291 research outputs found
Electron capture on iron group nuclei
We present Gamow-Teller strength distributions from shell model Monte Carlo
studies of fp-shell nuclei that may play an important role in the pre-collapse
evolution of supernovae. We then use these strength distributions to calculate
the electron-capture cross sections and rates in the zero-momentum transfer
limit. We also discuss the thermal behavior of the cross sections. We find
large differences in these cross sections and rates when compared to the naive
single-particle estimates. These differences need to be taken into account for
improved modeling of the early stages of type II supernova evolution
Gamow-Teller strength distributions in fp-shell nuclei
We use the shell model Monte Carlo method to calculate complete 0f1p-shell
response functions for Gamow-Teller (GT) operators and obtain the corresponding
strength distributions using a Maximum Entropy technique. The approach is
validated against direct diagonalization for 48Ti. Calculated GT strength
distributions agree well with data from (n,p) and (p,n) reactions for nuclei
with A=48-64. We also calculate the temperature evolution of the GT+
distributions for representative nuclei and find that the GT+ distributions
broaden and the centroids shift to lower energies with increasing temperature
Presupernova collapse models with improved weak-interaction rates
Improved values for stellar weak interaction rates have been recently
calculated based upon a large shell model diagonalization. Using these new
rates (for both beta decay and electron capture), we have examined the
presupernova evolution of massive stars in the range 15-40 Msun. Comparing our
new models with a standard set of presupernova models by Woosley and Weaver, we
find significantly larger values for the electron-to-baryon ratio Ye at the
onset of collapse and iron core masses reduced by approximately 0.1 Msun. The
inclusion of beta-decay accounts for roughly half of the revisions, while the
other half is a consequence of the improved nuclear physics. These changes will
have important consequences for nucleosynthesis and the supernova explosion
mechanism.Comment: 4 pages, 2 figure
Analytical in vitro approach for studying cyto- and genotoxic effects of particulate airborne material
In the field of inhalation toxicology, progress in the development of in vitro methods and efficient exposure strategies now offers the implementation of cellular-based systems. These can be used to analyze the hazardous potency of airborne substances like gases, particles, and complex mixtures (combustion products). In addition, the regulatory authorities require the integration of such approaches to reduce or replace animal experiments. Although the animal experiment currently still has to provide the last proof of the toxicological potency and classification of a certain compound, in vitro testing is gaining more and more importance in toxicological considerations. This paper gives a brief characterization of the CULTEX® Radial Flow System exposure device, which allows the exposure of cultivated cells as well as bacteria under reproducible and stable conditions for studying cellular and genotoxic effects after the exposure at the air–liquid or air–agar interface, respectively. A commercial bronchial epithelial cell line (16HBE14o-) as well as Salmonella typhimurium tester strains were exposed to smoke of different research and commercial available cigarettes. A dose-dependent reduction of cell viability was found in the case of 16HBE14o- cells; S. typhimurium responded with a dose-dependent induction of revertants. The promising results recommend the integration of cellular studies in the field of inhalation toxicology and their regulatory acceptance by advancing appropriate validation studies
Spectral Distribution Studies of fp shell nuclei with modified Kuo--Brown Interaction
The structure of nuclei in the lower half of fp shell is investigated by the
spectral distribution method using the modified Kuo-Brown interaction. This
interaction recently showed success in reproducing observed properties through
detailed shell model studies. Spectral distribution studies avoid explicit
diagonalization and hold promise for applications to astrophysics.Comment: Latex, 19 pages, 1 figure, to be published in Physical Review
Half-lives and pre-supernova weak interaction rates for nuclei away from the stability line
A detailed model for the calculation of beta decay rates of the shell
nuclei for situations prevailing in pre-supernova and collapse phases of
evolution of the core of massive stars leading to supernova explosion has been
extended for electron-capture rates. It can also be used to determine the
half-lives of neutron-rich nuclei in the shell. The model uses an
averaged Gamow-Teller (GT) strength function. But it can also use the
experimental log ft values and GT strength function from reaction
studies wherever available. The calculated rate includes contributions from
each of the low-lying excited states of the mother including some specific
resonant states ("back resonance") having large GT matrix elements.Comment: 11 pages; Latex; no figs; version to appear in J. Phys.
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HADES, A Radiographic Simulation Code
We describe features of the HADES radiographic simulation code. We begin with a discussion of why it is useful to simulate transmission radiography. The capabilities of HADES are described, followed by an application of HADES to a dynamic experiment recently performed at the Los Alamos Neutron Science Center. We describe quantitative comparisons between experimental data and HADES simulations using a copper step wedge. We conclude with a short discussion of future work planned for HADES
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