5,812 research outputs found
Analytical computation of the off-axis Effective Area of grazing incidence X-ray mirrors
Focusing mirrors for X-ray telescopes in grazing incidence, introduced in the
70s, are characterized in terms of their performance by their imaging quality
and effective area, which in turn determines their sensitivity. Even though the
on-axis effective area is assumed in general to characterize the collecting
power of an X-ray optic, the telescope capability of imaging extended X-ray
sources is also determined by the variation in its effective area with the
off-axis angle. [...] The complex task of designing optics for future X-ray
telescopes entails detailed computations of both imaging quality and effective
area on- and off-axis. Because of their apparent complexity, both aspects have
been, so far, treated by using ray-tracing routines aimed at simulating the
interaction of X-ray photons with the reflecting surfaces of a given focusing
system. Although this approach has been widely exploited and proven to be
effective, it would also be attractive to regard the same problem from an
analytical viewpoint, to assess an optical design of an X-ray optical module
with a simpler calculation than a ray-tracing routine. [...] We have developed
useful analytical formulae for the off-axis effective area of a
double-reflection mirror in the double cone approximation, requiring only an
integration and the standard routines to calculate the X-ray coating
reflectivity for a given incidence angle. [...] Algebraic expressions are
provided for the mirror geometric area, as a function of the off-axis angle.
Finally, the results of the analytical computations presented here are
validated by comparison with the corresponding predictions of a ray-tracing
code.Comment: 12 pages, 11 figures, accepted for publication in "Astronomy &
Astrophysics", section "Instruments, observational techniques, and data
processing". Updated version after grammatical revision and typos correctio
Photon and Pomeron -- induced production of Dijets in , and collisions
In this paper we present a detailed comparison of the dijet production by
photon -- photon, photon -- pomeron and pomeron -- pomeron interactions in
, and collisions at the LHC energy. The transverse
momentum, pseudo -- rapidity and angular dependencies of the cross sections are
calculated at LHC energy using the Forward Physics Monte Carlo (FPMC), which
allows to obtain realistic predictions for the dijet production with two
leading intact hadrons. We obtain that \gamma \pom channel is dominant at
forward rapidities in collisions and in the full kinematical range in the
nuclear collisions of heavy nuclei. Our results indicate that the analysis of
dijet production at the LHC can be useful to test the Resolved Pomeron model as
well as to constrain the magnitude of the absorption effects.Comment: 11 pages, 6 figures, 1 table. Improved and enlarged version published
in European Physical Journal
Direct calorimetric measurements of isothermal entropy change on single crystal W-type hexaferrites at the spin reorientation transition
We report on the magnetic field induced isothermal entropy change, \Delta
s(Ha, T), of W-type ferrite with CoZn substitution. Entropy measurements are
performed by direct calorimetry. Single crystals of the composition
BaCoZnFeO, prepared by the flux method, are measured at
different fixed temperatures under an applied field perpendicular and parallel
to the c axis. At 296 K one deduces a value of K = 8.7 \times 10^{4} J
m for the first anisotropy constant, which is in good agreement with the
literature. The spin reorientation transition temperature is estimated to take
place between 200 and 220 K
Specific heat and entropy change at the first order phase transition of La(Fe-Mn-Si)(13)-H compounds
Longitudinal spin Seebeck coefficient: heat flux vs. temperature difference method
The determination of the longitudinal spin Seebeck effect (LSSE) coefficient
is currently plagued by a large uncertainty due to the poor reproducibility of
the experimental conditions used in its measurement. In this work we present a
detailed analysis of two different methods used for the determination of the
LSSE coefficient. We have performed LSSE experiments in different laboratories,
by using different setups and employing both the temperature difference method
and the heat flux method. We found that the lack of reproducibility can be
mainly attributed to the thermal contact resistance between the sample and the
thermal baths which generate the temperature gradient. Due to the variation of
the thermal resistance, we found that the scaling of the LSSE voltage to the
heat flux through the sample rather than to the temperature difference across
the sample greatly reduces the uncertainty. The characteristics of a single
YIG/Pt LSSE device obtained with two different setups was Vm/W and Vm/W with the heat flux method
and V/K and V/K
with the temperature difference method. This shows that systematic errors can
be considerably reduced with the heat flux method.Comment: PDFLaTeX, 10 pages, 6 figure
Is demagnetization an efficient optimization method?
Demagnetization, commonly employed to study ferromagnets, has been proposed
as the basis for an optimization tool, a method to find the ground state of a
disordered system. Here we present a detailed comparison between the ground
state and the demagnetized state in the random field Ising model, combing exact
results in and numerical solutions in . We show that there are
important differences between the two states that persist in the thermodynamic
limit and thus conclude that AC demagnetization is not an efficient
optimization method.Comment: 2 pages, 1 figur
Noise Measurement of Interacting Ferromagnetic Particles with High Resolution Hall Microprobes
We present our first experimental determination of the magnetic noise of a
superspinglass made of < 1 pico-liter frozen ferrofluid. The measurements were
performed with a local magnetic field sensor based on Hall microprobes operated
with the spinning current technique. The results obtained, though preliminary,
qualitatively agree with the theoretical predictions of Fluctuation-Dissipation
theorem (FDT) violation [1].Comment: 4pages, 2 figure
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