9,835 research outputs found
Proof of principle of a high-spatial-resolution, resonant-response gamma-ray detector for Gamma Resonance Absorption in 14N
The development of a mm-spatial-resolution, resonant-response detector based
on a micrometric glass capillary array filled with liquid scintillator is
described. This detector was developed for Gamma Resonance Absorption (GRA) in
14N. GRA is an automatic-decision radiographic screening technique that
combines high radiation penetration (the probe is a 9.17 MeV gamma ray) with
very good sensitivity and specificity to nitrogenous explosives. Detailed
simulation of the detector response to electrons and protons generated by the
9.17 MeV gamma-rays was followed by a proof-of-principle experiment, using a
mixed gamma-ray and neutron source. Towards this, a prototype capillary
detector was assembled, including the associated filling and readout systems.
Simulations and experimental results indeed show that proton tracks are
distinguishable from electron tracks at relevant energies, on the basis of a
criterion that combines track length and light intensity per unit length.Comment: 18 pages, 16 figure
Horizon effects with surface waves on moving water
Surface waves on a stationary flow of water are considered, in a linear model
that includes the surface tension of the fluid. The resulting gravity-capillary
waves experience a rich array of horizon effects when propagating against the
flow. In some cases three horizons (points where the group velocity of the wave
reverses) exist for waves with a single laboratory frequency. Some of these
effects are familiar in fluid mechanics under the name of wave blocking, but
other aspects, in particular waves with negative co-moving frequency and the
Hawking effect, were overlooked until surface waves were investigated as
examples of analogue gravity [Sch\"utzhold R and Unruh W G 2002 Phys. Rev. D 66
044019]. A comprehensive presentation of the various horizon effects for
gravity-capillary waves is given, with emphasis on the deep water/short
wavelength case kh>>1 where many analytical results can be derived. A
similarity of the state space of the waves to that of a thermodynamic system is
pointed out.Comment: 30 pages, 15 figures. Minor change
Poor-man's model of hollow-core anti-resonant fibers
We investigate various methods for extending the simple analytical capillary
model to describe the dispersion and loss of anti-resonant hollow-core fibers
without the need of detailed finite-element simulations across the desired
wavelength range. This poor-man's model can with a single fitting parameter
quite accurately mimic dispersion and loss resonances and anti-resonances from
full finite-element simulations. Due to the analytical basis of the model it is
easy to explore variations in core size and cladding wall thickness, and should
therefore provide a valuable tool for numerical simulations of the ultrafast
nonlinear dynamics of gas-filled hollow-core fibers.Comment: In preparatio
Optical guiding in meter-scale plasma waveguides
We demonstrate a new highly tunable technique for generating meter-scale low
density plasma waveguides. Such guides can enable electron acceleration to tens
of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by
optical field ionization induced by two time-separated Bessel beam pulses: The
first pulse, a J_0 beam, generates the core of the waveguide, while the delayed
second pulse, here a J_8 or J_16 beam, generates the waveguide cladding. We
demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths
with on axis plasma densities as low as N_e0=5x10^16 cm^-3
Evolving fracture patterns: columnar joints, mud cracks, and polygonal terrain
When cracks form in a thin contracting layer, they sequentially break the
layer into smaller and smaller pieces. A rectilinear crack pattern encodes
information about the order of crack formation, as later cracks tend to
intersect with earlier cracks at right angles. In a hexagonal pattern, in
contrast, the angles between all cracks at a vertex are near 120.
However, hexagonal crack patterns are typically only seen when a crack network
opens and heals repeatedly, in a thin layer, or advances by many intermittent
steps into a thick layer. Here it is shown how both types of pattern can arise
from identical forces, and how a rectilinear crack pattern evolves towards a
hexagonal one. Such an evolution is expected when cracks undergo many opening
cycles, where the cracks in any cycle are guided by the positions of cracks in
the previous cycle, but when they can slightly vary their position, and order
of opening. The general features of this evolution are outlined, and compared
to a review of the specific patterns of contraction cracks in dried mud,
polygonal terrain, columnar joints, and eroding gypsum-sand cementsComment: 19 pages, 9 figures, accepted for publication in Phil. Trans. R. Soc.
A; theme issue on Geophysical Pattern Formation (to appear 2013
A robust method for calculating interface curvature and normal vectors using an extracted local level set
The level-set method is a popular interface tracking method in two-phase flow
simulations. An often-cited reason for using it is that the method naturally
handles topological changes in the interface, e.g. merging drops, due to the
implicit formulation. It is also said that the interface curvature and normal
vectors are easily calculated. This last point is not, however, the case in the
moments during a topological change, as several authors have already pointed
out. Various methods have been employed to circumvent the problem. In this
paper, we present a new such method which retains the implicit level-set
representation of the surface and handles general interface configurations. It
is demonstrated that the method extends easily to 3D. The method is validated
on static interface configurations, and then applied to two-phase flow
simulations where the method outperforms the standard method and the results
agree well with experiments.Comment: 31 pages, 18 figure
Growth, microstructure, and failure of crazes in glassy polymers
We report on an extensive study of craze formation in glassy polymers.
Molecular dynamics simulations of a coarse-grained bead-spring model were
employed to investigate the molecular level processes during craze nucleation,
widening, and breakdown for a wide range of temperature, polymer chain length
, entanglement length and strength of adhesive interactions between
polymer chains. Craze widening proceeds via a fibril-drawing process at
constant drawing stress. The extension ratio is determined by the entanglement
length, and the characteristic length of stretched chain segments in the
polymer craze is . In the craze, tension is mostly carried by the
covalent backbone bonds, and the force distribution develops an exponential
tail at large tensile forces. The failure mode of crazes changes from
disentanglement to scission for , and breakdown through scission
is governed by large stress fluctuations. The simulations also reveal
inconsistencies with previous theoretical models of craze widening that were
based on continuum level hydrodynamics
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