7,208 research outputs found
Angular distribution of high-energy photoproduction close to the end of spectrum
We consider the differential cross section of electron-positron pair
production by a high-energy photon in a strong Coulomb field close to the end
of the electron or positron spectrum. When the momentum transfer largely
exceeds the electron mass, the cross section is obtained analytically in a
compact form. Coulomb corrections essentially modify the cross section even for
moderate values of the nuclear charge number . In the same kinematical
region, the angular distribution for bound-free pair production,
bremsstrahlung, and photorecombination is also obtained.Comment: 12 pages, 4 figure
Polarization effects in non-relativistic scattering
The cross section which addresses the spin-flip transitions of a proton
(antiproton) interacting with a polarized non-relativistic electron or positron
is calculated analytically. In the case of attraction, this cross section is
greatly enhanced for sufficiently small relative velocities as compared to the
result obtained in the Born approximation. However, it is still very small, so
that the beam polarization time turns out to be enormously large for the
parameters of beams available now. This practically rules out a use
of such beams to polarize stored antiprotons or protons.Comment: 9 pages; version accepted for publication in Nucl.Instr.Methods B
where cross sections without averaging over directions of relative velocities
are adde
Spin-dependent part of interaction cross section and Nijmegen potential
Low energy interaction is considered taking into account the
polarization of both particles. The corresponding cross sections are calculated
using the Nijmegen nucleon-antinucleon optical potential. Then they are applied
to the analysis of the polarization buildup which is due to the interaction of
stored antiprotons with polarized protons of a hydrogen target. It is shown
that, at realistic parameters of a storage ring and a target, the filtering
mechanism may provide a noticeable polarization in a time comparable with the
beam lifetime.Comment: 10 pages, 3 figure
High-energy photoproduction cross section close to the end of spectrum
We consider the cross section of electron-positron pair production by a
high-energy photon in a strong Coulomb field close to the end of electron or
positron spectrum. We show that the cross section essentially differs from the
result obtained in the Born approximation as well as form the result which
takes into account the Coulomb corrections under assumption that both electron
and positron are ultrarelativistic. The cross section of bremsstrahlung in a
strong Coulomb field by a high-energy electron is also obtained in the region
where the final electron is not ultrarelativistic.Comment: 20 pages, 4 figure
Girls Share Their Voice
Looks at the past and current leadership development needs of girls in Southern New Jersey and Philadelphia and offers recommendations for change
The Induced Charge Generated By The Potential Well In Graphene
The induced charge density, , generated in graphene by the
potential well of the finite radius is considered. The result for
is derived for large distances . We also obtained
the induced charges outside of the radius and inside of this radius
for subcritical and supercritical regimes. The consideration is based on the
convenient representation of the induced charge density via the Green's
function of electron in the field.Comment: 12 pages, 2 figures, version published in Phys.Rev.
Screening of Coulomb Impurities in Graphene
We calculate exactly the vacuum polarization charge density in the field of a
subcritical Coulomb impurity, , in graphene. Our analysis is based on
the exact electron Green's function, obtained by using the operator method, and
leads to results that are exact in the parameter , where is
the "fine structure constant" of graphene. Taking into account also
electron-electron interactions in the Hartree approximation, we solve the
problem self-consistently in the subcritical regime, where the impurity has an
effective charge , determined by the localized induced charge. We find
that an impurity with bare charge Z=1 remains subcritical, , for any , while impurities with and higher can become
supercritical at certain values of .Comment: 4 pages, 2 figure
The Calvin 28 cryptoexplosive disturbance, Cass County, Michigan: Evidence for impact origin
The Calvin 28 cryptoexplosive disturbance is an isolated, nearly circular subsurface structure of Late Ordovician age in southwestern Michigan. The structure is defined by 107 wells, is about 7.24 km in diameter and consists of a central dome, an annular depression and an encircling anticlinal rim. Seismic and geophysical well log data confirm that an intricate system of faults and structural derangement exists within the structure. Deformation decreases with depth and distance from the structure. U.S.G.S. topographic maps and aerial imagery show the structure is reflected as a subtle surface topographic rise controlling local drainage. Igneous or diapiric intrusion and solution collapse are rejected as possible origins for Calvin 28 on the basis of stratigraphic, structural and geophysical evidence. A volcanic origin is inconsistent with calculated energy requirements and an absence of igneous material. Although shock-metamorphic features are unidentified, microbreccias occur in deep wells that penetrate the structure. Morphology and structural parameters support an impact origin
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