Selfconsistent calculations of fission barriers in the Fm region

The fission barriers of the nuclei 254Fm, 256Fm, 258Fm, 258No and 260Rf are investigated in a fully microscopic way up to the scission point. The analysis is based on the constrained Hartree-Fock-Bogoliubov theory and Gogny's D1S force. The quadrupole, octupole and hexadecapole moments as well as the number of nucleons in the neck region are used as constraints. Two fission paths, corresponding to the bimodal fission, are found. The decrease with isotope mass of the half-life times of heavy Fm isotopes is also explained.Comment: 29 pages in LaTeX including 14 figure

Effect of differences in proton and neutron density distributions on fission barriers

The neutron and proton density distributions obtained in constrained Hartree-Fock-Bogolyubov calculations with the Gogny force along the fission paths of 232Th, 236U, 238U and 240Pu are analyzed. Significant differences in the multipole deformations of neutron and proton densities are found. The effect on potential energy surfaces and on barrier heights of an additional constraint imposing similar spatial distributions to neutrons and protons, as assumed in macroscopic-microscopic models, is studied.Comment: 5 pages in Latex, 4 figures in ep

Diamond thin Film Detectors for Beam Monitoring Devices

Diamonds offer radiation hard sensors, which can be used directly in primary beams. Here we report on the use of a polycrystalline CVD diamond strip sensor as beam monitor of heavy ion beams with up to 2.10^9 lead ions per bunch. The strips allow for a determination of the transverse beam profile to a fraction of the pitch of the strips, while the timing information yields the longitudinal bunch length with a resolution of the order of a few mm.Comment: 6 pages, 7 figures, to appear in the Proceedings of the Hasselt Diamond Workshop (Hasselt, Belgium, Feb. 2006), v4: accidentally submitted figure, appearing at end, remove

Light-particle emission from the fissioning nuclei 126Ba, 188Pt and (266,272,278)/110: theoretical predictions and experimental results

We present a comparison of our model treating fission dynamics in conjunction with light-particle (n, p, alpha) evaporation with the available experimental data for the nuclei 126Ba, 188Pt and three isotopes of the element Z=110. The dynamics of the symmetric fission process is described through the solution of a classical Langevin equation for a single collective variable characterizing the nuclear deformation along the fission path. A microscopic approach is used to evaluate the emission rates for pre-fission light particles. Entrance-channel effects are taken into account by generating an initial spin distribution of the compound nucleus formed by the fusion of two deformed nuclei with different relative orientations

The Neutron Halo in Heavy Nuclei Calculated with the Gogny Force

The proton and neutron density distributions, one- and two-neutron separation energies and radii of nuclei for which neutron halos are experimentally observed, are calculated using the self-consistent Hartree-Fock-Bogoliubov method with the effective interaction of Gogny. Halo factors are evaluated assuming hydrogen-like antiproton wave functions. The factors agree well with experimental data. They are close to those obtained with Skyrme forces and with the relativistic mean field approach.Comment: 13 pages in Latex and 17 figures in ep

Evidence for the Jacobi shape transition in hot 46Ti

The gamma-rays from the decay of the GDR in 46Ti compound nucleus formed in the 18O+28Si reaction at bombarding energy 105 MeV have been measured in an experiment using a setup consisting of the combined EUROBALL IV, HECTOR and EUCLIDES arrays. A comparison of the extracted GDR lineshape data with the predictions of the thermal shape fluctuation model shows evidence for the Jacobi shape transition in hot 46Ti. In addition to the previously found broad structure in the GDR lineshape region at 18-27 MeV caused by large deformations, the presence of a low energy component (around 10 MeV), due to the Coriolis splitting in prolate well deformed shape, has been identified for the first time.Comment: 8 pages, 4 figures, proceedings of the COMEX1 conference, June 2003, Paris; to be published in Nucl. Phys.

A Particle number conserving shell-correction method

The shell correction method is revisited. Contrary to the traditional Strutinsky method, the shell energy is evaluated by an averaging over the number of particles and not over the single-particle energies, which is more consistent with the definition of the macroscopic energy. In addition, the smooth background is subtracted before averaging the sum of single-particle energies, which significantly improves the plateau condition and allows to apply the method also for nuclei close to the proton or neutron drip lines. A significant difference between the shell correction energy obtained with the traditional and the new method is found in particular for highly degenerated single-particle spectra (as i.e. in magic nuclei) while for deformed nuclei (where the degeneracy is lifted to a large extent) both estimates are close, except in the region of super or hyper-deformed states.Comment: 11 pages in LaTeX, 7 figure

Stability of bubble nuclei through Shell-Effects

We investigate the shell structure of bubble nuclei in simple phenomenological shell models and study their binding energy as a function of the radii and of the number of neutron and protons using Strutinsky's method. Shell effects come about, on the one hand, by the high degeneracy of levels with large angular momentum and, on the other, by the big energy gaps between states with a different number of radial nodes. Shell energies down to -40 MeV are shown to occur for certain magic nuclei. Estimates demonstrate that the calculated shell effects for certain magic numbers of constituents are probably large enough to produce stability against fission, alpha-, and beta-decay. No bubble solutions are found for mass number A < 450.Comment: 9 pages and 9 figures in the eps format include

An ultra-thin diamond membrane as a transmission particle detector and vacuum window for external microbeams

Several applications of external microbeam techniques demand a very accurate and controlled dose delivery. To satisfy these requirements when post-sample ion detection is not feasible, we constructed a transmission single-ion detector based on an ultra-thin diamond membrane. The negligible intrinsic noise provides an excellent signal-to-noise ratio and enables a hit-detection efficiency of close to 100%, even for energetic protons, while the small thickness of the membrane limits beam spreading. Moreover, because of the superb mechanical stiffness of diamond, this membrane can simultaneously serve as a vacuum window and allow the extraction of an ion microbeam into the atmosphere

Contact-induced spin polarization in carbon nanotubes

Motivated by the possibility of combining spintronics with molecular structures, we investigate the conditions for the appearance of spin-polarization in low-dimensional tubular systems by contacting them to a magnetic substrate. We derive a set of general expressions describing the charge transfer between the tube and the substrate and the relative energy costs. The mean-field solution of the general expressions provides an insightful formula for the induced spin-polarization. Using a tight-binding model for the electronic structure we are able to estimate the magnitude and the stability of the induced moment. This indicates that a significant magnetic moment in carbon nanotubes can be observed.Comment: To appear in Phys. Rev. B (2003