arXiv subjects

C. Ray

Publications and source records attributed to C. Ray.

7 recordsLinked to original sources

arXiv2010

Electron acceleration in a helical waveguide

Travelling wave in a helical wave guide is considered for electron acceleration. A first determination of the travelling wave modes using a partial wave expansion (PWE) and a point matching method is presented. It gives a rapid solution for moderate deformation of the guide relative to a straight cylinder. Strong deformations will give higher accelerating gradient but the PWE diverges. Two methods overcoming this difficulties are suggested.

physics.acc-ph↗

arXiv2009

Particle Acceleration in a Helical Wave Guide

The electromagnetic wave field propagating in a helical wave guide is decomposed in an angular momentum basis. Eigenmodes are calculated using a truncation in $l$ and a discretisation of the boundary condition. Modes slightly slower than light are considered for relativistic particle acceleration.

physics.acc-ph↗

arXiv2008

Monitoring the Bragg peak location of 73 MeV/u carbon ions by means of prompt $γ$-ray measurements

By means of a time-of-flight technique, we measured the longitudinal profile of prompt $γ$-rays emitted by 73 MeV/u $^{13}$C ions irradiating a PMMA target. This technique allowed us to minimize the shielding against neutrons and scattered $γ$-rays, and to correlate prompt gamma emission to the ion path. This correlation, together with a high counting rate, paves the way toward real-time monitoring of the longitudinal dose profile during ion therapy treatments. Moreover, the time correlation between the prompt gamma detection and the transverse position of the incident ions measured by a beam monitor can provide real-time 3D control of the irradiation.

physics.atom-ph↗

arXiv2008

Interference and Shadow Effects in the Production of Light by Charged Particles in Optical Fibers

A charged particle passing through or near a narrow optical fiber induces, by polarisation, coherent light guided by the fiber. In the limit of zero crossing angle, the radiation tends towards a Cherenkov radiation with a discrete spectrum, studied by different authors. If the particle crosses a bent fiber at regularly spaced points, interference gives quasi-monochromatic lines. If the particle passes near an end of the fiber, light is produced by the capture of virtual photons through the end face. An alternative way consists in sticking a metallic ball to the fiber: the passing particle induces plasmons which are then evacuated as light in the fiber. Interferences can occur between lights from several ends or balls. Applications of these various light signals to beam diagnostics are discussed. The shadow effect, which reduces the photon yield when the particle runs parallel to a row of balls, is pointed out and an upper bound $-dE/dz\le C(Ze/b)^2$ for the particle energy loss is conjectured ($Ze$ is the particle charge, $b$ the impact parameter and $C$ a numerical constant). This bound should also apply to other kinds of light sources, in particular to Smith-Purcell radiation.

physics.ins-det↗

arXiv2006

Photon production by charged particles in narrow optical fibers

A charged particle passing through or by an optical fiber induces emission of light guided by the fiber. The formula giving the spontaneous emission amplitude are given in the general case when the particle trajectory is not parallel to the fiber axis. At small angle, the photon yield grows like the inverse power of the angle and in the parallel limiting case the fiber Cherenkov effect studied by Bogdankevich and Bolotovskii is recovered. Possible application to beam diagnostics are discussed, as well as resonance effects when the particle trajectory or the fiber is bent periodically.

hep-ph↗

arXiv2005

Complex networks in brain electrical activity

We analyze the complex networks associated with brain electrical activity. Multichannel EEG measurements are first processed to obtain 3D voxel activations using the tomographic algorithm LORETA. Then, the correlation of the current intensity activation between voxel pairs is computed to produce a voxel cross-correlation coefficient matrix. Using several correlation thresholds, the cross-correlation matrix is then transformed into a network connectivity matrix and analyzed. To study a specific example, we selected data from an earlier experiment focusing on the MMN brain wave. The resulting analysis highlights significant differences between the spatial activations associated with Standard and Deviant tones, with interesting physiological implications. When compared to random data networks, physiological networks are more connected, with longer links and shorter path lengths. Furthermore, as compared to the Deviant case, Standard data networks are more connected, with longer links and shorter path lengths--i.e., with a stronger ``small worlds'' character. The comparison between both networks shows that areas known to be activated in the MMN wave are connected. In particular, the analysis supports the idea that supra-temporal and inferior frontal data work together in the processing of the differences between sounds by highlighting an increased connectivity in the response to a novel sound.

physics.bio-ph↗

arXiv2005

Electron gas polarization effect induced by heavy H-like ions of moderate velocities channeled in a silicon crystal

We report on the observation of a strong perturbation of the electron gas induced by 20 MeV/u U$^{91+}$ ions and 13 MeV/u Pb$^{81+}$ ions channeled in silicon crystals. This collective response (wake effect) in-duces a shift of the continuum energy level by more than 100 eV, which is observed by means of Radiative Electron Capture into the K and L-shells of the projectiles. We also observe an increase of the REC probability by 20-50% relative to the probability in a non-perturbed electron gas. The energy shift is in agreement with calculations using the linear response theory, whereas the local electron density enhancement is much smaller than predicted by the same model. This shows that, for the small values of the adiabaticity parameter achieved in our experiments, the density fluctuations are not strongly localized at the vicinity of the heavy ions.

physics.atom-ph↗