7,550 research outputs found
Measurement of direct neutron capture by neutron-rich sulfur isotopes
Thermal neutron capture cross sections for S(n,)S and
S(n,)S have been measured and spectroscopic factors of
the final states have been extracted. The calculated direct-capture cross
sections reproduce the experimental data.Comment: 4 pages (uses espcrc1.sty), 1 postscript figure (uses psfig),
accepted for publication in Nucl. Phys. A (Suppl.), uuencoded tex-files and
postscript-files available at ftp://is1.kph.tuwien.ac.at/pub/ohu/Stherm.u
Direct neutron capture of 48Ca at kT = 52 keV
The neutron capture cross section of 48Ca was measured relative to the known
gold cross section at kT = 52 keV using the fast cyclic activation technique.
The experiment was performed at the Van-de-Graaff accelerator, Universitaet
Tuebingen. The new experimental result is in good agreement with a calculation
using the direct capture model. The 1/v behaviour of the capture cross section
at thermonuclear energies is confirmed, and the adopted reaction rate which is
based on several previous experimental investigations remains unchanged.Comment: 9 pages (uses Revtex), 2 postscript figures, accepted for publication
as Brief Report in Phys. Rev.
Perfection of materials technology for producing improved Gunn-effect devices Interim scientific report
Chemical vapor deposition of epitaxial gallium arsenid
A Possible Hidden Population of Spherical Planetary Nebulae
We argue that relative to non-spherical planetary nebulae (PNs), spherical
PNs are about an order of magnitude less likely to be detected, at distances of
several kiloparsecs. Noting the structure similarity of halos around
non-spherical PNs to that of observed spherical PNs, we assume that most
unobserved spherical PNs are also similar in structure to the spherical halos
around non-spherical PNs. The fraction of non-spherical PNs with detected
spherical halos around them, taken from a recent study, leads us to the claim
of a large (relative to that of non-spherical PNs) hidden population of
spherical PNs in the visible band. Building a toy model for the luminosity
evolution of PNs, we show that the claimed detection fraction of spherical PNs
based on halos around non-spherical PNs, is compatible with observational
sensitivities. We use this result to update earlier studies on the different PN
shaping routes in the binary model. We estimate that ~30% of all PNs are
spherical, namely, their progenitors did not interact with any binary
companion. This fraction is to be compared with the ~3% fraction of observed
spherical PNs among all observed PNs. From all PNs, ~15% owe their moderate
elliptical shape to the interaction of their progenitors with planets, while
\~55% of all PNs owe their elliptical or bipolar shapes to the interaction of
their progenitors with stellar companions.Comment: AJ, in pres
Measurement of neutron capture on Ca at thermal and thermonuclear energies
At the Karlsruhe pulsed 3.75\,MV Van de Graaff accelerator the thermonuclear
Ca(n,)Ca(8.72\,min) cross section was measured by the
fast cyclic activation technique via the 3084.5\,keV -ray line of the
Ca-decay. Samples of CaCO enriched in Ca by 77.87\,\% were
irradiated between two gold foils which served as capture standards. The
capture cross-section was measured at the neutron energies 25, 151, 176, and
218\,keV, respectively. Additionally, the thermal capture cross-section was
measured at the reactor BR1 in Mol, Belgium, via the prompt and decay
-ray lines using the same target material. The
Ca(n,)Ca cross-section in the thermonuclear and thermal
energy range has been calculated using the direct-capture model combined with
folding potentials. The potential strengths are adjusted to the scattering
length and the binding energies of the final states in Ca. The small
coherent elastic cross section of Ca+n is explained through the nuclear
Ramsauer effect. Spectroscopic factors of Ca have been extracted from
the thermal capture cross-section with better accuracy than from a recent (d,p)
experiment. Within the uncertainties both results are in agreement. The
non-resonant thermal and thermonuclear experimental data for this reaction can
be reproduced using the direct-capture model. A possible interference with a
resonant contribution is discussed. The neutron spectroscopic factors of
