6,295 research outputs found
Study of an attitude reference system utilizing an electrically suspended gyro final report, 1 aug. 1964 - 31 mar. 1965
Miniature electrically suspended gyroscope for spacecraft attitude reference syste
Test-retest reliability of structural brain networks from diffusion MRI
Structural brain networks constructed from diffusion MRI (dMRI) and tractography have been demonstrated in healthy volunteers and more recently in various disorders affecting brain connectivity. However, few studies have addressed the reproducibility of the resulting networks. We measured the test–retest properties of such networks by varying several factors affecting network construction using ten healthy volunteers who underwent a dMRI protocol at 1.5 T on two separate occasions. Each T1-weighted brain was parcellated into 84 regions-of-interest and network connections were identified using dMRI and two alternative tractography algorithms, two alternative seeding strategies, a white matter waypoint constraint and three alternative network weightings. In each case, four common graph-theoretic measures were obtained. Network properties were assessed both node-wise and per network in terms of the intraclass correlation coefficient (ICC) and by comparing within- and between-subject differences. Our findings suggest that test–retest performance was improved when: 1) seeding from white matter, rather than grey; and 2) using probabilistic tractography with a two-fibre model and sufficient streamlines, rather than deterministic tensor tractography. In terms of network weighting, a measure of streamline density produced better test–retest performance than tract-averaged diffusion anisotropy, although it remains unclear which is a more accurate representation of the underlying connectivity. For the best performing configuration, the global within-subject differences were between 3.2% and 11.9% with ICCs between 0.62 and 0.76. The mean nodal within-subject differences were between 5.2% and 24.2% with mean ICCs between 0.46 and 0.62. For 83.3% (70/84) of nodes, the within-subject differences were smaller than between-subject differences. Overall, these findings suggest that whilst current techniques produce networks capable of characterising the genuine between-subject differences in connectivity, future work must be undertaken to improve network reliability
Proteins and polymers
Proteins, chain molecules of amino acids, behave in ways which are similar to
each other yet quite distinct from standard compact polymers. We demonstrate
that the Flory theorem, derived for polymer melts, holds for compact protein
native state structures and is not incompatible with the existence of
structured building blocks such as -helices and -strands. We
present a discussion on how the notion of the thickness of a polymer chain,
besides being useful in describing a chain molecule in the continuum limit,
plays a vital role in interpolating between conventional polymer physics and
the phase of matter associated with protein structures.Comment: 7 pages, 6 figure
Eikonal profile functions and amplitudes for and scattering
The eikonal profile function obtained from the Model of the Stochastic
Vacuum is parametrized in a form suitable for comparison with experiment. The
amplitude and the extended profile function (including imaginary and real
parts) are determined directly from the complete pp and p elastic
scattering data at high energies. Full and accurate representation of the data
is presented, with smooth energy dependence of all parameters. The changes
needed in the original profile function required for description of scattering
beyond the forward direction are described.Comment: Latex, 28 pages and 16 figure
Predicting narrow states in the spectrum of a nucleus beyond the proton drip line
Properties of particle-unstable nuclei lying beyond the proton drip line can
be ascertained by considering those (usually known) properties of its mirror
neutron-rich system. We have used a multi-channel algebraic scattering theory
to map the known properties of the neutron-C system to those of the
proton-O one from which we deduce that the particle-unstable
F will have a spectrum of two low lying broad resonances of positive
parity and, at higher excitation, three narrow negative parity ones. A key
feature is to use coupling to Pauli-hindered states in the target.Comment: 5 pages, 3 figure
The Brieva-Rook Localization of the Microscopic Nucleon-Nucleus Potential
The nonlocality of the microscopic nucleon-nucleus optical potential is
commonly localized by the Brieva-Rook approximation. The validity of the
localization is tested for the proton+Zr scattering at the incident
energies from 65 MeV to 800 MeV. The localization is valid in the wide
incident-energy range.Comment: 20 pages, 8 figure
Inclusion of non-spherical components of the Pauli blocking operator in (p,p') reactions
We present the first calculations of proton elastic and inelastic scattering
in which the Pauli blocking operator contains the leading non-spherical
components as well as the usual spherical (angle-averaged) part. We develop a
formalism for including the contributions to the effective nucleon-nucleon
interaction from the resulting new G-matrix elements that extend the usual
two-nucleon spin structure and may not conserve angular momentum. We explore
the consequences of parity conservation, time reversal invariance, and
nucleon-nucleon antisymmetrization for the new effective interaction. Changes
to the calculated cross section and spin observables are small in the energy
range from 100 to 200 MeV.Comment: 24 pages, 4 figures, to be published in Physical Review
Global optical potential for nucleus-nucleus systems from 50 MeV/u to 400 MeV/u
We present a new global optical potential (GOP) for nucleus-nucleus systems,
including neutron-rich and proton-rich isotopes, in the energy range of MeV/u. The GOP is derived from the microscopic folding model with the
complex -matrix interaction CEG07 and the global density presented by S{\~
a}o Paulo group. The folding model well accounts for realistic complex optical
potentials of nucleus-nucleus systems and reproduces the existing elastic
scattering data for stable heavy-ion projectiles at incident energies above 50
MeV/u. We then calculate the folding-model potentials (FMPs) for projectiles of
even-even isotopes, C, O, Ne, Mg,
Si, S, Ar, and Ca, scattered by stable
target nuclei of C, O, Si, Ca Ni, Zr,
Sn, and Pb at the incident energy of 50, 60, 70, 80, 100, 120,
140, 160, 180, 200, 250, 300, 350, and 400 MeV/u. The calculated FMP is
represented, with a sufficient accuracy, by a linear combination of 10-range
Gaussian functions. The expansion coefficients depend on the incident energy,
the projectile and target mass numbers and the projectile atomic number, while
the range parameters are taken to depend only on the projectile and target mass
numbers. The adequate mass region of the present GOP by the global density is
inspected in comparison with FMP by realistic density. The full set of the
range parameters and the coefficients for all the projectile-target
combinations at each incident energy are provided on a permanent open-access
website together with a Fortran program for calculating the microscopic-basis
GOP (MGOP) for a desired projectile nucleus by the spline interpolation over
the incident energy and the target mass number.Comment: 25 pages, 13 figure
Comparison of optical model results from a microscopic Schr\"odinger approach to nucleon-nucleus elastic scattering with those from a global Dirac phenomenology
Comparisons are made between results of calculations for intermediate energy
nucleon-nucleus scattering for 12C, 16O, 40Ca, 90Zr, and 208Pb, using optical
potentials obtained from global Dirac phenomenology and from a microscopic
Schr\"odinger model. Differential cross sections and spin observables for
scattering from the set of five nuclei at 65 MeV and 200 MeV have been studied
to assess the relative merits of each approach. Total reaction cross sections
from proton-nucleus and total cross sections from neutron-nucleus scattering
have been evaluated and compared with data for those five targets in the energy
range 20 MeV to 800 MeV. The methods of analyses give results that compare well
with experimental data in those energy regimes for which the procedures are
suited.Comment: 22 pages, 12 figure
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