2,125 research outputs found
Effect of reactor irradiation on properties of a nongalling alloy
Effect of reactor irradiation on properties of nongalling alloy
Energy dependence of nucleus-nucleus potential close to the Coulomb barrier
The nucleus-nucleus interaction potentials in heavy-ion fusion reactions are
extracted from the microscopic time-dependent Hartree-Fock theory for mass
symmetric reactions OO, CaCa,
CaCa and mass asymmetric reactions OCa,
CaCa, O+Pb, Ca+Zr. When the
center-of-mass energy is much higher than the Coulomb barrier energy,
potentials deduced with the microscopic theory identify with the frozen density
approximation. As the center-of-mass energy decreases and approaches the
Coulomb barrier, potentials become energy dependent. This dependence signs
dynamical reorganization of internal degrees of freedom and leads to a
reduction of the "apparent" barrier felt by the two nuclei during fusion of the
order of compared to the frozen density case. Several examples
illustrate that the potential landscape changes rapidly when the center-of-mass
energy is in the vicinity of the Coulomb barrier energy. The energy dependence
is expected to have a significant role on fusion around the Coulomb barrier.Comment: 11 pages, 13 figures, 1 table, discussion of effects of
coordinate-dependent mass added, accepted for publication in Phys. Rev.
A GENERAL ITERATION SCHEME FOR THE CALCULATION OF LEVEL DENSITIES, AND RESULTS USING A SEMICLASSICAL APPROXIMATION
A general scheme is derived to calculate m-particle n-hole fermion level densities for any single particle Hamiltonian taking into account Pauli exclusion. This technique is applied to obtain level densities of the three dimensional isotropic Harmonic Oscillator semiclassically in the Thomas-Fermi approach
Arithmetic Spacetime Geometry from String Theory
An arithmetic framework to string compactification is described. The approach
is exemplified by formulating a strategy that allows to construct geometric
compactifications from exactly solvable theories at . It is shown that the
conformal field theoretic characters can be derived from the geometry of
spacetime, and that the geometry is uniquely determined by the two-dimensional
field theory on the world sheet. The modular forms that appear in these
constructions admit complex multiplication, and allow an interpretation as
generalized McKay-Thompson series associated to the Mathieu and Conway groups.
This leads to a string motivated notion of arithmetic moonshine.Comment: 36 page
A generalized linear Hubble law for an inhomogeneous barotropic Universe
In this work, I present a generalized linear Hubble law for a barotropic
spherically symmetric inhomogeneous spacetime, which is in principle compatible
with the acceleration of the cosmic expansion obtained as a result of high
redshift Supernovae data. The new Hubble function, defined by this law, has two
additional terms besides an expansion one, similar to the usual volume
expansion one of the FLRW models, but now due to an angular expansion. The
first additional term is dipolar and is a consequence of the existence of a
kinematic acceleration of the observer, generated by a negative gradient of
pressure or of mass-energy density. The second one is quadrupolar and due to
the shear. Both additional terms are anisotropic for off-centre observers,
because of to their dependence on a telescopic angle of observation. This
generalized linear Hubble law could explain, in a cosmological setting, the
observed large scale flow of matter, without to have recourse to peculiar
velocity-type newtonian models. It is pointed out also, that the matter dipole
direction should coincide with the CBR dipole one.Comment: 9 pages, LaTeX, to be published in Class. Quantum Gra
Observation of Three-dimensional Long-range Order in Smaller Ion Coulomb Crystals in an rf Trap
Three-dimensional long-range ordered structures in smaller and
near-spherically symmetric Coulomb crystals of ^{40}Ca^+ ions confined in a
linear rf Paul trap have been observed when the number of ions exceeds ~1000
ions. This result is unexpected from ground state molecular dynamics (MD)
simulations, but found to be in agreement with MD simulations of metastable ion
configurations. Previously, three-dimensional long-range ordered structures
have only been reported in Penning traps in systems of ~50,000 ions or more.Comment: 5 pages; 4 figures; to appear in Phys. Rev. Lett.; changed content
From finite nuclei to the nuclear liquid drop: leptodermous expansion based on the self-consistent mean-field theory
The parameters of the nuclear liquid drop model, such as the volume, surface,
