2,407 research outputs found
A new approach to barrier-top fission dynamics
We proposed a calculational framework for describing induced fission that
avoids the Bohr-Wheeler assumption of well-defined fission channels. The
building blocks of our approach are configurations that form a discrete,
orthogonal basis and can be characterized by both energy and shape. The
dynamics is to be determined by interaction matrix elements between the states
rather than by a Hill-Wheeler construction of a collective coordinate. Within
our approach, several simple limits can be seen: diffusion; quantized
conductance; and ordinary decay through channels. The specific proposal for the
discrete basis is to use the quantum numbers of the axially symmetric
Hartree-Fock approximation to generate the configurations. Fission paths would
be determined by hopping from configuration to configuration via the residual
interaction. We show as an example the configurations needed to describe a
fictitious fission decay . We also examine the geometry of the path for fission of U,
measuring distances by the number of jumps needed to go to a new
partition.Comment: Write-up of a talk given at the Workshop "Compound-nuclear reactions
2015" Tokyo, Oct. 19-23, 2015; 11 pages and 11 figures. To be published in
European Journal of Physics, Web of Conference
Unitary Fermi gas at finite temperature in the epsilon expansion
Thermodynamics of the unitary Fermi gas at finite temperature is investigated
from the perspective of the expansion over epsilon=4-d with d being the
dimensionality of space. We show that the thermodynamics is dominated by
bosonic excitations in the low temperature region T<<Tc. Analytic formulas for
the thermodynamic functions as functions of the temperature are derived to the
lowest order in epsilon in this region. In the high temperature region where T
Tc, bosonic and fermionic quasiparticles are excited. We determine the critical
temperature Tc of the superfluid phase transition and the thermodynamic
functions around Tc to the leading and next-to-leading orders in epsilon.Comment: 13 pages, 7 figures, revtex4; version to appear in Phys. Rev.
Application of time-dependent density-functional theory to electron-ion couplng in ethylene
To examine the applicability of the time-dependent density-functional theory
(TDDFT) for treating the electron-nucleus coupling in excited states, we
calculate the strength distribution associated with the pi-pi* transition in
ethylene. The observed optical transition strength at 7-8.5 eV region shows a
complex structure arising from coupling to C-C stretch motion, to torsional
motion, and to Rydberg excitations. The mean energy of the observed peak is
reproduced to about 0.2 eV accuracy by the TDDFT in the local density
approximation (LDA). The reflection approximation is used to calculate the peak
broadening. Roughly half of the broadening can be attributed to the fluctuation
in the C-C coordinate. The asymmetry in the line shape is also qualitatively
reproduced by the C-C coordinate fluctuation. We find, in agreement with other
theoretical studies, that the torsional motion is responsible for the
progression of weak transition strength extending from the peak down to about 6
eV. The LDA reproduces the strength in this region to about factor of 3. We
conclude that the TDDFT is rather promising for calculating the electron
nucleus coupling at short times.Comment: 14 pages and 4 figures: an error is corrected in Table
Independent Orbiter Assessment (IOA): Assessment of the electrical power generation/fuel cell powerplant subsystem FMEA/CIL
Results of the Independent Orbiter Assessment (IOA) of the Failure Modes and Effects Analysis (FMEA) and Critical Items List (CIL) are presented. The IOA effort first completed an analysis of the Electrical Power Generation/Fuel Cell Powerplant (EPG/FCP) hardware, generating draft failure modes and potential critical items. To preserve independence, this analysis was accomplished without reliance upon the results contained within the NASA FMEA/CIL documentation. The IOA results were then compared to the proposed Post 51-L NASA FMEA/CIL baseline. A resolution of each discrepancy from the comparison was provided through additional analysis as required. This report documents the results of that comparison for the Orbiter EPG/FCP hardware
Independent Orbiter Assessment (IOA): Analysis of the electrical power generation/fuel cell powerplant subsystem
Results of the Independent Orbiter Assessment (IOA) of the Failure Modes and Effects Analysis (FMEA) and Critical Items List (CIL) are presented. The IOA approach features a top-down analysis of the hardware to determine failure modes, criticality, and potential critical items. To preserve independence, this analysis was accomplished without reliance upon the results contained within the NASA FMEA/CIL documentation. This report documents the independent analysis results corresponding to the Orbiter Electrical Power Generation (EPG)/Fuel Cell Powerplant (FCP) hardware. The EPG/FCP hardware is required for performing functions of electrical power generation and product water distribution in the Orbiter. Specifically, the EPG/FCP hardware consists of the following divisions: (1) Power Section Assembly (PSA); (2) Reactant Control Subsystem (RCS); (3) Thermal Control Subsystem (TCS); and (4) Water Removal Subsystem (WRS). The IOA analysis process utilized available EPG/FCP hardware drawings and schematics for defining hardware assemblies, components, and hardware items. Each level of hardware was evaluated and analyzed for possible failure modes and effects. Criticality was assigned based upon the severity of the effect for each failure mode
Photoabsorption spectra in the continuum of molecules and atomic clusters
We present linear response theories in the continuum capable of describing
photoionization spectra and dynamic polarizabilities of finite systems with no
spatial symmetry. Our formulations are based on the time-dependent local
density approximation with uniform grid representation in the three-dimensional
Cartesian coordinate. Effects of the continuum are taken into account either
with a Green's function method or with a complex absorbing potential in a
real-time method. The two methods are applied to a negatively charged cluster
in the spherical jellium model and to some small molecules (silane, acetylene
and ethylene).Comment: 13 pages, 9 figure
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