherence. It is shown that the state of coherence affects the contributions of the homogeneous as well as the evanescent waves of the emitted field. The near-field spectra are studied in detail. The analysis is illustrated by examples.