symmetry, and curvature constants, as well as bulk radii, are extracted from
the non-relativistic and relativistic energy density functionals used in
microscopic calculations for finite nuclei. The microscopic liquid drop energy,
obtained self-consistently for a large sample of finite, spherical nuclei, has
been expanded in terms of powers of A^{-1/3} (or inverse nuclear radius) and
the isospin excess (or neutron-to-proton asymmetry). In order to perform a
reliable extrapolation in the inverse radius, the calculations have been
carried out for nuclei with huge numbers of nucleons, of the order of 10^6. The
Coulomb interaction has been ignored to be able to approach nuclei of arbitrary
sizes and to avoid radial instabilities characteristic of systems with very
large atomic numbers. The main contribution to the fluctuating part of the
binding energy has been removed using the Green's function method to calculate
the shell correction. The limitations of applying the leptodermous expansion to
actual nuclei are discussed. While the leading terms in the macroscopic energy
expansion can be extracted very precisely, the higher-order, isospin-dependent
terms are prone to large uncertainties due to finite-size effects.Comment: 13 pages revtex4, 7 eps figures, submitted to Phys. Rev.
Effective moment of inertia for several fission reaction systems induced by nucleons, light particles and heavy ions
Compound nucleus effective moment of inertia has been calculated for several
fission reaction systems induced by nucleons, light particles, and heavy ions.
Determination of this quantity for these systems is based upon the comparison
between the experimental data of the fission fragment angular distributions as
well as the prediction of the standard saddle-point statistical model (SSPSM).
For the systems, the two cases, namely with and without neutron emission
corrections were considered. In these calculations, it is assumed that all the
neutrons are emitted before reaching the saddle point.It should be noted that
the above method for determining of the effective moment of inertia had not
been reported until now and this method is used for the first time to determine
compound nucleus effective moment of inertia. Hence, our calculations are of
particular importance in obtaining this quantity, and have a significant rule
in the field of fission physics. Afterwards, our theoretical results have been
compared with the data obtained from the rotational liquid drop model as well
as the Sierk model, and satisfactory agreements were found. Finally, we have
considered the effective moment of inertia of compound nuclei for the systems
that formed similar compound nuclei at similar excitation energies.Comment: 9 pages, 2 Figures, 2 Table
Microscopic Description of Nuclear Fission Dynamics
We discuss possible avenues to study fission dynamics starting from a
time-dependent mean-field approach. Previous attempts to study fission dynamics
using the time-dependent Hartree-Fock (TDHF) theory are analyzed. We argue that
different initial conditions may be needed to describe fission dynamics
depending on the specifics of the fission phenomenon and propose various
approaches towards this goal. In particular, we provide preliminary
calculations for studying fission following a heavy-ion reaction using TDHF
with a density contraint. Regarding prompt muon-induced fission, we also
suggest a new approach for combining the time-evolution of the muonic wave
function with a microscopic treatment of fission dynamics via TDHF
Influence of uniform electron clouds on the coupling impedance
The contribution to the longitudinal coupling impedance from an electron cloud in the form of a uniformly distributed non-neutral plasma of electrons is investigated analytically. The beam-pipe is assumed to be of circular cross section with a thick resistive wall. The beam charge distribution is uniform in the transverse direction. The electron contribution to the charge and current densities are obtained from the collective electron response to the beam passage through the pipe. We obtain the radial differential equation governing the field variation in the presence of the electron background and a general closed formula for the longitudinal coupling impedance is derived. The depletion of the coupling impedance with the density of the electron cloud is discussed for the examples of GSI SIS-18 and SIS- 100, CERN SPS and PS, and the KEKB LER, and conditions for the minimum excitation frequency as a function of the electron density are derived. Furthermore, the case of over-dense plasmas is also studied
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