Volume Reflection of a Proton Beam in a Bent Crystal

Volume reflection predicted in the mid-1980s by Taratin and Vorobiev has been observed for the first time in the interactions of a 70 GeV proton beam with a short bent crystal. Incident protons deviate from convex atomic planes in the bulk of the crystal as a result of coherent interaction with bent lattice around the tangency point of particle trajectory with a curved atomic plane. The deflection angle $2{\theta }_R$ was found to be $(39.5\pm 2.0)\mu \mathrm{rad}$, or $(1.65\pm 0.08){\theta }_c$ in terms of the critical angle for channeling. The process has a large probability with respect to channeling and takes place in the angular range equal to the bend angle of atomic planes. It could possibly open new fields of application of crystals in high-energy particle beam optics.

Figure 1

Bent crystal used in the experiment. Proton beam crosses the crystal in the $Z$ direction.

Figure 2

Layout of the experiment. S1, S2, S3—scintillation counters.

Figure 3

Part of emulsion 2 with the profile of the proton beam. The white dashed line indicates the trace of the measurement with a microscope. The black dashed lines show $X$ readings, which correspond to the borders of the lines A, B, and C.

Figure 4

Proton trajectories crossing the crystal and emulsions in the horizontal plane (top view). The incident beam is shown as parallel (distance between the effective “source” of the beam and the crystal is much larger than a saddle radius of the crystal) with a uniform intensity distribution along the $X$ direction. The dots within the crystal indicate the tangency points of the incident proton trajectories with the (111) atomic planes where volume reflection takes place.