8,654 research outputs found
Elevated temperature fatigue of TZC MOLYBDENUM alloy under high frequency and high vacuum conditions
Elevated temperature fatigue of TZC molybdenum alloy determined in high frequency and high vacuum test
Many-Body Corrections to Charged-Current Neutrino Absorption Rates in Nuclear Matter
Including nucleon--nucleon correlations due to both Fermi statistics and
nuclear forces, we have developed a general formalism for calculating the
charged--current neutrino--nucleon absorption rates in nuclear matter. We find
that at one half nuclear density many--body effects alone suppress the rates by
a factor of two and that the suppression factors increase to 5 at
g cm. The associated increase in the neutrino--matter
mean--free--paths parallels that found for neutral--current interactions and
opens up interesting possibilities in the context of the delayed supernova
mechanism and protoneutron star cooling.Comment: 11 pages, APS REVTeX format, 1 PostScript figure, uuencoded
compressed, and tarred, submitted to Physical Review Letter
The impact of mutation and gene conversion on the local diversification of antigen genes in African trypanosomes
Patterns of genetic diversity in parasite antigen gene families hold important information about their potential to generate antigenic variation within and between hosts. The evolution of such gene families is typically driven by gene duplication, followed by point mutation and gene conversion. There is great interest in estimating the rates of these processes from molecular sequences for understanding the evolution of the pathogen and its significance for infection processes. In this study, a series of models are constructed to investigate hypotheses about the nucleotide diversity patterns between closely related gene sequences from the antigen gene archive of the African trypanosome, the protozoan parasite causative of human sleeping sickness in Equatorial Africa. We use a hidden Markov model approach to identify two scales of diversification: clustering of sequence mismatches, a putative indicator of gene conversion events with other lower-identity donor genes in the archive, and at a sparser scale, isolated mismatches, likely arising from independent point mutations. In addition to quantifying the respective probabilities of occurrence of these two processes, our approach yields estimates for the gene conversion tract length distribution and the average diversity contributed locally by conversion events. Model fitting is conducted using a Bayesian framework. We find that diversifying gene conversion events with lower-identity partners occur at least five times less frequently than point mutations on variant surface glycoprotein (VSG) pairs, and the average imported conversion tract is between 14 and 25 nucleotides long. However, because of the high diversity introduced by gene conversion, the two processes have almost equal impact on the per-nucleotide rate of sequence diversification between VSG subfamily members. We are able to disentangle the most likely locations of point mutations and conversions on each aligned gene pair
Numerical Toy-Model Calculation of the Nucleon Spin Autocorrelation Function in a Supernova Core
We develop a simple model for the evolution of a nucleon spin in a hot and
dense nuclear medium. A given nucleon is limited to one-dimensional motion in a
distribution of external, spin-dependent scattering potentials. We calculate
the nucleon spin autocorrelation function numerically for a variety of
potential densities and distributions which are meant to bracket realistic
conditions in a supernova core. For all plausible configurations the width of
the spin-density structure function is found to be less than the temperature.
