Microscopic Enhancement of Heavy-Element Production

Realistic fusion barriers are calculated in a macroscopic-microscopic model for several soft-fusion heavy-ion reactions leading to heavy and superheavy elements. The results obtained in such a realistic picture are very different from those obtained in a purely macroscopic model. For reactions on 208:Pb 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 soft-fusion reactions leading to compound systems near Z = 110. These results show that no significant ``extra-extra-push'' 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.Comment: 7 pages. LaTeX. Submitted to Zeitschrift fur Physik A. 5 figures not included here. Complete preprint, including device-independent (dvi), PostScript, and LaTeX versions of the text, plus PostScript files of the figures, available at http://t2.lanl.gov/publications/publications.html or at ftp://t2.lanl.gov/pub/publications/mehe

Global microscopic calculations of ground-state spin and parity for odd-mass nuclei

Systematic calculations of ground-state spin and parity of odd-mass nuclei have been performed within the Hartree--Fock--BCS (HFBCS) approach and the Finite-Range Droplet Model for nuclei for which experimental data are available. The unpaired nucleon has been treated perturbatively, and axial and left-right reflection symmetries have been assumed. As for the HFBCS approach, three different Skyrme forces have been used in the particle-hole channel, whereas the particle-particle matrix elements have been approximated by a seniority force. The calculations have been done for the 621 nuclei for which the Nubase 2003 data set give assignments of spin and parity with strong arguments. The agreement of both spin and parity in the self-consistent model reaches about 80% for spherical nuclei, and about 40% for well-deformed nuclei regardless of the Skyrme force used. As for the macroscopic-microscopic approach, the agreement for spherical nuclei is about 90% and about 40% for well-deformed nuclei, with different sets of spherical and deformed nuclei found in each model.Comment: 5 pages, 4 figures (three in color), 1 table, to be submitted to Physical Review

Paired composite fermion wavefunctions

We construct a family of BCS paired composite fermion wavefunctions that generalize, but remain in the same topological phase as, the Moore-Read Pfaffian state for the half-filled Landau level. It is shown that for a wide range of experimentally relevant inter-electron interactions the groundstate can be very accurately represented in this form.Comment: 4 pages, 2 figure

A simple, ultrahigh vacuum compatible scanning tunneling microscope for use at variable temperatures

We present the construction of a very compact scanning tunneling microscope (STM) which can be operated at temperatures between 4 and 350 K. The tip and a tiny tip holder are the only movable parts, whereas the sample and the piezoscanner are rigidly attached to the body of the STM. This leads to an excellent mechanical stability. The coarse approach system relies on the slip-stick principle and is operated by the same piezotube which is used for scanning. As an example of the performance of the device, images of a NbSe2 surface with atomic resolution are obtained

Nuclear Ground-State Masses and Deformations

We tabulate the atomic mass excesses and nuclear ground-state deformations of 8979 nuclei ranging from $^{16}$O to $A=339$. The calculations are based on the finite-range droplet macroscopic model and the folded-Yukawa single-particle microscopic model. Relative to our 1981 mass table the current results are obtained with an improved macroscopic model, an improved pairing model with a new form for the effective-interaction pairing gap, and minimization of the ground-state energy with respect to additional shape degrees of freedom. The values of only 9 constants are determined directly from a least-squares adjustment to the ground-state masses of 1654 nuclei ranging from $^{16}$O to $^{263}$106 and to 28 fission-barrier heights. The error of the mass model is 0.669~MeV for the entire region of nuclei considered, but is only 0.448~MeV for the region above $N=65$.Comment: 50 pages plus 20 PostScript figures and 160-page table obtainable by anonymous ftp from t2.lanl.gov in directory masses, LA-UR-93-308