Beam Steering and Radiation Generation of Electrons in Bent Crystals in the Sub-GeV Domain

TL;DR

This work addresses beam steering and high-intensity radiation generation from sub-GeV electrons in bent Si crystals. It combines targeted experiments at 855, 600, and 300 MeV with Geant4-based simulations to quantify channeling, dechanneling, rechanneling, and volume capture, linking microscopic dynamics to macroscopic deflection and photon emission. Key findings include channeling efficiencies exceeding 50% at 300 MeV, radiation enhancements up to about 6x relative to random orientation, and a dominant role for dechanneling and rechanneling in radiative output at low energy. These results establish bent crystals as feasible tools for compact beam manipulation and tunable photon sources at low energies and provide a framework for optimizing crystal parameters and beam energy in accelerator facilities.

Abstract

We present an investigation into beam steering and radiation emission by sub-GeV electrons traversing bent silicon crystals. Using 855, 600, and 300~MeV electron beams at the Mainz Microtron (MAMI), we explored orientational coherent effects and particle dynamics in a 15~$μ$m-thick crystal bent along the (111) planes. Combined experimental and simulation analyses enabled the classification and quantitative assessment of the contributions from channeling, dechanneling, rechanneling, and volume capture to both beam deflection and radiation emission. Crystal steering remained effective even at 300~MeV, with measured channeling efficiencies exceeding 50\%, a record at such low energy. Channeling and volume reflection enhanced radiation emission by up to a factor of six compared to the misaligned orientation, highlighting strong orientational coherence effects in the sub-GeV regime. These findings confirm the feasibility of using bent crystals for efficient beam manipulation and high-intensity photon generation at low energies, supporting the development of novel light sources and beam control strategies at accelerator facilities operating in this energy range.

Figures (7)

• Figure 1: (a) Potential energy of bent Si(111) planes for 300 (red), 600 (green), and 855 (blue) MeV electrons. The horizontal line at $x=0$ indicates the reference potential, highlighting how the potential-well amplitude increases as the beam energy decreases. (b) Illustration of the main dynamical regimes experienced by charged particles: Over-barrier (OB), Volume Reflection (VR), Channeling (CH), Dechanneling (DCH), Rechanneling (RCH), and Volume Capture (VC).
• Figure 2: Schematic representation of the dechanneling process in a bent crystal. Curved black lines: atomic planes; colored lines: representative particle trajectories. The mean dechanneling length $\langle L_d\rangle$ and the corresponding angle $\langle\vartheta_\text{dech}\rangle$ are related through the crystal curvature $R_{\text{bending}}$.
• Figure 3: (a) Scheme of the experimental setup at MAMI B, adapted from bandiera-2021.(b) Geometry of the bent Si(111) crystal mounted on the holder that imparts the deformation
• Figure 4: Experimental (a–c) and simulated (d–f) angular scans at the investigated energies. Six regions can be distinguished: (1)–(6) amorphous, (2) channeling, (3) dechanneling, (4) volume reflection, and (5) volume capture.
• Figure 5: Profiles of the deflected beam under channeling conditions. Experimental data (solid blue) are compared with simulated distributions (dashed), whose sub-components correspond to distinct mechanisms within the bent crystal. The contribution labeled "dechanneling + rechanneling" includes all trajectories that underwent at least one dechanneling event.