This is in contrast with a naive perturbative calculation based on the one-pion
exchange potential which overestimates the width and thus suggests a large
suppression of the neutrino opacities by nucleon spin fluctuations. Our results
suggest that it may be justified to neglect the collisional broadening of the
spin-density structure function for the purpose of estimating the neutrino
opacities in the deep inner core of a supernova. On the other hand, we find no
indication that processes such as axion or neutrino pair emission, which depend
on nucleon spin fluctuations, are substantially suppressed beyond the
multiple-scattering effect already discussed in the literature. Aside from
these practical conclusions, our model reveals a number of interesting and
unexpected insights. For example, the spin-relaxation rate saturates with
increasing potential strength only if bound states are not allowed to form by
including a repulsive core. There is no saturation with increasing density of
scattering potentials until localized eigenstates of energy begin to form.Comment: 14 latex pages in two-column format, 15 postscript figures included,
uses revtex.sty and epsf.sty. Submitted to Physical Review
Composition of Haar Paraproducts: The Random Case
When is the composition of paraproducts bounded? This is an important, and
difficult question, related to to a question of Sarason on composition of
Hankel matrices, and the two-weight problem for the Hilbert transform. We
consider randomized variants of this question, finding non-classical
characterizations, for dyadic paraproducts.Comment: 13 pages. Submitted. v2: \showkeys commented out, with other minor
change
The top 500 mathematics pins: Analysis of elementary mathematics activities on Pinterest
A 2017 study found that 87% of elementary teachers reported consulting Pinterest when planning mathematical lessons (Hertel & Wessman-Enzinger, 2017). When searching for resources on Pinterest, preservice teachers identified looking at the number of pins to determine their quality (Sawyer & Meyers, 2018). This leaves teacher educators wondering, what is the quality of materials that preservice and inservice teachers are finding on Pinterest? We conducted a document analysis on the top 500 elementary mathematics pins found on Pinterest to determine what kinds of elementary mathematics materials are available, what mathematics topics are represented, the level of cognitive demand of the elementary mathematics activities, and how the image found on the activities relates to the level of cognitive demand. We found that less than two percent of activities are the highest level of cognitive demand and decorative images are correlated with lower level elementary mathematics activities. With this information, teacher educators could help prepare teachers to decide which resources they should use and what they should look for to increase the level of cognitive demand of elementary mathematics activities they implement in their classroom
A Framework to Manage the Complex Organisation of Collaborating: Its Application to Autonomous Systems
In this paper we present an analysis of the complexities of large group
collaboration and its application to develop detailed requirements for
collaboration schema for Autonomous Systems (AS). These requirements flow from
our development of a framework for collaboration that provides a basis for
designing, supporting and managing complex collaborative systems that can be
applied and tested in various real world settings. We present the concepts of
"collaborative flow" and "working as one" as descriptive expressions of what
good collaborative teamwork can be in such scenarios. The paper considers the
application of the framework within different scenarios and discuses the
utility of the framework in modelling and supporting collaboration in complex
organisational structures
Photon Propagation in Dense Media
Using thermal field theory, we derive simple analytic expressions for the
spectral density of photons in degenerate QED plasmas, without assuming the
usual non or ultra-relativistic limit. We recover the standard results in both
cases. Although very similar in ultra-relativistic plasmas, transverse and
longitudinal excitations behave very differently as the electron Fermi momentum
decreases.Comment: 12pp (3 PS figures available upon request), ENSLAPP-A-412/9
Phases of QCD at High Baryon Density
We review recent work on the phase structure of QCD at very high baryon
density. We introduce the phenomenon of color superconductivity and discuss how
the quark masses and chemical potentials determine the structure of the
superfluid quark phase. We comment on the possibility of kaon condensation at
very high baryon density and study the competition between superfluid, density
wave, and chiral crystal phases at intermediate density.Comment: 15 pages. To appear in the proceedings of the ECT Workshop on Neutron
Star Interiors, Trento, Italy, June 200
Response of selected plant and insect species to simulated solid rocket exhaust mixtures and to exhaust components from solid rocket fuels
The effects of solid rocket fuel (SRF) exhaust on selected plant and and insect species in the Merritt Island, Florida area was investigated in order to determine if the exhaust clouds generated by shuttle launches would adversely affect the native, plants of the Merritt Island Wildlife Refuge, the citrus production, or the beekeeping industry of the island. Conditions were simulated in greenhouse exposure chambers and field chambers constructed to model the ideal continuous stirred tank reactor. A plant exposure system was developed for dispensing and monitoring the two major chemicals in SRF exhaust, HCl and Al203, and for dispensing and monitoring SRF exhaust (controlled fuel burns). Plants native to Merritt Island, Florida were grown and used as test species. Dose-response relationships were determined for short term exposure of selected plant species to HCl, Al203, and mixtures of the two to SRF exhaust
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