61,173 research outputs found
Performance evaluation of the Boron Coated Straws detector with Geant4
The last decade has witnessed the development of several alternative neutron
detector technologies, as a consequence of upcoming neutron sources and
upgrades, as well the world-wide shortage of He. One branch of development
is the family of B-based gaseous detectors. This work focuses on the
boron coated straws (BCS) by Proportional Technologies Inc., a commercial
solution designed for use in homeland security and neutron science. A detailed
Geant4 simulation study of the BCS is presented, which investigates various
aspects of the detector performance, e.g. efficiency, activation, absorption
and the impact of scattering on the measured signal. The suitability of the BCS
detector for Small Angle Neutron Scattering (SANS), direct chopper spectrometry
and imaging is discussed.Comment: 50 pages, 37 figures, minor changes after review, results unchange
Aerodynamics of lift fan V/STOL aircraft
Aerodynamic characteristics of lift fan installation for direct lift V/STOL aircraf
Optical transfer cavity stabilization using current-modulated injection-locked diode lasers
It is demonstrated that RF current modulation of a frequency stabilized
injection-locked diode laser allows the stabilization of an optical cavity to
adjustable lengths, by variation of the RF frequency. This transfer cavity may
be used to stabilize another laser at an arbitrary wavelength, in the absence
of atomic or molecular transitions suitable for stabilization. Implementation
involves equipment and techniques commonly used in laser cooling and trapping
laboratories, and does not require electro- or acousto-optic modulators. With
this technique we stabilize a transfer cavity using a RF current-modulated
diode laser which is injection locked to a 780 nm reference diode laser. The
reference laser is stabilized using polarization spectroscopy in a Rb cell. A
Ti:sapphire ring laser at 960 nm is locked to this transfer cavity and may be
precisely scanned by varying the RF modulation frequency. We demonstrate the
suitability of this system for the excitation of laser cooled Rb atoms to
Rydberg states
Finitely generated infinite simple groups of infinite square width and vanishing stable commutator length
It is shown that there exist finitely generated infinite simple groups of
infinite commutator width and infinite square width on which there exists no
stably unbounded conjugation-invariant norm, and in particular stable
commutator length vanishes. Moreover, a recursive presentation of such a group
with decidable word and conjugacy problems is constructed.Comment: v4: 41 pages, 6 figures rescaled at 120%; references updated, typos
corrected, other minor corrections. v3: minor changes to the title, text and
figures. v2: 41 pages, 6 figures; correction: Ore's conjecture was proved in
2008; 2 references added. v1: 40 pages, 6 figure
A two-compartment mechanochemical model of the roles of\ud transforming growth factor β and tissue tension in dermal wound healing
The repair of dermal tissue is a complex process of interconnected phenomena, where cellular, chemical and mechanical aspects all play a role, both in an autocrine and in a paracrine fashion. Recent experimental results have shown that transforming growth factor−β (TGFβ) and tissue mechanics play roles in regulating cell proliferation, differentiation and the production of extracellular materials. We have developed a 1D mathematical model that considers the interaction between the cellular, chemical and mechanical phenomena, allowing the combination of TGFβ and tissue stress to inform the activation of fibroblasts to myofibroblasts. Additionally, our model incorporates the observed feature of residual stress by considering the changing zero-stress state in the formulation for effective strain. Using this model, we predict that the continued presence of TGFβ in dermal wounds will produce contractures due to the persistence of myofibroblasts; in contrast, early elimination of TGFβ significantly reduces the myofibroblast numbers resulting in an increase in wound size. Similar results were obtained by varying the rate at which fibroblasts differentiate to myofibroblasts and by changing the myofibroblast apoptotic rate. Taken together, the implication is that elevated levels of myofibroblasts is the key factor behind wounds healing with excessive contraction, suggesting that clinical strategies which aim to reduce the myofibroblast density may reduce the appearance of contractures
A fibrocontractive mechanochemical model of dermal wound\ud closure incorporating realistic growth factor kinetics
