Ch. Trikalinos

Influence of channeling on scattering of low-energy antiprotons transmitted through thin crystals

Abstract A computer simulation study of the transmission of low-energy antiprotons through a thin single crystal, along one of the axial directions, is reported. The angular distribution of the scattered antiprotons behind the crystal is analysed. As an example the scattering of 300 keV antiprotons moving along the 〈110〉 axis in 500 A and 2000 A thick silicon crystal is considered. The results of the calculation show the formation of the ring-shaped patterns (doughnuts) for the incident angles of the order of the critical angle for channeling. A comparison of the angular distribution of antiprotons with that of protons is made. It is shown that the randomization of the transverse momentum is much quicker for antiprotons than for protons, which leads to a more uniform azimuthal angular distribution. The blocking effect in the transverse plane, known for protons, is not found for antiprotons. The influence of the doughnut distribution on the results of the transmission experiments with small acceptance angle detectors is discussed.

Energy loss of axially channeled antiprotons

Abstract A detailed study of energy spectra (channeled fraction, most probable energy loss, shape of the spectra) of low energy antiprotons moving in a thin crystal, under axial channeling conditions is performed using a Monte Carlo computer simulation. The results of calculations for antiprotons are compared with those for protons. It was observed that the channeled fraction of the antiproton beam is considerably smaller than that for protons due to strong dechanneling. This fraction as a function of incident angle shows a maximum at angles near ψ = 0.5ψL. Whereas for protons, well aligned with a crystal axis, the energy loss is onsiderably reduced as compared to the random beam, for antiprotons the energy loss is nearly the same as for those in a random direction. The spectra for antiprotons are wider than those for protons and have a high-loss tail. In general the directional effects in energy spectra are less pronounced for antiprotons than for protons.

Channeling of protons in radially compressed carbon nanotubes

Channeling of 10 MeV protons in radially compressed chiral carbon nanotubes is considered. Monte Carlo simulation program is used for the calculation of the trajectories, energy losses and angular distributions of protons in nanotubes of various lengths, where the potential in Doyle-Turner approximation is used to describe the interaction between a proton and a nanotube. Calculations were carried out for different incident angles between proton beam and the nanotube axis. The results show that a decreased angular distribution of the beam is observed, compared with propagation through a straight nanotube, in case when it enters from the compressed end of the nanotube. The energy and spatial distribution of channeled protons in compressed nanotubes is examined.

Analytical and numerical calculations of the elastic force fluctuation in planar channeling

Abstract The fluctuation of the force of elastic interaction of a channeled particle with atomic planes in a crystal due to discreteness and thermal vibrations of atoms is studied. We give an analytical formula and results calculated by Monte-Carlo method. We observe good agreement between two methods. Such comparison has been carried out also for the case of axial channeling.

INVESTIGATION OF THE SAMARIUM ROLE IN THE PHYSICAL PROPERTIES OF Sm0.5Re0.5Ba2Cu3O7−δ COMPOUNDS IN THE ORTHORHOMBIC AND TETRAGONAL STRUCTURES

Perovskites of type Sm0.5Re0.5Ba2Cu3O7−δ (Re=Y, La, Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu) have been investigated by XRD, magnetic and EPR measurements in both orthorhombic and tetragonal phases. In the former case the usual superconducting phase transition in the range 90 to 95 K has been detected. In all the compounds an EPR spectrum of Cu2+ ions has been observed. In the tetragonal phase, the EPR spectra of trivalent rare earth ions have been recorded for some members of the series. The observation of a broad EPR signal, tentatively attributed to superexchange interaction over the oxygen bridges <O2), has also been recorded. Furthermore, the intensity of low-field microwave absorption has been found to depend strongly on the nature of the substituted rare earth ion at the sites of Y ions of the YBa2Cu3O7−δ type compounds.

The improved point matching method for triangular metal gratings

Abstract In this paper, we present the “point matching method” (PMM) adapted for the calculation of Smith–Purcell (SP) radiation parameters in H-polarization, generated by relativistic electron beams (REB) passing close to a triangular metal grating. A truncated expression Hdy,n(N;β,ω) was used in order to find the coefficients Hdy,n(β,ω) in the infinite Rayleigh expansion of the diffracted field of a moving electron. A linear system of 2N+1 equations was solved, increasing the number N of harmonics until convergence solutions were achieved. When N→∞ then lim N→∞ H d y,n (N;β,ω)=H d y,n (β,ω) and the diffracted field can be calculated. SP radiation factors were calculated using the modified PMM for electron energies in 1–30 MeV domain and metal gratings with period length in 1–10 mm range.

Monte Carlo calculations and experimental setup for a Smith-Purcell experiment in Bucharest and Athens

Abstract A study of energy spectra (Smith–Purcell power, energy peaks, and shape of the spectra) is performed using a Monte Carlo simulation method. This was done in order to examine the dependence from the experimental parameters. The setup for an experiment looking for Smith–Purcell radiation in the IR and FIR region is also presented. Experiment is taking place at the beginning of year 2000 in Bucharest (Romania) at the National Institute for Laser, Plasma and Radiation Physics, at the betatron facilities of the Electron Accelerator Laboratory with an electron beam up to 30 MeV. This experiment will be continued later on in Athens (Greece), at the Institute of Accelerating Systems and Applications (IASA) with a linac delivering up to 10 MeV CW electron beam.

Critical radius in axial channeling

Abstract When studying phenomena related to the channeling of charged particles it is of great importance to consider them as functions of the impact parameter. This point of view plays an important role to the interpretation of experimental results as well as to computer simulations. Thus the distance of minimum approach, for which channeling conditions still hold, either under axial or planar channeling is of importance. In the present work we use the Monte-Carlo method to investigate the correlation of the distance of minimum approach of axially channeled particles with the exit angle of those particles from the crystal. We calculated the distance of minimum approach for several incidence angles at different temperatures and for many crystal thickness. The results we obtained prove that the distance of minimum approach has two main properties. (a) It is independent of the crystal thickness and depends only slightly on the incidence angle (as long as the incidence angle does not exceed the Lindhard angle) and slightly decreases with increasing temperature. (b) It agrees with Lindhard’s theory predictions but at the same time it seems that some particles approach too close to the crystal axis without being dechanneled.

CHANNELING RADIATION RELATION WITH THE MEAN NUMBER OF CHANNELED PARTICLES

Abstract We considered planar positron channeling in {110} planes in Silicon. We used the appropriate Fokker-Planck type kinetic equation to calculated the values of the flux distribution function at several depths for the energies of 1 GeV, 5 GeV and 10 GeV and incidence angles 0, 0.5 Ψ L , and 0.8 Ψ L , where Ψ L is the Lindhard angle. Using the values of the flux distribution function at the critical transverse energy, we calculated the mean number of particles with transverse energies equal to the critical one. The first maximum of the radiation intensity is proportional to this number. We conclude that the first maximum of the radiation intensity versus particle is expected to decrease dramatically with increasing depth.