164 research outputs found
Negative Particle Planar and Axial Channeling and Channeling Collimation
While information exists on high energy negative particle channeling there
has been little study of the challenges of negative particle bending and
channeling collimation. Partly this is because negative dechanneling lengths
are relatively much shorter. Electrons are not particularly useful for
investigating negative particle channeling effects because their material
interactions are dominated by channeling radiation. Another important factor is
that the current central challenge in channeling collimation is the
proton-proton Large Hadron Collider (LHC) where both beams are positive. On the
other hand in the future the collimation question might reemerge for
electron-positron or muon colliders. Dechanneling lengths increase at higher
energies so that part of the negative particle experimental challenge
diminishes. In the article different approaches to determining negative
dechanneling lengths are reviewed. The more complicated case for axial
channeling is also discussed. Muon channeling as a tool to investigate
dechanneling is also discussed. While it is now possible to study muon
channeling it will probably not illuminate the study of negative dechanneling.Comment: 15 pages, 1 figure, docx fil
Photon emission by an ultra-relativistic particle channeling in a periodically bent crystal
This paper is devoted to a detailed analysis of the new type of the undulator
radiation generated by an ultra-relativistic charged particle channeling along
a crystal plane, which is periodically bent by a transverse acoustic wave, as
well as to the conditions limiting the observation of this phenomenon. This
mechanism makes feasible the generation of electromagnetic radiation, both
spontaneous and stimulated, emitted in a wide range of the photon energies,
from X- up to gamma-rays
Full average radiation of electrons and positrons channeled between the planes of a crystal
Electron-based crystalline undulator
We discuss the features of a crystalline undulator of the novel type based on
the effect of a planar channeling of ultra-relativistic electrons in a
periodically bent crystals. It is demonstrated that an electron-based undulator
is feasible in the tens of GeV range of the beam energies, which is noticeably
higher than the energy interval allowed in a positron-based undulator.
Numerical analysis of the main parameters of the undulator as well as the
characteristics of the emitted undulator radiation is carried out for 20 and 50
GeV electrons channeling in diamond and silicon crystals along the (111)
crystallographic planes.Comment: 16 pages, 8 figures, Latex, IOP styl
A Simple Explanation for DAMA with Moderate Channeling
We consider the possibility that the DAMA signal arises from channeled events
in simple models where the dark matter interaction with nuclei is suppressed at
small momenta. As with the standard WIMP, these models have two parameters (the
dark matter mass and the size of the cross-section), without the need to
introduce an additional energy threshold type of parameter. We find that they
can be consistent with channeling fractions as low as about ~ 15%, so long as
at least ~70% of the nuclear recoil energy for channeled events is deposited
electronically. Given that there are reasons not to expect very large
channeling fractions, these scenarios make the channeling explanation of DAMA
much more compelling.Comment: 6 pages, 2 figure
Total spectrum of photon emission by an ultra-relativistic positron channeling in a periodically bent crystal
We present the results of numerical calculations of the channelling and
undulator radiation generated by an ultra-relativistic positron channelling
along a crystal plane, which is periodically bent. The bending might be due
either to the propagation of a transverse acoustic wave through the crystal, or
due to the static strain as it occurs in superlattices. The periodically bent
crystal serves as an undulator. We investigate the dependence of the
intensities of both the ordinary channelling and the undulator radiations on
the parameters of the periodically bent channel with simultaneous account for
the dechannelling effect of the positrons. We demonstrate that there is a range
of parameters in which the undulator radiation dominates over the channelling
one and the characteristic frequencies of both types of radiation are well
separated. This result is important, because the undulator radiation can be
used to create a tunable source of X-ray and gamma-radiation.Comment: published in J. Phys. G: Nucl. Part. Phys. 26 (2000) L87-L95,
http://www.iop.org ; 12 pages, 4 figures, LaTe
Channeling of Positrons through Periodically Bent Crystals: on Feasibility of Crystalline Undulator and Gamma-Laser
The electromagnetic radiation generated by ultra-relativistic positrons
channelling in a crystalline undulator is discussed. The crystalline undulator
is a crystal whose planes are bent periodically with the amplitude much larger
than the interplanar spacing. Various conditions and criteria to be fulfilled
for the crystalline undulator operation are established. Different methods of
the crystal bending are described. We present the results of numeric
calculations of spectral distributions of the spontaneous radiation emitted in
the crystalline undulator and discuss the possibility to create the stimulated
emission in such a system in analogy with the free electron laser. A careful
literature survey covering the formulation of all essential ideas in this field
is given. Our investigation shows that the proposed mechanism provides an
efficient source for high energy photons, which is worth to study
experimentally.Comment: 52 pages, MikTeX, 14 figure
Nonlinear interaction of charged particles with a free electron gas beyond the random-phase approximation
A nonlinear description of the interaction of charged particles penetrating a
solid has become of basic importance in the interpretation of a variety of
physical phenomena. Here we develop a many-body theoretical approach to the
quadratic decay rate, energy loss, and wake potential of charged particles
moving in an interacting free electron gas. Explicit expressions for these
quantities are obtained either within the random-phase approximation (RPA) or
with full inclusion of short-range exchange and correlation effects. The Z^3
correction to the energy loss of ions is evaluated beyond RPA, in the limit of
low velocities.Comment: 5 pages, 2 figures To appear in Phys. Rev.
The influence of the dechanneling process on the photon emission by an ultra-relativistc positron channeling in a periodically bent crystal
We investigate, both analytically and numerically, the influence of the
dechanneling process on the parameters of undulator radiation generated by
ultra-relativistic positron channelling along a crystal plane, which is
periodically bent. The bending might be due either to the propagation of a
transverse acoustic wave through the crystal, or due to the static strain as it
occurs in superlattices. In either case the periodically bent crystal serves as
an undulator which allows to generate X-ray and gamma-radiation.
We propose the scheme for accurate quantitative treatment of the radiation in
presence of the dechanneling. The scheme includes (i) the analytic expression
for spectral-angular distribution which contains, as a parameter, the
dechanneling length, (ii) the simulation procedure of the dechanneling process
of a positron in periodically bent crystals. Using these we calculate the
dechanneling lengths of 5 GeV positrons channeling in Si, Ge and W crystals,
and the spectral-angular and spectral distributions of the undulator over broad
ranges of the photons. The calculations are performed for various parameters of
the channel bending.Comment: published in J. Phys. G: Nucl. Part. Phys. 27 (2001) 95-125,
http://www.iop.or
Modelisation of transition and noble metal vicinal surfaces: energetics, vibrations and stability
The energetics of transition and noble metal (Rh, Pd, Cu) vicinal surfaces,
i.e., surface energy, step energy, kink energy and electronic interactions
between steps, is studied at 0K from electronic structure calculations in the
tight-binding approximation using a {\it s, p} and {\it d} valence orbital
basis set. Then, the surface phonon spectra of copper are investigated in the
harmonic approximation with the help of a semi-empirical inter-atomic
potential. This allows to derive the contribution of phonons at finite
temperatures to the step free energy and to the interactions between steps. The
last part is devoted to the stability of vicinal surfaces relative to faceting
with special attention to the domain of orientations (100)-(111).
Semi-empirical potentials are shown to be not realistic enough to give a
reliable answer to this problem. The results derived from electronic structure
calculations predict a variety of behaviors and, in particular, a possible
faceting into two other vicinal orientations. Finally, temperature effects are
discussed. Comparisons are made with other theoretical works and available
experiments
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