7,345 research outputs found
Nucleosynthesis in the Outflow from Gamma Ray Burst Accretion Disks
We examine the nucleosynthesis products that are produced in the outflow from
rapidly accreting disks. We find that the type of element synthesis varies
dramatically with the degree of neutrino trapping in the disk and therefore the
accretion rate of the disk. Disks with relatively high accretion rates such as
10 M_solar/s can produce very neutron rich nuclei that are found in the r
process. Disks with more moderate accretion rates can produce copious amounts
of Nickel as well as the light elements such as Lithium and Boron. Disks with
lower accretion rates such as 0.1 M_solar/s produce large amounts of Nickel as
well as some unusual nuclei such as Ti-49, Sc-45, Zn-64, and Mo-92. This wide
array of potential nucleosynthesis products is due to the varying influence of
electron neutrinos and antineutrinos emitted from the disk on the
neutron-to-proton ratio in the outflow. We use a parameterization for the
outflow and discuss our results in terms of entropy and outflow acceleration.Comment: 12 pages, 12 figures; submitted to Ap
Prospects for Detecting Supernova Neutrino Flavor Oscillations
The neutrinos from a Type II supernova provide perhaps our best opportunity
to probe cosmologically interesting muon and/or tauon neutrino masses. This is
because matter enhanced neutrino oscillations can lead to an anomalously hot
nu_e spectrum, and thus to enhanced charged current cross sections in
terrestrial detectors. Two recently proposed supernova neutrino observatories,
OMNIS and LAND, will detect neutrons spalled from target nuclei by neutral and
charged current neutrino interactions. As this signal is not flavor specific,
it is not immediately clear whether a convincing neutrino oscillation signal
can be extracted from such experiments. To address this issue we examine the
responses of a series of possible light and heavy mass targets, 9Be, 23Na,
35Cl, and 208Pb. We find that strategies for detecting oscillations which use
only neutron count rates are problematic at best, even if cross sections are
determined by ancillary experiments. Plausible uncertainties in supernova
neutrino spectra tend to obscure rate enhancements due to oscillations.
However, in the case of 208Pb, a signal emerges that is largely flavor specific
and extraordinarily sensitive to the nu_e temperature, the emission of two
neutrons. This signal and its flavor specificity are associated with the
strength and location of the first-forbidden responses for neutral and charge
current reactions, aspects of the 208Pb neutrino cross section that have not
been discussed previously. Hadronic spin transfer experiments might be helpful
in confirming some of the nuclear structure physics underlying our conclusions.Comment: 27 pages, RevTeX, 2 figure
Neutrinos, Fission Cycling, and the r-process
It has long been suggested that fission cycling may play an important role in
the r-process. Fission cycling can only occur in a very neutron rich
environment. In traditional calculations of the neutrino driven wind of the
core-collapse supernova, the environment is not sufficiently neutron rich to
produce the r-process elements. However, we show that with a reduction of the
electron neutrino flux coming from the supernova, fission cycling does occur
and furthermore it produces an abundance pattern which is consistent with
observed r-process abundance pattern in halo stars. Such a reduction can be
caused by active-sterile neutrino oscillations or other new physics.Comment: Typos corrected. Presented at NIC-IX, International Symposium on
Nuclear Astrophysics - Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland,
25-30 June, 200
K-shell x-ray spectroscopy of atomic nitrogen
Absolute {\it K}-shell photoionization cross sections for atomic nitrogen
have been obtained from both experiment and state-of-the-art theoretical
techniques. Due to the difficulty of creating a target of neutral atomic
nitrogen, no high-resolution {\it K}-edge spectroscopy measurements have been
reported for this important atom. Interplay between theory and experiment
enabled identification and characterization of the strong
resonance features throughout the threshold region. An experimental value
of 409.64 0.02 eV was determined for the {\it K}-shell binding energy.Comment: 4 pages, 2 graphs, 1 tabl
Kinetic analysis of an efficient DNA-dependent TNA polymerase.
alpha-l-Threofuranosyl nucleoside triphosphates (tNTPs) are tetrafuranose nucleoside derivatives and potential progenitors of present-day beta-d-2'-deoxyribofuranosyl nucleoside triphosphates (dNTPs). Therminator DNA polymerase, a variant of the 9 degrees N DNA polymerase, is an efficient DNA-directed threosyl nucleic acid (TNA) polymerase. Here we report a detailed kinetic comparison of Therminator-catalyzed TNA and DNA syntheses. We examined the rate of single-nucleotide incorporation for all four tNTPs and dNTPs from a DNA primer-template complex and carried out parallel experiments with a chimeric DNA-TNA primer-DNA template containing five TNA residues at the primer 3'-terminus. Remarkably, no drop in the rate of TNA incorporation was observed in comparing the DNA-TNA primer to the all-DNA primer, suggesting that few primer-enzyme contacts are lost with a TNA primer. Moreover, comparison of the catalytic efficiency of TNA synthesis relative to DNA synthesis at the downstream positions reveals a difference of no greater than 5-fold in favor of the natural DNA substrate. This disparity becomes negligible when the TNA synthesis reaction mixture is supplemented with 1.25 mM MnCl(2). These results indicate that Therminator DNA polymerase can recognize both a TNA primer and tNTP substrates and is an effective catalyst of TNA polymerization despite changes in the geometry of the reactants
Monte Carlo Neutrino Oscillations
We demonstrate that the effects of matter upon neutrino propagation may be
recast as the scattering of the initial neutrino wavefunction. Exchanging the
differential, Schrodinger equation for an integral equation for the scattering
matrix S permits a Monte Carlo method for the computation of S that removes
many of the numerical difficulties associated with direct integration
techniques
Neutron capture rates and r-process nucleosynthesis
Simulations of r-process nucleosynthesis require nuclear physics information
for thousands of neutron-rich nuclear species from the line of stability to the
neutron drip line. While arguably the most important pieces of nuclear data for
the r-process are the masses and beta decay rates, individual neutron capture
rates can also be of key importance in setting the final r-process abundance
pattern. Here we consider the influence of neutron capture rates in forming the
A~80 and rare earth peaks.Comment: 10 pages, 5 figures, appears in the Proceedings of the 14th
International Symposium on Capture Gamma-Ray Spectroscopy and Related Topic
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