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

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