Coherent Effects of High-Energy Particles in a GradedSi1−xGexCrystal

Abstract

A graded Si(1-x)Ge(x) crystal has been manufactured for operation with high-energy protons to excite coherent interactions of the particles with the crystal such as channeling and volume reflection. The crystal had the shape of a parallelepiped though its (111) atomic planes were curved at a radius of 25.6 m because of the graded Ge content. The crystal was exposed to a 400 GeV/c proton beam at the external lines of CERN Super Proton Synchrotron to probe its capability to steer high-energy particles. Measured deflection efficiency was 62.0% under planar channeling and 96.0% under volume reflection. Such values are critically compared to their counterparts for a standard bent Si crystal under peer conditions. A Monte Carlo simulation of the dynamics of channeled and volume reflected particles in a graded crystal including the effect of Ge impurities and of lattice dislocations has been carried out. We found that the effect of crystal imperfections spoiled the efficiency of channeling while it negligibly affected the performance of volume reflection. We finally propose the usage of the graded crystal as a primary scatterer to aid halo collimation for the new generation of hadronic machines. As a unique feature, a properly cut graded crystal circumvents the problem of the miscut angle, which is currently a severe limitation for implementation of crystal-assisted collimation.

Experimental evidence of planar channeling in a periodically bent crystal

“…In order to manufacture a working CU, a crystal with very low density of defects should be used, together with high accuracy in bending the crystallographic planes. Indeed, the capture efficiency of channeling is strongly reduced by the presence of crystalline defects [20], and the coherence of the emitted X-and γ-rays relies on the accuracy in the periodically bending of the crystallographic planes. Various methods have been proposed for the realization of a periodically bent crystal, such as acoustic-wave transmission [16], periodically-graded Si 1−x Ge x structures [21], periodic-surface deformations obtainable via superficial grooves [22,23], laser ablation [24], or film deposition [25,26].…”

Simulation of orientational effects in crystals with structural defects through DYNECHARM++

“…An application example can be found in Ref. [10]. Interplanar potential for a perfect crystal 3 vacancies per cm 22 Interplanar potential for a crystal with 10 Nuclei density distribution Interstitial defect density distribution Fig.…”

“…As an example, Ge crystals with a sufficiently high crystalline perfection demonstrated to overcome the performances of Si crystals under the same condition [8,9]. Recently, a new collimation scheme based on self-bent graded Si x Ge 1Àx crystal was proposed, by exploiting the volume reflection effect [10]. Although the designed scheme looks promising for a low cost upgrade of actual collimators, due to the presence of dislocations the efficiency of deflection for channeling was limited, pointing out that few defects of the crystalline structure, 10 2 per cm 2 dislocations, may affect the efficiency of orientational effects.…”

Orientational Coherent Effects of High-Energy Particles in aLiNbO3Crystal

“…The intrinsic curvature of such systems would restrain the detrimental impact of any mechanical bending device. However, the presence of ∼10 2 dislocations per cm 2 induced by the growth process reduces the deflection efficiency for channeling to 62.5% against the predicted 76.5%, while leaving almost unaltered the efficiency for volume reflection at 96.0% [32].…”

“…However, particular care is needed in the selection of starting material in terms of defect concentration. Recently, coherent effects at high energy in a self-bent graded Si 1−x Ge x crystals were observed [32]. The intrinsic curvature of such systems would restrain the detrimental impact of any mechanical bending device.…”