Steering of a Sub-GeV Electron Beam through Planar Channeling Enhanced by Rechanneling

A. Mazzolari, E. Bagli, L. Bandiera, V. Guidi, H. Backe, W. Lauth, V. Tikhomirov, A. Berra, D. Lietti, M. Prest, E. Vallazza, and D. De Salvador

Phys. Rev. Lett. 112, 135503 – Published 2 April 2014

Abstract

We report the observation of efficient steering of a 855 MeV electron beam at MAMI (MAinzer MIkrotron) facilities by means of planar channeling and volume reflection in a bent silicon crystal. A $30.5 μ m$ thick plate of (211) oriented Si was bent to cause quasimosaic deformation of the (111) crystallographic planes, which were used for coherent interaction with the electron beam. The experimental results are analogous to those recorded some years ago at energy higher than 100 GeV, which is the only comparable study to date. Monte Carlo simulations demonstrated that rechanneling plays a considerable role in a particle’s dynamics and hinders the spoiling of channeled particles. These results allow a better understanding of the dynamics of electrons subject to coherent interactions in a bent silicon crystal in the sub-GeV energy range, which is relevant for realization of innovative x-ray sources based on channeling in periodically bent crystals.

Received 2 December 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.135503

Authors & Affiliations

A. Mazzolari, E. Bagli, L. Bandiera, and V. Guidi^*

INFN Sezione di Ferrara, Dipartimento di Fisica e Scienze della Terra, Università di Ferrara Via Saragat 1, 44100 Ferrara, Italy

H. Backe and W. Lauth

Institut für Kernphysik der Universität Mainz, Fachbereich Physik, Mathematik und Informatik, D-55099 Mainz, Germany

V. Tikhomirov

Research Institute for Nuclear Problems, Belarusian State University, Bobruiskaya street, 11, Minsk 220030, Belarus

A. Berra, D. Lietti, and M. Prest

Università dell’Insubria, via Valleggio 11, 22100 Como, Italy and INFN Sezione di Milano Bicocca, Piazza della Scienza 3, 20126 Milano, Italy

E. Vallazza

INFN Sezione di Trieste, Via Valerio 2, 34127 Trieste, Italy

D. De Salvador

INFN Laboratori Nazionali di Legnaro, Viale dell’Università 2, 35020 Legnaro, Italy and Dipartimento di Fisica, Università di Padova, Via Marzolo 8, 35131 Padova, Italy

^*Corresponding author. guidi@fe.infn.it

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Vol. 112, Iss. 13 — 4 April 2014

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Images

Figure 1
(a) Bending of a silicon platelike crystal with properly chosen crystallographic orientations generates the quasimosaic effect, resulting in a secondary bending of the planes lying in the crystal thickness. (b) Sketch of the experimental setup. The dashed arrow indicates the incoming beam, impinging on the crystal mounted on a high-precision goniometer (G). The solid-black arrow indicates particles deflected thanks to planar channeling, while solid-white arrow correspond to overbarrier particles. A silicon detector, (D), reconstructs the beam profile after interaction with the crystal.
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Figure 2
(a) An “angular scan” recorded during the interaction between the crystal and the electron beam. Six different regions can be distinguished: (1) and (6) nonchanneling, (2) channeling, (3) dechanneling, (4) volume reflection, and (5) volume capture. (b) The bent crystal is aligned with respect to the charged particle beam to excite planar channeling. Overbarrier particles are deflected toward the opposite side as that of crystal bending. Underbarrier particles are captured under the channeling regime (CH). Because of multiple scattering, channeled particles may suffer dechanneling (DCH). A fraction of dechanneled particles experiences rechanneling (RCH). (c) Bent crystal is aligned with respect to the incoming particle beam (red-dashed arrow) in such a way that the beam trajectory becomes tangent to the atomic planes inside the crystal. Under such regime, either VR or the competitive process of VC occurs.
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Figure 3
(a) Beam profile with the crystal aligned for VR and VC: solid line for experimental and dash line with dots for simulation. A largest fraction of the beam is steered toward the direction opposite of channeling (left peak). VR efficiency is limited by VC (smaller right peak), deflecting particles along the crystal bending. (b) Beam profile with the crystal aligned for channeling (solid line experimental, line with squares simulation). A large fraction of the beam is deflected to the nominal deflection of the crystal (right peak). Particles found in over barrier states at the crystal entry face populate the left peak in the distribution, while the region between the two peaks is populated by particles suffering dechanneling.
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Figure 4
(a) Monte Carlo simulation of the contributions to the angular distribution originated from rechanneled particles. The red line represents the whole distribution; black line the particles that have never been rechanneled; green, blue, and light blue lines the distribution of the particles rechanneled 1, 2, and 3 times, respectively. (b) The red curve represents the fraction of channeled particles summing up the contribution of never dechanneled particles and rechanneled particles. The magenta curve represents the fraction of never dechanneled particles. Because of crystal bending the fraction of channeled particles at the crystal entry face is not equal to unity because the centrifugal force renders asymmetric the potential, and some of the particles (impact parameter between 0 and $\sim 0.6 Å$ ) are not channeled (see the inset).
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