24,249 research outputs found

### Effect of nuclear structure on Type Ia supernova nucleosynthesis

The relationship among nuclear structure, the weak processes in nuclei, and
astrophysics becomes quite apparent in supernova explosion and nucleosynthesis
studies. In this brief article, I report on progress made in the last few years
on calculating electron capture and beta-decay rates in iron-group nuclei. I
also report on applications of these rates to Type-Ia nucleosynthesis studies.Comment: To appear in the proceedings of Nuclei In the Cosmos 200

### Renormalization of Drift and Diffusivity in Random Gradient Flows

We investigate the relationship between the effective diffusivity and
effective drift of a particle moving in a random medium. The velocity of the
particle combines a white noise diffusion process with a local drift term that
depends linearly on the gradient of a gaussian random field with homogeneous
statistics. The theoretical analysis is confirmed by numerical simulation. For
the purely isotropic case the simulation, which measures the effective drift
directly in a constant gradient background field, confirms the result
previously obtained theoretically, that the effective diffusivity and effective
drift are renormalized by the same factor from their local values. For this
isotropic case we provide an intuitive explanation, based on a {\it spatial}
average of local drift, for the renormalization of the effective drift
parameter relative to its local value. We also investigate situations in which
the isotropy is broken by the tensorial relationship of the local drift to the
gradient of the random field. We find that the numerical simulation confirms a
relatively simple renormalization group calculation for the effective
diffusivity and drift tensors.Comment: Latex 16 pages, 5 figures ep

### Monte Carlo methods and applications for the nuclear shell model

The shell-model Monte Carlo (SMMC) technique transforms the traditional
nuclear shell-model problem into a path-integral over auxiliary fields. We
describe below the method and its applications to four physics issues:
calculations of sdpf- shell nuclei, a discussion of electron-capture rates in
pf-shell nuclei, exploration of pairing correlations in unstable nuclei, and
level densities in rare earth systems.Comment: Proceedings of the Nuclear Structure '98 conference, Gatlinburg, TN,
10-15 August 199

### Calculation of exciton densities in SMMC

We develop a shell-model Monte Carlo (SMMC) method to calculate densities of
states with varying exciton (particle-hole) number. We then apply this method
to the doubly closed-shell nucleus 40Ca in a full 0s-1d-0f-1p shell-model space
and compare our results to those found using approximate analytic expressions
for the partial densities. We find that the effective one-body level density is
reduced by approximately 22% when a residual two-body interaction is included
in the shell model calculation.Comment: 10 pages, 4 figure

### Spin-Dependent Neutralino-Nucleus Scattering for $A \sim 127$ Nuclei

We perform nuclear shell model calculations of the neutralino-nucleus cross
section for several nuclei in the A = 127 region. Each of the four nuclei
considered is a primary target in a direct dark matter detection experiment.
The calculations are valid for all relevant values of the momentum transfer.
Our calculations are performed in the $3s 2d 1g_{7/2} 1h_{11/2}$ model space
using extremely large bases, allowing us to include all relevant correlations.
We also study the dependence of the nuclear response upon the assumed nuclear
Hamiltonian and find it to be small. We find good agreement with the observed
magnetic moment as well as other obervables for the four nuclei considered:
^{127}I, ^{129,131}Xe, and ^{125}Te.Comment: 23 pages + 7 postscript figures. LaTeX uses RevTe

### Dynamical transition for a particle in a squared Gaussian potential

We study the problem of a Brownian particle diffusing in finite dimensions in
a potential given by $\psi= \phi^2/2$ where $\phi$ is Gaussian random field.
Exact results for the diffusion constant in the high temperature phase are
given in one and two dimensions and it is shown to vanish in a power-law
fashion at the dynamical transition temperature. Our results are confronted
with numerical simulations where the Gaussian field is constructed, in a
standard way, as a sum over random Fourier modes. We show that when the number
of Fourier modes is finite the low temperature diffusion constant becomes
non-zero and has an Arrhenius form. Thus we have a simple model with a fully
understood finite size scaling theory for the dynamical transition. In addition
we analyse the nature of the anomalous diffusion in the low temperature regime
and show that the anomalous exponent agrees with that predicted by a trap
model.Comment: 18 pages, 4 figures .eps, JPA styl

### Some observations on the renormalization of membrane rigidity by long-range interactions

We consider the renormalization of the bending and Gaussian rigidity of model
membranes induced by long-range interactions between the components making up
the membrane. In particular we analyze the effect of a finite membrane
thickness on the renormalization of the bending and Gaussian rigidity by
long-range interactions. Particular attention is paid to the case where the
interactions are of a van der Waals type.Comment: 11 pages RexTex, no figure

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