15,715 research outputs found
Channeling of electrons and positrons in straight and periodically bent diamond(110) crystals
In this paper we present the results of a systematic numerical analysis of
the channeling properties of electrons and positrons in oriented straight and
periodically bent diamond(110) crystals. We analyse dependence of the intensity
of the radiation emitted on the projectile energy as well as on the bending
amplitude. The analysis presented is based on the grounds of accurate numerical
simulations of the channeling process. The simulation parameters, such as the
crystal orientation, thickness and bending parameters of the crystals as well
as the energy of the projectiles, were chosen to match those used in past and
ongoing experiments. The peculiarities which appear in the radiation spectra
are attributed to the interplay of various radiation mechanisms. The analysis
performed can be used to predict and explain future experimental results.Comment: 14 pages, 8 figures, 1 tabl
Self-gravitating spheres of anisotropic fluid in geodesic flow
The fluid models mentioned in the title are classified. All characteristics
of the fluid are expressed through a master potential, satisfying an ordinary
second order differential equation. Different constraints are imposed on this
core of relations, finding new solutions and deriving the classical results for
perfect fluids and dust as particular cases. Many uncharged and charged
anisotropic solutions, all conformally flat and some uniform density solutions
are found. A number of solutions with linear equation among the two pressures
are derived, including the case of vanishing tangential pressure.Comment: 21 page
Analytical representation of elastic scattering cross sections of low energy electrons by atmospheric gases
Analytical representations of the elastic scattering cross sections of electrons with energies of 0.01-1 keV in atmospheric gases of N2, O2, O are given. These representations are suitable for the Monte Carlo method
Electrostatic Point Charge Fitting as an Inverse Problem: Revealing the Underlying Ill-Conditioning
Atom-centered point charge model of the molecular electrostatics---a major
workhorse of the atomistic biomolecular simulations---is usually parameterized
by least-squares (LS) fitting of the point charge values to a reference
electrostatic potential, a procedure that suffers from numerical instabilities
due to the ill-conditioned nature of the LS problem. Here, to reveal the
origins of this ill-conditioning, we start with a general treatment of the
point charge fitting problem as an inverse problem, and construct an
analytically soluble model with the point charges spherically arranged
according to Lebedev quadrature naturally suited for the inverse electrostatic
problem. This analytical model is contrasted to the atom-centered point-charge
model that can be viewed as an irregular quadrature poorly suited for the
problem. This analysis shows that the numerical problems of the point charge
fitting are due to the decay of the curvatures corresponding to the
eigenvectors of LS sum Hessian matrix. In part, this ill-conditioning is
intrinsic to the problem and related to decreasing electrostatic contribution
of the higher multipole moments, that are, in the case of Lebedev grid model,
directly associated with the Hessian eigenvectors. For the atom-centered model,
this association breaks down beyond the first few eigenvectors related to the
high-curvature monopole and dipole terms; this leads to even wider spread-out
of the Hessian curvature values. Using these insights, it is possible to
alleviate the ill-conditioning of the LS point-charge fitting without
introducing external restraints and/or constraints. Also, as the analytical
Lebedev grid PC model proposed here can reproduce multipole moments up to a
given rank, it may provide a promising alternative to including explicit
multipole terms in a force field
Structural heterogeneity of ultrafine-grained fcc metals processed through equal-channel angular pressing on meso- and macroscale levels
Y-Scaling Analysis of the Deuteron Within the Light-Front Dynamics Method
The concept of relativistic scaling is applied to describe the most recent
data from inclusive electron-deuteron scattering at large momentum transfer. We
calculate the asymptotic scaling function f(y) of the deuteron using its
relationship with the nucleon momentum distribution. The latter is obtained in
the framework of the relativistic light-front dynamics (LFD) method, in which
the deuteron is described by six invariant functions f_{i} (i=1,...,6) instead
of two (S and D waves) in the nonrelativistic case. Comparison of the LFD
asymptotic scaling function with other calculations using and waves
corresponding to various nucleon-nucleon potentials, as well as with the
Bethe-Salpeter result is made. It is shown that for |y|> 400 MeV/c the
differences between the LFD and the nonrelativistic scaling functions become
larger.Comment: 7 pages, 5 figures, Talk at 21-st International Workshop on Nuclear
Theory, Rila Mountains, Bulgaria, June 10-15, 200
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