587 research outputs found
Microscopic multicluster description of neutron-halo nuclei with a stochastic variational method
To test a multicluster approach for halo nuclei, we give a unified
description for the ground states of He and He in a model comprising an
cluster and single-neutron clusters. The intercluster wave function is
taken a superposition of terms belonging to different arrangements, each
defined by a set of Jacobi coordinates. Each term is then a superposition of
products of gaussian functions of the individual Jacobi coordinates with
different widths, projected to angular momenta or 1. To avoid excessively
large dimensions and ``overcompleteness", stochastic methods were tested for
selecting the gaussians spanning the basis. For He, we were able to
calculate ground-state energies that are virtully exact within the subspace
defined by the arrangements and values, and we found that preselected
random sets of bases (with or without simulated annealing) yield excellent
numerical convergence to this ``exact" value with thoroughly truncated bases.
For He good energy convergence was achieved in a state space comprising
three arrangements with all , and there are indications showing that the
contributions of other subspaces are likely to be small. The He and He
energies are reproduced by the same effective force very well, and the matter
radii obtained are similar to those of other sophisticated calculations.Comment: Latex , 8 figures available on request, ATOMKI-4-1993-
Relativistic description of electron scattering on the deuteron
Within a quasipotential framework a relativistic analysis is presented of the
deuteron current. Assuming that the singularities from the nucleon propagators
are important, a so-called equal time approximation of the current is
constructed. This is applied to both elastic and inelastic electron scattering.
As dynamical model the relativistic one boson exchange model is used.
Reasonable agreement is found with a previous relativistic calculation of the
elastic electromagnetic form factors of the deuteron. For the unpolarized
inelastic electron scattering effects of final state interactions and
relativistic corrections to the structure functions are considered in the
impulse approximation. Two specific kinematic situations are studied as
examples.Comment: (19 pages in revtex + 15 figures not included, available upon
request.) report THU-93-10
Vortex Lattice Melting into Disentangled Liquid Followed by the 3D-2D Decoupling Transition in YBa_2Cu_4O_8 Single Crystals
A sharp resistance drop associated with vortex lattice melting was observed
in high quality YBa_2Cu_4O_8 single crystals. The melting line is well
described well by the anisotropic GL theory. Two thermally activated flux flow
regions, which were separated by a crossover line B_cr=1406.5(1-T/T_c)/T
(T_c=79.0 K, B_cr in T), were observed in the vortex liquid phase. Activation
energy for each region was obtained and the corresponding dissipation mechanism
was discussed. Our results suggest that the vortex lattice in YBa_2Cu_4O_8
single crystal melts into disentangled liquid, which then undergoes a 3D-2D
decoupling transition.Comment: 5 pages, 4 eps figures, RevTex (Latex2.09
Pion photoproduction on the nucleon in the quark model
We present a detailed quark-model study of pion photoproduction within the
effective Lagrangian approach. Cross sections and single-polarization
observables are investigated for the four charge channels, , , , and .
Leaving the coupling strength to be a free parameter, we obtain a
reasonably consistent description of these four channels from threshold to the
first resonance region. Within this effective Lagrangian approach, strongly
constrainted by the quark model, we consider the issue of double-counting which
may occur if additional {\it t}-channel contributions are included.Comment: Revtex, 35 pages, 16 eps figures; version to appear on PR
Sub-barrier fusion cross section measurements with STELLA
The experimental setup STELLA (STELlar LAboratory) is designed for the measurement of deep sub-barrier light heavy ion fusion cross sections. For background suppression the γ-particle coincidence technique is used. In this project, LaBr3 detectors from the UK FATIMA (FAst TIMing Array) collaboration are combined with annular silicon strip detectors customized at IPHC-CNRS, Strasbourg, and the setup is located at Andromède, IPN, Orsay. The commissioning of the experimental approach as well as a sub-barrier 12C +12C → 24Mg∗ cross section measurement campaign are carried out
Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results
The chromosphere is a thin layer of the solar atmosphere that bridges the
relatively cool photosphere and the intensely heated transition region and
corona. Compressible and incompressible waves propagating through the
chromosphere can supply significant amounts of energy to the interface region
and corona. In recent years an abundance of high-resolution observations from
state-of-the-art facilities have provided new and exciting ways of
disentangling the characteristics of oscillatory phenomena propagating through
the dynamic chromosphere. Coupled with rapid advancements in
magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly
investigate the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting in the
solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review
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