X. Artru

Parametric X-rays and diffracted transition radiation in perfect and mosaic crystals

Abstract The amplitude of X-ray emission by relativistic electrons in a single crystal, calculated in the kinematical approach, is decomposed unambiguously in Diffracted Transition Radiation (DTR) and Parametric X-rays (PXR). DTR becomes significant for γ ≳ ω P ,γ being the Lorentz factor and ωP the plasma frequency. It is more collimated than PXR and, above threshold, its flux increases logarithmically with γ. However, it saturates with thickness at the Bragg primary extinction length le. This saturation is accounted for only in the dynamical approach, the formulas of which are compared to the kinematical ones. The respective contributions of DTR and PXR are calculated for a simple model of mosaic crystal, taking into account saturation of DTR with thickness. The PXR flux is basically the same as in a perfect crystal. If the size of the domains is larger than le, the DTR flux is multiplied by the number of domains crossed by the electron. For domains smaller than le and γ ≳ ω P , the DTR and PXR fluxes are of the same order of magnitude, up to logarithmic factors. In any case, mosaicity increases the total yield of X-ray photons.

RESOLUTION POWER OF OPTICAL TRANSITION RADIATION : THEORETICAL CONSIDERATIONS

Abstract The spatial resolution power of Optical Transition Radiation (OTR), applied to beam profile measurements at high Lorentz factor γ, is studied taking into account diffraction and self-diffraction effects. Microscopic and macroscopic points of view about the different geometrical natures of the forward and backward OTR sources are presented. Properties of the impact parameter profile I(b) of the OTR emitted by one electron are described. Curves for the modulation transfer function and for the OTR profile of a laminar beam are given. Two methods of improving the resolution are investigated : (a) putting a mask to eliminate small-angle photons, (b) using a polarizer.

Positron source using channeling in a tungsten crystal

Abstract A non-conventional positron source using 1.5–20 GeV electron beam impinging on a tungsten crystal oriented along its 〈111〉 axis is described. Computer simulation taking into account photon creation in the crystal, positron generation in a subsequent amorphous target and also positron capture and transport, allowed extensive investigation of this novel type of source. The possible performances are discussed hereafter. Positron yield has been calculated for different operating conditions. Comparisons with classical sources are presented.

Axial channeling of relativistic electrons in crystals as a source for positron production

Abstract In the framework of studies of positron sources dedicated to linear colliders, crystals are considered as photon radiators in association with thin amorphous converters or as compact positron sources where photon production and pair creation occur in the same medium. The main features of such sources in comparison with conventional amorphous sources are described. Taking linear collider requirements for such sources, a crystal converter suited for the Japanese Linear Collider (JLC) is considered. Limitations caused by radiation damage are mentioned and indications on critical fluences are given. A proof-of-principle experiment has been undertaken in Orsay, observing radiation enhancement in a tungsten crystal oriented along the 〈111〉 axis and submitted to a 2 GeV electron beam. The main results are reported.

Parametric X-ray radiation for calibration of X-ray space telescopes and generation of several X-ray beams

Abstract The setup based on a moderate energy linear accelerator (linac) is proposed to provide users with a tunable, quasi-monochromatic, linearly polarized X-ray beam. The effect of parametric X-ray radiation (PXR) from relativistic electrons in a crystal is used in an X-ray source. The application of the setup for calibration of X-ray space telescopes and other equipment is considered. The setup allows calibration of angular, spectral, polarization parameters of telescopes with smooth tuning of X-ray energy from several keV to hundreds of keV. Estimates of X-ray flux intensities and spectral line widths on the telescope aperture at a distance of several hundred meters are presented. Besides, the PXR-based facility for simultaneous generation of several X-ray beams is proposed.

Set-up optimization for an experimental test of a positron source using channeling

Abstract A new experiment is suggested in order to investigate the positron yield at the exit of crystal tungsten targets of different thicknesses submitted to 10 GeV electron beam at one of the SPS CERN transfer lines. The Set-up consists of the beam control counters, targets mounted on the goniometer, positron detector and photon detector. The positron detector is based mainly on a drift chamber inserted between the poles of the magnet. It allows to reconstruct the track of the charged particles, to define the vertex coordinates and initial angle of the particles at the exit of the target, to measure the total and transverse momenta. A Monte Carlo simulation corresponding to the theoretical description of chanelling radiation and also numerical calculation were provided for the estimations of the expected distributions and detector response. The optimization of the Set-up is considered, as well as the status of the project in general.

Advantages of axially aligned crystals used in positron production at future linear colliders

The characteristics of the electron-photon showers initiated by 2 to 10 GeV electrons aligned along the

Generation of circularly polarized photons for a linear collider polarized positron source

⟨111⟩

Experimental study of a crystal positron source

axis of tungsten crystals are compared with those for the amorphous tungsten. In this energy range, as known, the positron yield at the optimal target thicknesses is larger in a crystal case only by several percent. However, the amount of the energy deposition in a crystal turns out to be considerably (by 20%\char21{}50%) lower than in an amorphous target providing the same positron yield, while the peak energy-deposition density is approximately of the same magnitude in both cases.

Experimental investigations on geometrical resolution of optical transition radiation (OTR)

Various methods of obtaining longitudinally polarized positrons for future linear colliders are reviewed. Special attention is paid to the schemes using circularly polarized high-energy photons for positron production. Most effectively, such photons are obtained from electrons passing through a helical undulator or colliding with a circularly polarized laser wave. Spectrum and polarization of radiation emitted during helical motion of electrons are considered in detail. A new simple presentation of known formulas is used to account for the influence of the wave intensity, the electron-beam angular divergence, the collimation of radiation, and the lateral and temporal profiles of the laser bunch on the radiation properties.