Ca determined from shell-model calculations are compared with the values
extracted from the experimental cross sections for Ca(d,p)Ca and
Ca(n,)Ca.Comment: 15 pages (uses Revtex), 7 postscript figures (uses psfig), accepted
for publication in PRC, uuencoded tex-files and postscript-files also
available at ftp://is1.kph.tuwien.ac.at/pub/ohu/Ca.u
Photosynthesis-controlled calcification in a hypersaline microbial mat
We investigated the hypothesis that sulfate reduction rather than oxygenic photosynthesis promotes calcification in a hypersaline microbial mat by increasing the ion concentration product: ICP 5 [Ca2+] X [CO32-]. Pore‐water 3 calcium concentration profiles directly measured with microsensors show that calcium concentration in the photic zone decreased in illuminated mats and increased slightly in dark mats. High pH values in the photic zone of illuminated mats resulted in higher carbonate concentrations (2.25 mmol L-1) than in dark mats (0.75 mmol L-1), although the dissolved inorganic carbon (DIC) pore‐water concentration in the former was much lower (5.9 mmol L-1) than in the latter (9.9 mmol L-1). The pH‐induced rise in carbonate concentration in the light was the main factor influencing the ICP, while changes in Ca-1 concentration played a subsidiary role. Sulfate reduction did not result in a net pH increase in these mats, as rates in the photic zone were comparable between illuminated and dark mats (4 and 5 nmol cm-2 h-1, respectively), and pH increased in illuminated mats but not in dark mats. Calcium carbonate precipitation in the photic zone of these hypersaline mats is primarily controlled by photosynthesisinduced pH and carbonate concentration increases. However, heterotrophic bacteria, including sulfate reducers, play an important complementary role in calcification because they maintain high concentrations of DIC in the mat pore water
Evidence for water-mediated mechanisms in coral-algal interactions
Although many coral reefs have shifted from coral-to-algal dominance, the consequence of such a transition for coral algal interactions and their underlying mechanisms remain poorly understood. At the microscale, it is unclear how diffusive boundary layers (DBLs) and surface oxygen concentrations at the coral algal interface vary with algal competitors and competitiveness. Using field observations and microsensor measurements in a flow chamber, we show that coral (massive Porites) interfaces with thick turf algae, macro algae, and cyanobacteria, which are successful competitors against coral in the field, are characterized by a thick DBL and hypoxia at night. In contrast, coral interfaces with crustose coralline algae, conspecifics, and thin turf algae, which are poorer competitors, have a thin DBL and low hypoxia at night. Furthermore, DBL thickness and hypoxia at the interface with turf decreased with increasing flow speed, but not when thick turf was upstream. Our results support the importance of water-mediated transport mechanisms in coral algal interactions. Shifts towards algal dominance, particularly dense assemblages, may lead to thicker DBLs, higher hypoxia, and higher concentrations of harmful metabolites and pathogens along coral borders, which in turn may facilitate algal overgrowth of live corals. These effects may be mediated by flow speed and orientation
Dependence of direct neutron capture on nuclear-structure models
The prediction of cross sections for nuclei far off stability is crucial in
the field of nuclear astrophysics. We calculate direct neutron capture on the
even-even isotopes Sn and Pb with energy levels,
masses, and nuclear density distributions taken from different
nuclear-structure models. The utilized structure models are a
Hartree-Fock-Bogoliubov model, a relativistic mean field theory, and a
macroscopic-microscopic model based on the finite-range droplet model and a
folded-Yukawa single-particle potential. Due to the differences in the
resulting neutron separation and level energies, the investigated models yield
capture cross sections sometimes differing by orders of magnitude. This may
also lead to differences in the predicted astrophysical r-process paths.
Astrophysical implications are discussed.Comment: 25 pages including 12 figures, RevTeX, to appear in Phys. Rev.
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