Deflection of high-energy negative particles in a bent crystal through axial channeling and multiple volume reflection stimulated by doughnut scattering.

Different kinds of deflection in a silicon crystal bent along the ⟨111⟩ axis was observedfor 150GeV/c negative particles, mainly π− mesons, at one of the secondary beams of the CERN SPS. The whole beam was deflected to one side in quasi-bound states of doughnut scattering (DSB) by atomic strings with the efficiency (95.4 ±0.2)% and with the peak position close to the bend crystal angle, α=185μrad. It was observed volume capture of π− mesons into the DSB states with a probability higher than 7%. A beam deflection opposite to the crystal bend was observed for some orientations of the crystal axis due to doughnut scattering and subsequent multiple volume reflections of π− mesons by different bent planes crossing the axis.