2,914 research outputs found
The air shower maximum probed by Cherenkov effects from radio emission
Radio detection of cosmic-ray-induced air showers has come to a flight the
last decade. Along with the experimental efforts, several theoretical models
were developed. The main radio-emission mechanisms are established to be the
geomagnetic emission due to deflection of electrons and positrons in Earth's
magnetic field and the charge-excess emission due to a net electron excess in
the air shower front. It was only recently shown that Cherenkov effects play an
important role in the radio emission from air showers. In this article we show
the importance of these effects to extract quantitatively the position of the
shower maximum from the radio signal, which is a sensitive measure for the mass
of the initial cosmic ray. We also show that the relative magnitude of the
charge-excess and geomagnetic emission changes considerably at small observer
distances where Cherenkov effects apply
Macroscopic Geo-Magnetic Radiation Model; Polarization effects and finite volume calculations
An ultra-high-energy cosmic ray (UHECR) colliding with the Earth's atmosphere
gives rise to an Extensive Air Shower (EAS). Due to different charge separation
mechanisms within the thin shower front coherent electromagnetic radiation will
be emitted within the radio frequency range. A small deviation of the index of
refraction from unity will give rise to Cherenkov radiation up to distances of
100 meters from the shower core and therefore has to be included in a complete
description of the radio emission from an EAS. Interference between the
different radiation mechanisms, in combination with different polarization
behavior will reflect in a lateral distribution function (LDF) depending on the
orientation of the observer and a non-trivial fall-off of the radio signal as
function of distance to the shower core.Comment: Proceedings of the ARENA2010 conference, Nantes, Franc
Diffraction-contrast imaging of cold atoms
We consider the inverse problem of in-line holography, applied to
minimally-destructive imaging of cold atom clouds. Absorption imaging
near-resonance provides a simple, but destructive measurement of atom column
density. Imaging off resonance greatly reduces heating, and sequential images
may be taken. Under the conditions required for off-resonant imaging, the
generally-intractable inverse problem may be linearized. A
minimally-destructive, quantitative and high-resolution image of the atom cloud
column density is then retrieved from a single diffraction pattern.Comment: 4 pages, 3 figures v2: minor changes in response to referee reports,
mostly additional experimental detail v3: revisions to figure 3: added trace
and changed image. Minor text and referencing changes. Accepted by Phys Rev A
(Rapid Commun
Analytic Calculation of Radio Emission from Extensive Air Showers subjected to Atmospheric Electric Fields
We have developed a code that semi-analytically calculates the radio
footprint (intensity and polarization) of an extensive air shower subject to
atmospheric electric fields. This can be used to reconstruct the height
dependence of atmospheric electric field from the measured radio footprint. The
various parameterizations of the spatial extent of the induced currents are
based on the results of Monte-Carlo shower simulations. The calculated radio
footprints agree well with microscopic CoREAS simulations.Comment: Contribution to the proceedings of the ARENA conference, Groningen,
The Netherlands, June 7-10, 201
Surface modification of ZrO<sub>2</sub> - Y<sub>2</sub>O<sub>3</sub> ss by ion implantation
With 56Fe+ implantations in oxygen ion conducting solid solutions of 0.86ZrO2-0.14YO1.5 [ZY14] electronic conductivity is introduced in a surface layer of less than 70 nm. The theoretically expected Gaussian distributions of Fe ions as calculated by computer simulation are compared with results of analyses by AES and RBS. Mean projected ranges (Rp) agree but remarkable differences in peakheight and halfwidth are found. Surface concentrations up to 6∗1021 ZY Fe/cm3 have been achieved using mediate doses of 2–4∗1016 Fe/cm2 and energies ranging from 15–110 KeV. Concurrent sputtering during high dose implantations (8–40∗1016 Fe/cm2, 15 and 110 KeV) causes a peakshift towards the solid-gas surface. With these high doses high concentrations up to 24∗1021 Fe/cm3 were obtained. The implanted profile shapes remain stable under heat treatments up to 900°C. Higher temperatures cause a decrease in topconcentration and broadening of the distribution. After annealing during 25 hrs. at 1500°C all implanted iron was dissolved in the ZY matrix and no precipitates were formed as indicated by XRD
Discounting by intervals: a generalized model of intertemporal choice
According to most models of intertemporal choice, an agent's discount rate is a function of how far the outcomes are removed from the present, and nothing else. This view has been challenged by recent studies, which show that discount rates tend to be higher the closer the outcomes are to one another (subadditive discounting) and that this can give rise to intransitive intertemporal choice. We develop and test a generalized model of intertemporal choice, the Discounting By Intervals (DBI) model, according to which the discount rate is a function of both how far outcomes are removed from the present and how far the outcomes are removed from one another. The model addresses past challenges to other models, most of which it includes as special cases, as well as the new challenges presented in this paper: Our studies show that when the interval between outcomes is very short, discount rate tends to increase with interval length (superadditive discounting). In the discussion we place our model and evidence in a broader theoretical context
Quasi dynamical symmetry in an interacting boson model phase transition
The oft-observed persistence of symmetry properties in the face of strong
symmetry-breaking interactions is examined in the SO(5)-invariant interacting
boson model. This model exhibits a transition between two phases associated
with U(5) and O(6) symmetries, respectively, as the value of a control
parameter progresses from 0 to 1. The remarkable fact is that, for intermediate
values of the control parameter, the model states exhibit the characteristics
of its closest symmetry limit for all but a relatively narrow transition region
that becomes progressively narrower as the particle number of the model
increases. This phenomenon is explained in terms of quasi-dynamical symmetry.Comment: 4 figure
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