631 research outputs found
Fission modes of mercury isotopes
Background: Recent experiments on beta-delayed fission in the mercury-lead
region and the discovery of asym- metric fission in 180 Hg [1] have stimulated
theoretical interest in the mechanism of fission in heavy nuclei. Purpose: We
study fission modes and fusion valleys in 180 Hg and 198 Hg to reveal the role
of shell effects in pre-scission region and explain the experimentally observed
fragment mass asymmetry and its variation with A. Methods: We use the
self-consistent nuclear density functional theory employing Skyrme and Gogny
energy density functionals. Results: The potential energy surfaces in
multi-dimensional space of collective coordinates, including elongation,
triaxiality, reflection-asymmetry, and necking, are calculated for 180 Hg and
198 Hg. The asymmetric fission valleys - well separated from fusion valleys
associated with nearly spherical fragments - are found in in both cases. The
density distributions at scission configurations are studied and related to the
experimentally observed mass splits. Conclusions: The energy density
functionals SkM\ast and D1S give a very consistent description of the fission
process in 180 Hg and 198 Hg. We predict a transition from asymmetric fission
in 180 Hg towards more symmetric distribution of fission fragments in 198 Hg.
For 180 Hg, both models yield 100 Ru/80 Kr as the most probable split. For 198
Hg, the most likely split is 108 Ru/90 Kr in HFB-D1S and 110 Ru/88 Kr in
HFB-SkM\ast.Comment: 6 pages, 5 figures, to be published in Physical Review
The contrasting fission potential-energy structure of actinides and mercury isotopes
Fission-fragment mass distributions are asymmetric in fission of typical
actinide nuclei for nucleon number in the range
and proton number in the range . For somewhat
lighter systems it has been observed that fission mass distributions are
usually symmetric. However, a recent experiment showed that fission of
Hg following electron capture on Tl is asymmetric. We calculate
potential-energy surfaces for a typical actinide nucleus and for 12 even
isotopes in the range Hg--Hg, to investigate the similarities
and differences of actinide compared to mercury potential surfaces and to what
extent fission-fragment properties, in particular shell structure, relate to
the structure of the static potential-energy surfaces. Potential-energy
surfaces are calculated in the macroscopic-microscopic approach as functions of
fiveshape coordinates for more than five million shapes. The structure of the
surfaces are investigated by use of an immersion technique. We determine
properties of minima, saddle points, valleys, and ridges between valleys in the
5D shape-coordinate space. Along the mercury isotope chain the barrier heights
and the ridge heights and persistence with elongation vary significantly and
show no obvious connection to possible fragment shell structure, in contrast to
the actinide region, where there is a deep asymmetric valley extending from the
saddle point to scission. The mechanism of asymmetric fission must be very
different in the lighter proton-rich mercury isotopes compared to the actinide
region and is apparently unrelated to fragment shell structure. Isotopes
lighter than Hg have the saddle point blocked from a deep symmetric
valley by a significant ridge. The ridge vanishes for the heavier Hg isotopes,
for which we would expect a qualitatively different asymmetry of the fragments.Comment: 8 pages, 9 figure
Effect of physical and chemical doping on optical spectra of SWNT's
We discuss the use of far-infrared spectroscopy in the characterization of doped and
functionalized nanotube derivatives
Mass distributions for induced fission of different Hg isotopes
With the improved scission-point model the mass distributions are calculated
for induced fission of different Hg isotopes with the masses 180-196. The
drastic change in the shape of the mass distribution from asymmetric to
symmetric is revealed with increasing mass number of the fissioning Hg isotope,
and the reactions are proposed to verify this prediction experimentally. The
asymmetric mass distribution of fission fragments observed in the recent
experiment on the fission of 180Hg is explained. The calculated mass
distribution and mean total kinetic energy of fission fragments are in a good
agreement with the available experimental data
Anomalous Electron Transport in Field-Effect Transistors with Titanium Ditelluride Semimetal Thin-Film Channels
We report on "graphene-like" mechanical exfoliation of thin films of titanium
ditelluride and investigation of their electronic properties. The exfoliated
crystalline TiTe2 films were used as the channel layers in the back-gated
field-effect transistors fabricated with Ti/Al/Au metal contacts on SiO2/Si
substrates. The room-temperature current-voltage characteristics revealed
strongly non-linear behavior with signatures of the source-drain threshold
voltage similar to those observed in the charge-density-wave devices. The
drain-current showed an unusual non-monotonic dependence on the gate bias
characterized by the presence of multiple peaks. The obtained results can be
potentially used for implementation of the non-Boolean logic gates.Comment: 11 pages, 4 figure
Non-Markovian large amplitude motion and nuclear fission
The general problem of dissipation in macroscopic large-amplitude collective
motion and its relation to energy diffusion of intrinsic degrees of freedom of
a nucleus is studied. By applying the cranking approach to the nuclear many
body system, a set of coupled dynamical equations for the collective classical
variables and the quantum mechanical occupancies of the intrinsic nuclear
states is derived. Different dynamical regimes of the intrinsic nuclear motion
and its consequences on time properties of collective dissipation are
discussed. The approach is applied to the descant of the nucleus from the
fission barrier.Comment: 9 pages and 3 figure
Multiple EphB receptor tyrosine kinases shape dendritic spines in the hippocampus
Here, using a genetic approach, we dissect the roles of EphB receptor tyrosine kinases in dendritic spine development. Analysis of EphB1, EphB2, and EphB3 double and triple mutant mice lacking these receptors in different combinations indicates that all three, although to varying degrees, are involved in dendritic spine morphogenesis and synapse formation in the hippocampus. Hippocampal neurons lacking EphB expression fail to form dendritic spines in vitro and they develop abnormal spines in vivo. Defective spine formation in the mutants is associated with a drastic reduction in excitatory glutamatergic synapses and the clustering of NMDA and AMPA receptors. We show further that a kinase-defective, truncating mutation in EphB2 also results in abnormal spine development and that ephrin-B2–mediated activation of the EphB receptors accelerates dendritic spine development. These results indicate EphB receptor cell autonomous forward signaling is responsible for dendritic spine formation and synaptic maturation in hippocampal neurons
Influence of entrance-channel magicity and isospin on quasi-fission
The role of spherical quantum shells in the competition between fusion and
quasi-fission is studied for reactions forming heavy elements. Measurements of
fission fragment mass distributions for different reactions leading to similar
compound nuclei have been made near the fusion barrier. In general, more
quasi-fission is observed for reactions with non-magic nuclei. However, the
Ca+Pb reaction is an exception, showing strong evidence for
quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock
calculations predict fast equilibration of in the two fragments early in
the collision. This transfer of nucleons breaks the shell effect, causing this
reaction to behave more like a non-magic one in the competition between fusion
and quasi-fission. Future measurements of fission in reactions with exotic
beams should be able to test this idea with larger asymmetries.Comment: accepted for publication in Physics Letters
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