683 research outputs found
Three component model of cosmic ray spectra from 10 GeV to 100 PeV
A model to describe cosmic ray spectra in the energy region from 10 GeV to
100 PeV is suggested based on the assumption that Galactic cosmic ray flux is a
mixture of fluxes accelerated by shocks from nova and supernova of different
types. We analyze recent experimental data on cosmic ray spectra obtained in
direct measurements above the atmosphere and data obtained with ground
Extensive Air Shower arrays. The model of the three classes of cosmic ray
sources is consistent with direct experimental data on cosmic ray elemental
spectra and gives a smooth transition from the all particle spectrum measured
in the direct experiments to the all particle spectrum measured with EAS.Comment: Revised version accepted for publication in Astronomy and
Astrophysics, 5 pages, 6 figures, aa.cl
Stability of boron-doped graphene/copper interface: DFT, XPS and OSEE studies
Two different types of boron-doped graphene/copper interfaces synthesized
using two different flow rates of Ar through the bubbler containing the boron
source were studied. X-ray photoelectron spectra (XPS) and optically stimulated
electron emission (OSEE) measurements have demonstrated that boron-doped
graphene coating provides a high corrosion resistivity of Cu-substrate with the
light traces of the oxidation of carbon cover. The density functional theory
calculations suggest that for the case of substitutional (graphitic)
boron-defect only the oxidation near boron impurity is energetically favorable
and creation of the vacancies that can induce the oxidation of copper substrate
is energetically unfavorable. In the case of non-graphitic boron defects
oxidation of the area, a nearby impurity is metastable that not only prevent
oxidation but makes boron-doped graphene. Modeling of oxygen reduction reaction
demonstrates high catalytic performance of these materials.Comment: 15 pages, 8 figures, to appear in Appl. Surf. Sc
Uncommon 2D Diamond-like Carbon Nanodots Derived from Nanotubes: Atomic Structure, Electronic States and Photonic Properties
In this article, we report the results of relatively facile fabrication of
carbon nanodots from single-walled and multi-walled carbon nanotubes (SWCNT and
MWCNT). The results of X-ray photoelectron spectroscopy (XPS) and Raman
measurements show that the obtained carbon nanodots are quasi-two-dimensional
objects with a diamond-like structure. Based on the characterization results, a
theoretical model of synthesized carbon nanodots was developed. The measured
absorption spectra demonstrate the similarity of the local atomic structure of
carbon nanodots synthesized from single-walled and multi-walled carbon
nanotubes. However, the photoluminescence (PL) spectra of nanodots synthesized
from both sources turned out to be completely different. Carbon dots fabricated
from MWCNTs exhibit PL spectra similar to nanoscale carbon systems with sp3
hybridization and a valuable edge contribution. At the same time nanodots
synthesized from SWCNTs exhibit PL spectra which are typical for quantum dots
with an estimated size of ~0.6-1.3 nm.Comment: 22 pages, 9 figures, to appear in PCC
Towards a model of population of astrophysical sources of ultra-high-energy cosmic rays
We construct and discuss a toy model of the population of numerous
non-identical extragalactic sources of ultra-high-energy cosmic rays. In the
model, cosmic-ray particles are accelerated in magnetospheres of supermassive
black holes in galactic nuclei, the key parameter of acceleration being the
black-hole mass. We use astrophysical data on the redshift-dependent black-hole
mass function to describe the population of these cosmic-ray accelerators, from
weak to powerful, and confront the model with cosmic-ray data.Comment: 9 pages, 4 figures, Revtex 4.
Deep shower interpretation of the cosmic ray events observed in excess of the Greisen-Zatsepin-Kuzmin energy
We consider the possibility that the ultra-high-energy cosmic ray flux has a
small component of exotic particles which create showers much deeper in the
atmosphere than ordinary hadronic primaries. It is shown that applying the
conventional AGASA/HiRes/Auger data analysis procedures to such exotic events
results in large systematic biases in the energy spectrum measurement. SubGZK
exotic showers may be mis-reconstructed with much higher energies and mimick
superGZK events. Alternatively, superGZK exotic showers may elude detection by
conventional fluorescence analysis techniques.Comment: 22 pages, 5 figure
Statistics of clustering of ultra-high energy cosmic rays and the number of their sources
Observation of clustering of ultra-high energy cosmic rays (UHECR) suggests
that they are emitted by compact sources. Assuming small deflection of UHECR
during the propagation, the statistical analysis of clustering allows to
estimate the spatial density of the sources, h, including those which have not
yet been observed directly. When applied to astrophysical models involving
extra-galactic sources, the estimate based on 14 events with energy E>10^{20}
eV gives h ~ 6 X 10^{-3} Mps^{-3}. With increasing statistics, this estimate
may lead to exclusion of the models which associate the production of UHECR
with exceptional galaxies such as AGN, powerful radio-galaxies, dead quasars,
and models based on gamma ray bursts.Comment: The version accepted for publication in Phys. Rev. Lett. Notations
changed to conventional ones. The estimate of the effective GZK radius
replaced by the result of numerical simulatio
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