High-Efficiency Deflection of High-Energy Protons through Axial Channeling in a Bent Crystal

Beam deflection due to axial channeling in a silicon crystal bent along the $⟨111⟩$ axis was observed with $400\mathrm{GeV}/c$ protons at the CERN Super Proton Synchrotron. The condition for doughnut scattering of protons by the atomic strings of the crystal was attained. Such a condition allowed one to observe a beam deflection of $50\mu \mathrm{rad}$ with about 30% efficiency. The contribution of hyperchanneled states of protons to the observed beam deflection was less than 2% according to simulation results.

Figure 1

(a) Schematic picture of a silicon crystal bent by anticlastic deformation along the $⟨111⟩$ axis. (b) Cross-sectional view of the crystal. Dimensions are expressed in millimeters. ${\theta }_v$ and ${\theta }_h$ are the orientation angles of the $⟨111⟩$ axis relative to the proton beam direction.

Figure 2

Beam intensity distribution as a function of the horizontal and vertical deflections at some orientation angles (${\theta }_v$) of the $⟨111⟩$ axis with respect to the incident beam direction: (a) ${\theta }_v=-40\mu \mathrm{rad}$, (b) ${\theta }_v=-15\mu \mathrm{rad}$, and (c) ${\theta }_v=0\mu \mathrm{rad}$. The beam particles shown in the figure are only those hitting the entry face of the crystal and satisfying the condition $({\sigma }_x^2+{\sigma }_y^2)<(5\mu \mathrm{rad}{)}^2$, where ${\sigma }_x$ and ${\sigma }_y$ are the horizontal and vertical values, respectively, of the single particle divergence at the crystal position. (d) Simulation of experimental conditions in case of perfect alignment of the $⟨111⟩$ axis with the beam direction, for comparison with the experimental case (c).

Figure 3

Projection on the horizontal plane of the unperturbed beam profile (curve 1) and of the distribution shown in Fig. 2c (curve 2). Each distribution is normalized to unity. The figure highlights that most of the particles in the incoming beam were shifted apart by significant angles through interaction with the crystal.