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Electron scattering cross sections pertinent to electron microscopy
Some elements of the physics that determine cross sections are discussed, and various sources of data are indicated that should be useful for analytical microscopy. Atoms, molecules, and to some extent, solids are considered. Inelastic and elastic scattering of electrons and some solid-state effects are treated. 30 references. (JFP
Radially restricted linear energy transfer for high-energy protons: A new analytical approach
Radially restricted linear energy transfer (LET) is a basic physical parameter relevant to radiation biology and radiation protection. In this report a convenient method is presented for the analytical computation of this quantity without the need for complicated simulation. The method uses the energy-re-stricted LETL, as recently redefined in a 1993 ICRU draft document and supplements it by a relatively simple term that represents the energy of fast rays lost within distancer from the track core. The method provides a better fit than other models and is valid over the entire range of radial distance from track center to the maximum radial distance traveled by the most energetic secondary electrons.L r computed by this approach differs only a few percent from the values
Contribution to the international symposium on heavy ions research: space, radiation protection and therapy, 21â24 March 1994, Sophia-Antipolis, Franc
Computation of microdosimetric distributions for small sites
Object of this study is the computation of microdosimetric functions for sites which are too small to permit experimental determination of the distributions by Rossi-counters. The calculations are performed on simulated tracks generated by Monte-Carlo techniques.
The first part of the article deals with the computational procedure. The second part presents numerical results for protons of energies 0.5, 5, 20 MeV and for site diameters of 5, 10, 100 nm
Back-to-back emission of the electrons in double photoionization of helium
We calculate the double differential distributions and distributions in
recoil momenta for the high energy non-relativistic double photoionization of
helium. We show that the results of recent experiments is the pioneering
experimental manifestation of the quasifree mechanism for the double
photoionization, predicted long ago in our papers. This mechanism provides a
surplus in distribution over the recoil momenta at small values of the latter,
corresponding to nearly "back-to-back" emission of the electrons. Also in
agreement with previous analysis the surplus is due to the quadrupole terms of
the photon-electron interaction. We present the characteristic angular
distribution for the "back-to-back" electron emission. The confirmation of the
quasifree mechanism opens a new area of exiting experiments, which are expected
to increase our understanding of the electron dynamics and of the bound states
structure. The results of this Letter along with the recent experiments open a
new field for studies of two-electron ionization not only by photons but by
other projectiles, e.g. by fast electrons or heavy ions.Comment: 10 pages, 2 figure
Qualitative difference between the angular anisotropy parameters in fast electron scattering and photoionization
It is demonstrated for the first time that in spite of well known big
similarities between atomic ionization by photons and fast electrons, a
qualitative difference exists in angular anisotropy parameters of electrons
knocked out in these processes. The difference is disclosed here and attributed
to distinction between normal (transverse) and virtual (longitudinal) photons.
Formulas are derived for dipole and non-dipole angular anisotropy parameters in
fast electronatom scattering. The ratio of quadrupole-to-dipole matrix elements
is determined by the parameter \omega R/v << 1 where \omega is the transferred
in collision energy, R is the ionized shell radius and v is the speed of
projectile. This factor can be much bigger than in the case of photoionization,
where one has the speed of light c that is much bigger than v . We illustrate
general formulas by concrete results for outer s-subshells of noble gas atoms
Ar and Xe. Even for very small transferred momentum q, in the so-called optical
limit, the deviation from photoionization case is prominent and instructive.Comment: 8 pages, 3 figures. arXiv admin note: substantial text overlap with
arXiv:1012.546
Electron impact double ionization of helium from classical trajectory calculations
With a recently proposed quasiclassical ansatz [Geyer and Rost, J. Phys. B 35
(2002) 1479] it is possible to perform classical trajectory ionization
calculations on many electron targets. The autoionization of the target is
prevented by a M\o{}ller type backward--forward propagation scheme and allows
to consider all interactions between all particles without additional
stabilization. The application of the quasiclassical ansatz for helium targets
is explained and total and partially differential cross sections for electron
impact double ionization are calculated. In the high energy regime the
classical description fails to describe the dominant TS1 process, which leads
to big deviations, whereas for low energies the total cross section is
reproduced well. Differential cross sections calculated at 250 eV await their
experimental confirmation.Comment: LaTeX, 22 pages, 10 figures, submitted to J. Phys.
Density functional theory calculations of the carbon ELNES of small diameter armchair and zigzag nanotubes: core-hole, curvature and momentum transfer orientation effects
We perform density functional theory calculations on a series of armchair and
zigzag nanotubes of diameters less than 1nm using the all-electron
Full-Potential(-Linearised)-Augmented-Plane-Wave (FPLAPW) method. Emphasis is
laid on the effects of curvature, the electron beam orientation and the
inclusion of the core-hole on the carbon electron energy loss K-edge. The
electron energy loss near-edge spectra of all the studied tubes show strong
curvature effects compared to that of flat graphene. The curvature induced
hybridisation is shown to have a more drastic effect on the
electronic properties of zigzag tubes than on those of armchair tubes. We show
that the core-hole effect must be accounted for in order to correctly reproduce
electron energy loss measurements. We also find that, the energy loss near edge
spectra of these carbon systems are dominantly dipole selected and that they
can be expressed simply as a proportionality with the local momentum projected
density of states, thus portraying the weak energy dependence of the transition
matrix elements. Compared to graphite, the ELNES of carbon nanotubes show a
reduced anisotropy.Comment: 25 pages, 15 figures, revtex4 submitted for publication to Phys. Rev.
The yield of air fluorescence induced by electrons
The fluorescence yield for dry air and pure nitrogen excited by electrons is
calculated using a combination of well-established molecular properties and
experimental data of the involved cross sections. Particular attention has been
paid to the role of secondary electrons from ionization processes. At high
pressure and high energy, observed fluorescence turns out to be proportional to
the ionization cross section which follows the Born-Bethe law. Predictions on
fluorescence yields in a very wide interval of electron energies (eV - GeV) and
pressures (1 and 1013 hPa) as expected from laboratory measurements are
presented. Experimental results at energies over 1 MeV are in very good
agreement with our calculations for pure nitrogen while discrepancies of about
20% are found for dry air, very likely associated to uncertainties in the
available data on quenching cross sections. The relationship between
fluorescence emission, stopping power and deposited energy is discussed.Comment: 27 pages, 12 figures, 64 references. Accepted in Astroparticle
Physic
Time-Fractional KdV Equation: Formulation and Solution using Variational Methods
In this work, the semi-inverse method has been used to derive the Lagrangian
of the Korteweg-de Vries (KdV) equation. Then, the time operator of the
Lagrangian of the KdV equation has been transformed into fractional domain in
terms of the left-Riemann-Liouville fractional differential operator. The
variational of the functional of this Lagrangian leads neatly to Euler-Lagrange
equation. Via Agrawal's method, one can easily derive the time-fractional KdV
equation from this Euler-Lagrange equation. Remarkably, the time-fractional
term in the resulting KdV equation is obtained in Riesz fractional derivative
in a direct manner. As a second step, the derived time-fractional KdV equation
is solved using He's variational-iteration method. The calculations are carried
out using initial condition depends on the nonlinear and dispersion
coefficients of the KdV equation. We remark that more pronounced effects and
deeper insight into the formation and properties of the resulting solitary wave
by additionally considering the fractional order derivative beside the
nonlinearity and dispersion terms.Comment: The paper has been rewritten, 12 pages, 3 figure
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