14,270 research outputs found
Extracting (n,g) direct capture cross sections from Coulomb dissociation: application to C(n,)C
A methodology for extracting neutron direct capture rates from Coulomb
dissociation data is developed and applied to the Coulomb dissociation of 15C
on 208Pb at 68 MeV/nucleon. Full Continuum Discretized Coupled Channel
calculations are performed and an asymptotic normalization coefficient is
determined from a fit to the breakup data. Direct neutron capture calculations
using the extracted asymptotic normalization coefficient provide
cross sections consistent with direct measurements. Our results show that the
Coulomb Dissociation data can be reliably used for extracting the cross section
for 14C(n,g)15C if the appropriate reaction theory is used. The resulting error
bars are of comparable magnitude to those from the direct measurement. This
procedure can be used more generally to extract capture cross sections from
breakup reactions whenever the desired capture process is fully peripheral.Comment: submitted to Phys. Rev. C (R
Understanding low energy reaction with exotic nuclei
Recent developments on the understanding of low energy reactions are
highlighted. Emphasis is given to the CDCC framework where the breakup channels
of the projectile are included explicitly. Properties of the breakup couplings
are presented. Comments are given with regard to the separation between the
nuclear and the Coulomb contributions to breakup cross sections as well as the
dependence on the optical potentials. A discussion on the sensitivity of the
CDCC basis is discussed, by comparing pure breakup results with transfer to the
continuum calculations. Finally, some remaining controversies show the need to
go beyond the single particle picture for the projectile.Comment: Proceedings from 'Nuclei at the limits', ANL 26-30 July 2004, 6 pages
and 8 figure
Sensitivity of 8B breakup cross section to projectile structure in CDCC calculations
Given the Astrophysical interest of BeB, there have been
several experiments applying the Coulomb dissociation method for extracting the
capture rate. Measurements at Michigan State are dominated by
contributions but have a small component. On the other hand, a lower
energy measurement at Notre Dame has a much stronger contribution. The
expectation was that the two measurements would tie down the and thus
allow for an accurate extraction of the relevant for the capture process.
The aim of this brief report is to show that the factor in breakup
reactions does not translate into a scaling of the contribution in the
corresponding capture reaction. We show that changes to the B single
particle parameters, which are directly related to the component in the
capture reaction, do not effect the corresponding breakup reactions, using the
present reaction theory.Comment: 4 pages, 6 figures, revtex
Remarks on Causality in Relativistic Quantum Field Theory
It is shown that the correlations predicted by relativistic quantum field
theory in locally normal states between projections in local von Neumann
algebras \cA(V_1),\cA(V_2) associated with spacelike separated spacetime
regions have a (Reichenbachian) common cause located in the union of
the backward light cones of and . Further comments on causality and
independence in quantum field theory are made.Comment: 10 pages, Latex, Quantum Structures 2002 Conference Proceedings
submission. Minor revision of the order of definitions on p.
GALAXY DYNAMICS IN CLUSTERS
We use high resolution simulations to study the formation and distribution of
galaxies within a cluster which forms hierarchically. We follow both dark
matter and baryonic gas which is subject to thermal pressure, shocks and
radiative cooling. Galaxy formation is identified with the dissipative collapse
of the gas into cold, compact knots. We examine two extreme representations of
galaxies during subsequent cluster evolution --- one purely gaseous and the
other purely stellar. The results are quite sensitive to this choice.
Gas-galaxies merge efficiently with a dominant central object while
star-galaxies merge less frequently. Thus, simulations in which galaxies remain
gaseous appear to suffer an ``overmerging'' problem, but this problem is much
less severe if the gas is allowed to turn into stars. We compare the kinematics
of the galaxy population in these two representations to that of dark halos and
of the underlying dark matter distribution. Galaxies in the stellar
representation are positively biased (\ie over-represented in the cluster) both
by number and by mass fraction. Both representations predict the galaxies to be
more centrally concentrated than the dark matter, whereas the dark halo
population is more extended. A modest velocity bias also exists in both
representations, with the largest effect, , found for the more massive star-galaxies. Phase diagrams show that the
galaxy population has a substantial net inflow in the gas representation, while
in the stellar case it is roughly in hydrostatic equilibrium. Virial mass
estimators can underestimate the true cluster mass by up to a factor of 5. The
discrepancy is largest if only the most massive galaxies are used, reflecting
significant mass segregation.Comment: 30 pages, self-unpacking (via uufiles) postscript file without
figures. Eighteen figures (and slick color version of figure 3) and entire
paper available at ftp://oahu.physics.lsa.umich.edu/groups/astro/fews Total
size of paper with figures is ~9.0 Mb uncompressed. Submitted to Ap.J
Nonlocal hydrodynamic influence on the dynamic contact angle: Slip models versus experiment
Experiments reported by Blake et al. [Phys. Fluids. 11, 1995 (1999)] suggest that the dynamic contact angle formed between the free surface of a liquid and a moving solid boundary at a fixed contact-line speed depends on the flow field/geometry near the moving contact line. The present paper examines quantitatively whether or not it is possible to attribute this effect to bending of the free surface due to hydrodynamic stresses acting upon it and hence interpret the results in terms of the so-called ``apparent'' contact angle. It is shown that this is not the case. Numerical analysis of the problem demonstrates that, at the spatial
resolution reported in the experiments, the variations of the ``apparent'' contact angle (defined in two different ways) caused by variations in the flow field, at a fixed contact-line speed, are too small to account for the observed effect. The results clearly indicate that the actual (macroscopic) dynamic contact angle, i.e.\ the one used in fluid mechanics as a boundary condition for the equation determining the free surface shape, must be regarded as dependent not only on the contact-line speed but also on the flow field/geometry in the vicinity of the moving contact line
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