The volume reflection phenomenon was detected while investigating 400 GeV proton interactions with bent silicon crystals in the external beam H8 of the CERN Super Proton Synchrotron. Such a process was observed for a wide interval of crystal orientations relative to the beam axis, and its efficiency exceeds 95%, thereby surpassing any previously observed value. These observations suggest new perspectives for the manipulation of high-energy beams, e.g., for collimation and extraction in new-generation hadron colliders, such as the CERN Large Hadron Collider.

Abstract. IHEP Protvino has pioneered the wide practical use of bent crystals as optical elements in high-energy beams for beam extraction and deflection on permanent basis since 1989. In the course of IHEP experiments, crystal channeling has been developed into efficient instrument for particle steering at accelerators, working in predictable, reliable manner with beams of very high intensity over years. Crystal systems extract 70 GeV protons from IHEP main ring with efficiency of 85% at intensity of 10 12 , basing on multi-pass mechanism of channeling proposed theoretically and realised experimentally at IHEP. Today, six locations on the IHEP 70-GeV main ring of the accelerator facility are equipped by crystal extraction systems, serving mostly for routine applications rather than for research and allowing a simultaneous run of several particle physics experiments, thus significantly enriching the IHEP physics program. The long successful history of large-scale crystal exploitation at IHEP should help to incorporate channeling crystals into accelerator systems worldwide in order to create unique systems for beam delivery. We report recent results from the research and exploitation of crystal extraction systems at IHEP.

A silicon crystal was used to channel and extract 70 GeV protons from the U-70 accelerator with an efficiency of 85.3 6 2.8%, as measured for a beam of ϳ10 12 protons directed towards crystals of ϳ2 mm length in spills of ϳ2 s duration. The experimental data follow very well the prediction of Monte Carlo simulations. This demonstration is important in devising a more efficient use of the U-70 accelerator in Protvino and provides crucial support for implementing crystal-assisted slow extraction and collimation in other machines, such as the Tevatron, RHIC, the AGS, the SNS, COSY, and the LHC.

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 R was found to be 39:5 2:0 rad, or 1:65 0:08 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. DOI: 10.1103/PhysRevLett.97.144801 PACS numbers: 29.27.ÿa, 61.85.+p Charged particles, incident to a properly oriented bent crystal, can be captured in the channeling regime and follow the crystal curvature, deviating from the initial path [1]. Capture occurs with the particles traveling in a direction tangential to the atomic planes. As the tangency point lies at the entry face of the crystal a surface capture occurs, while for volume capture the tangency point is inside the crystal volume. It was found [2] that only a small part of the particles with the appropriate parameters for being volume captured are indeed channeled. Nonvolume-captured particles were called reflected particles by Sumbaev [3].In the mid-1980s, Taratin and Vorobiev developed an accurate model of the particle motion in the bent crystal and studied with this model the volume capture process. They explained [4] the volume capture as a result from the multiple scattering of the incident particles on electrons and nuclei of the crystal material. But in addition to the volume capture process, they found [5,6] that interaction of the particles with the continuous periodical potential of the bent crystal lattice in the vicinity of the tangency point should lead to an angular deflection of the reflected particles, almost twice the channeling critical angle in the direction, opposite the atomic plane bending. They called this process a volume reflection effect.We have carried out a crystal channeling experiment at IHEP (Protvino) in which we observed, in the first time, the volume reflection phenomenon. In this Letter, we present a main result of the experiment; the more detailed description of the study can be found in [7].In the experiment we applied a very short (0.72 mm along the beam) bent silicon crystal prepared using the elastic quasimosaicity effect [8][9][10]. This effect originates from crystal anisotropy and results in curving of the normal cross sections of the crystal plate under bending. The crystal plate was cut from silicon ingot as described in [11]. The plate sizes were 0:72 20 60 mm 3 , with the channeling (111) planes parallel to the 0:72 60 mm 2 face. The plate was bent in the YZ plane with a radius of curvature of 48 cm (Fig. 1) inducing a quasimosaic curvature of the atomic (111) planes in th...

This paper presents a detailed study of the deflection phenomena of a 400 GeV=c proton beam impinging on a new generation of bent silicon crystals; the tests have been performed at the CERN Super Proton Synchrotron H8 beam line. Channeling and volume reflection angles are measured with an extremely precise goniometer and with high resolution silicon microstrip detectors. Volume reflection has been observed and measured for the first time at this energy, with a single-pass efficiency as large as 98%, in good agreement with the simulation results. This efficiency makes volume reflection a possible candidate for collimation with bent crystals at the CERN Large Hadron Collider.