Fibroblasts and their activated phenotype, myofibroblasts, are the primary cell types involved in the contraction associated with dermal wound healing. Recent experimental evidence indicates that the transformation from fibroblasts to myofibroblasts involves two distinct processes: the cells are stimulated to change phenotype by the combined actions of transforming growth factor β (TGFβ) and mechanical tension. This observation indicates a need for a detailed exploration of the effect of the strong interactions between the mechanical changes and growth factors in dermal wound healing. We review the experimental findings in detail and develop a model of dermal wound healing that incorporates these phenomena. Our model includes the interactions between TGFβ and collagenase, providing a more biologically realistic form for the growth factor kinetics than those included in previous mechanochemical descriptions. A comparison is made between the model predictions and experimental data on human dermal wound healing and all the essential features are well matched
Aircraft control via variable cant-angle winglets
Copyright @ 2008 American Institute of Aeronautics and AstronauticsThis paper investigates a novel method for the control of "morphing" aircraft. The concept consists of a pair of winglets; with adjustable cant angle, independently actuated and mounted at the tips of a baseline flying wing. The general philosophy behind the concept was that for specific flight conditions such as a coordinated turn, the use of two control devices would be sufficient for adequate control. Computations with a vortex lattice model and subsequent wind-tunnel tests demonstrate the viability of the concept, with individual and/or dual winglet deflection producing multi-axis coupled control moments. Comparisons between the experimental and computational results showed reasonable to good agreement, with the major discrepancies thought to be due to wind-tunnel model aeroelastic effects.This work has been supported by a Marie Curie excellence research grant funded by the European Commission
On the Theory of Killing Orbits in Space-Time
This paper gives a theoretical discussion of the orbits and isotropies which
arise in a space-time which admits a Lie algebra of Killing vector fields. The
submanifold structure of the orbits is explored together with their induced
Killing vector structure. A general decomposition of a space-time in terms of
the nature and dimension of its orbits is given and the concept of stability
and instability for orbits introduced. A general relation is shown linking the
dimensions of the Killing algebra, the orbits and the isotropies. The
well-behaved nature of "stable" orbits and the possible miss-behaviour of the
"unstable" ones is pointed out and, in particular, the fact that independent
Killing vector fields in space-time may not induce independent such vector
fields on unstable orbits. Several examples are presented to exhibit these
features. Finally, an appendix is given which revisits and attempts to clarify
the well-known theorem of Fubini on the dimension of Killing orbits.Comment: Latex, 19 pages, no figur
Suppression of intrinsic neutron background in the Multi-Grid detector
One of the key requirements for neutron scattering instruments is the
Signal-to-Background ratio (SBR). This is as well a design driving requirement
for many instruments at the European Spallation Source (ESS), which aspires to
be the brightest neutron source of the world. The SBR can be effectively
improved with background reduction. The Multi-Grid, a large-area thermal
neutron detector with a solid boron carbide converter, is a novel solution for
chopper spectrometers. This detector will be installed for the three
prospective chopper spectrometers at the ESS. As the Multi-Grid detector is a
large area detector with a complex structure, its intrinsic background and its
suppression via advanced shielding design should be investigated in its
complexity, as it cannot be naively calculated. The intrinsic scattered neutron
background and its effect on the SBR is determined via a detailed Monte Carlo
simulation for the Multi-Grid detector module, designed for the CSPEC
instrument at the ESS. The impact of the detector vessel and the neutron
entrance window on scattering is determined, revealing the importance of an
optimised internal detector shielding. The background-reducing capacity of
common shielding geometries, like side-shielding and end-shielding is
determined by using perfect absorber as shielding material, and common
shielding materials, like BC and Cd are also tested. On the basis of the
comparison of the effectiveness of the different shielding topologies and
materials, recommendations are given for a combined shielding of the Multi-Grid
detector module, optimised for increased SBR.Comment: 26 pages, 18 figures, revise
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