8,500 research outputs found
Moments of inertia for solids of revolution and variational methods
We present some formulae for the moments of inertia of homogeneous solids of
revolution in terms of the functions that generate the solids. The development
of these expressions exploits the cylindrical symmetry of these objects, and
avoids the explicit use of multiple integration, providing an easy and
pedagogical approach. The explicit use of the functions that generate the solid
gives the possibility of writing the moment of inertia as a functional, which
in turn allows us to utilize the calculus of variations to obtain a new insight
into some properties of this fundamental quantity. In particular, minimization
of moments of inertia under certain restrictions is possible by using
variational methods.Comment: 6 pages, 6 figures, LaTeX2e. Two paragraphs added. Minor typos
corrected. Version to appear in European Journal of Physic
A causal model of radiating stellar collapse
We find a simple exact model of radiating stellar collapse, with a shear-free
and non-accelerating interior matched to a Vaidya exterior. The heat flux is
subject to causal thermodynamics, leading to self-consistent determination of
the temperature . We solve for exactly when the mean collision time
is constant, and perturbatively in a more realistic case of variable
. Causal thermodynamics predicts temperature behaviour that can
differ significantly from the predictions of non-causal theory. In particular,
the causal theory gives a higher central temperature and greater temperature
gradient.Comment: Latex [ioplppt style] 9 pages; to appear Class. Quantum Gra
Design of the EURISOL multi-MW target assembly: radiation and safety issues
The multi-MW target proposed for the EURISOL facility will be based on
fission of uranium (or thorium) compounds to produce rare isotopes far from
stability. A two-step process is used for the isotope production. First,
neutrons are generated in a liquid mercury target, irradiated by the 1 GeV
proton or deuteron beam, provided by the EURISOL linac driver. Then, the
neutrons induce fission in a surrounding assembly of uranium carbide. R&D
projects on several aspects of the target assembly are ongoing. Key criteria
for the target design are a maximum beam power capability of 4 MW, a remote
handling system with minimum downtime and maximum reliability, as well as
radiation safety, minimization of hazards and the classification of the
facility. In the framework of the ongoing radiation characterization and safety
studies, radiation transport simulations have been performed to calculate the
prompt radiation dose in the target and surrounding materials, as well as to
determine shielding material and angle-dependent parameters. In this paper, we
report the results of these studies and the proposed radiation shield design
for the multi-MW target area. Furthermore, accurate estimates have been
performed of the amount of fissile elements being produced in the uranium
target assembly, for typical running conditions, in order to understand the
implications for the classification of the facility. The results are reported
and briefly discussed.Comment: 11 pages,5 figures, Invited talk at the SATIF-8 Workshop, May
22-24,2006, Pohang Accelerator Laboratory, Rep. Of Kore
Space-time inhomogeneity, anisotropy and gravitational collapse
We investigate the evolution of non-adiabatic collapse of a shear-free
spherically symmetric stellar configuration with anisotropic stresses
accompanied with radial heat flux. The collapse begins from a curvature
singularity with infinite mass and size on an inhomogeneous space-time
background. The collapse is found to proceed without formation of an even
horizon to singularity when the collapsing configuration radiates all its mass
energy. The impact of inhomogeneity on various parameters of the collapsing
stellar configuration is examined in some specific space-time backgrounds.Comment: To appear in Gen. Relativ. Gra
Estudio del apoyo metatarsal en bipedestación mediante podoscopio electrónico de alta resolución.
Con el jin de estudiar el apoyo metatarsal en bipedestación se
han realizado 780 registros en la plataforma PDS 93, correspondientes
a 390 individuos, no seleccionados, sin patología de la
marcha, 234 varones y 156 mujeres, con edades comprendidas
entre 24 y 41 años (edad media de 29,9 años).
En cada registro se ha recogido la presión ejercida por cada metatarsiano,
relacionando la carga obtenida con el peso, conjiguración
mecánica del pie y huella plantar: Los resultados obtenidos se
han procesado estadísticamente por medio del programa SPSS.
El porcentaje de las cargas medias obtenidas para cada metatarsiano
han sido: 16,85, 21,25, 23,88, 20,24 y 17,78%, existiendo
dqerencias sign$cativas para el apoyo de los metatarsianos
centrales (p < 0,001).
Todos los metatarsianos soportan carga en bipedestación, de
acuerdo con las teorías actuales. Sin embargo, aunque la distribución
del peso para el primer metatarsiano se aproxima al doble
de los otros, la carga máxima en g/cm2 se realiza en los metatarsianos
centrales
Study of scintillation in natural and synthetic quartz and methacrylate
Samples from different materials typically used as optical windows or light
guides in scintillation detectors were studied in a very low background
environment, at the Canfranc Underground Laboratory, searching for
scintillation. A positive result can be confirmed for natural quartz: two
distinct scintillation components have been identified, not being excited by an
external gamma source. Although similar effect has not been observed neither
for synthetic quartz nor for methacrylate, a fast light emission excited by
intense gamma flux is evidenced for all the samples in our measurements. These
results could affect the use of these materials in low energy applications of
scintillation detectors requiring low radioactive background conditions, as
they entail a source of background.Comment: Accepted for publication in Optical Material
The nature of domain walls in ultrathin ferromagnets revealed by scanning nanomagnetometry
The recent observation of current-induced domain wall (DW) motion with large
velocity in ultrathin magnetic wires has opened new opportunities for
spintronic devices. However, there is still no consensus on the underlying
mechanisms of DW motion. Key to this debate is the DW structure, which can be
of Bloch or N\'eel type, and dramatically affects the efficiency of the
different proposed mechanisms. To date, most experiments aiming to address this
question have relied on deducing the DW structure and chirality from its motion
under additional in-plane applied fields, which is indirect and involves strong
assumptions on its dynamics. Here we introduce a general method enabling
direct, in situ, determination of the DW structure in ultrathin ferromagnets.
It relies on local measurements of the stray field distribution above the DW
using a scanning nanomagnetometer based on the Nitrogen-Vacancy defect in
diamond. We first apply the method to a Ta/Co40Fe40B20(1 nm)/MgO magnetic wire
and find clear signature of pure Bloch DWs. In contrast, we observe left-handed
N\'eel DWs in a Pt/Co(0.6 nm)/AlOx wire, providing direct evidence for the
presence of a sizable Dzyaloshinskii-Moriya interaction (DMI) at the Pt/Co
interface. This method offers a new path for exploring interfacial DMI in
ultrathin ferromagnets and elucidating the physics of DW motion under current.Comment: Main text and Supplementary Information, 33 pages and 12 figure
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