6,710 research outputs found
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 O to . 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 O to
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 .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
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