A. Baurichter

Channeling of high-energy particles in bent crystals - Experiments at the CERN SPS

Abstract During the latest decade, experiments have been performed at the CERN SPS to investigate the use of high-energy channeled nuclei in bent crystals for extraction, beam splitting and beam bending. An understanding of channeling in a bent crystal with extraction and deflection efficiencies for different energies, crystal types and ions has been developed. Furthermore, the long-standing question of radiation damage has been addressed with encouraging outcome. This makes extrapolations possible for the construction of, e.g., an extraction device for the LHC at CERN, RHIC at Brookhaven or new splitting elements in high-energy beams.We present the main results obtained and discuss existing and future applications of bent crystals in high-energy physics.

Measurement of pair production by high-energy photons in an aligned tungsten crystal

Abstract A new measurement has been made of the rate of pair-production in a 3.2 mm thick tungsten crystal, exposed to photons with energies in the range 10 to 150 GeV, for angles of incidence up to 10 mrad from the crystal axis. A strong enhancement of the pair-production rate is observed when the beam is aligned along the 〈100〉 crystal axis, as compared to a random orientation. This effect can be exploited in the NA48 CP-violation experiment by using a thin crystal rather than an amorphous material to convert photons, thus minimising the scattering of kaons in the converter.

NEW RESULTS FROM THE CERN-SPS BEAM DEFLECTION EXPERIMENTS WITH BENT CRYSTALS

Abstract Results from five distinct bending experiments performed recently in the H8 beam at CERN are presented. Firstly, deflection of a positive pion beam at 200 GeV/c is compared to the “standard” 450-GeV/c proton beam for a bending angle of 3.1 mrad along the (111) plane in a 50 mm silicon crystal. Second, deflection of negative pions at 200 GeV/c is investigated for the same crystal, for incidence along the (111) plane as well as the 〈110〉 axis. Small deflection effects are seen, but no negative particles are bent through the full bending angle of the crystal. Third, the first results from beam deflection at high energy using a germanium crystal are shown. Slightly higher deflection efficiencies than for silicon are seen for large bending angles, but significantly smaller than expected for such a crystal with higher atomic number. Fourth, deflection efficiencies using a strongly irradiated silicon crystal have been measured for the first time, and a small reduction in efficiency is seen in the irradiated region. Finally, deflection of positive particles using axial alignment of a bent silicon crystal has been investigated at 450 GeV/c. Qualitatively similar behaviour as in previous experiments at 12 GeV/c is seen; the beam splits into several beams corresponding to the different planes, and even weak planes are observed.

Conduction Cooled High Temperature Superconducting Dipole Magnet for Accelerator Applications

A 3T proof-of-principle dipole magnet for accelerator applications, based on 2nd generation high temperature superconducting tape was designed, built, and tested by a consortium under the lead of Danfysik. The magnet was designed to have a straight, circular bore with a good field region of radius r = 25 mm, and a magnetic length of 250 mm. A total length of 2.5 km YBCO-based copper stabilized conductor supplied by SuperPower Inc., NY, USA, was isolated with 0.025 mm of epoxy and subsequently wound into 14 saddle coils and 4 racetrack coils with a cosine theta like configuration. The coils were impregnated with epoxy and mounted between a support of stainless steel and a collar made from aluminum. The cold mass consisting of the coil assembly and a laminated steel yoke is cooled by two cryocoolers from via copper bars to below 20 K. Current leads were made from the same batch of HTS tape. Cryogen-liquid free operation of an HTS accelerator magnet was demonstrated. The cold mass support design permits magnet orientation under arbitrary angles. Careful choice of materials in terms of magnetic, heat conducting and mechanical properties resulted in a robust and compact solution which opens up for accelerator applications in many fields, in particular where cryogenic liquid cooling is not an option.

Enhanced electromagnetic showers initiated by 20–180 GeV gamma rays on aligned thick germanium crystals

Abstract The distribution of the energy released in a silicon detector placed on the downstream side of thick germanium single crystals bombarded by 20–180 GeV gamma rays along directions close to the 〈1 1 0〉 axis or along a random direction has been investigated. A large enhancement of the shower for axial incidence of the gamma rays has been found. The response of the system composed of a germanium crystal and a silicon detector to single gamma rays as a function of their energy has been deduced and compared with existing Monte Carlo simulations.

Enhancement of electromagnetic showers initiated by ultrarelativistic electrons in aligned thick germanium crystals

Abstract The distribution of the energy deposited in thin silicon detectors placed on the downstream side of a thick germanium single crystal bombarded with a 70, 150 and 250 GeV electron beam along directions close to the 〈110〉 axis or {110} and {100} planes has been measured. The enhancement of the shower with respect to random incidence, as reflected in the higher value of the centroid of the distribution, is studied as a function of the incidence angle to the axis or plane.

Superconducting Solenoids for RF Accelerators and Electron Guns

Compact solenoids are applied at an increasing rate for beam focusing in superconducting RF accelerating structures, for instance in secondary ion beam linacs and for free electron lasers. High field solenoids with a center field of 7.9 T for the HIE-ISOLDE high-beta cryomodules at CERN are currently being designed and manufactured at Danfysik. NbTi coils and a soft iron yoke are carefully designed for operation in a LHe bath at a temperature of 4.5 K at field integral requirements of ≤ 13.5 T2m. The quenching characteristics of this solenoid have been studied and simulated, and a passive quench protection circuit is part of the solenoid design. The yoke outer diameter is 200 mm; the total length 312 mm. An HTS solenoid based on second-generation high-temperature superconducting technology was designed, built, tested, and delivered by Danfysik to the free electron laser project of the University of Wisconsin. The compact 136-mm-long HTS solenoid with a center field of 0.2 T operates in the temperature range between 5 and 70 K. The field integral is 3.1 T2mm with very small integrated field errors. Conduction cooling of the compact solenoid makes it easy to assemble and operate very close to the RF cavity.

New results from the CERN SPS beam deflection experiments with bent crystals

Abstract Results from five distinct bending experiments performed recently in the H8 beam at CERN are presented. Firstly, deflection of a positive pion beam at 200 GeV/c is compared to the “standard” 450-GeV/c proton beam for a bending angle of 3.1 mrad along the (111) plane in a 50 mm silicon crystal. Second, deflection of negative pions at 200 GeV/c is investigated for the same crystal, for incidence along the (111) plane as well as the 〈110〉 axis. Small deflection effects are seen, but no negative particles are bent through the full bending angle of the crystal. Third, the first results from beam deflection at high energy using a germanium crystal are shown. Slightly higher deflection efficiencies than for silicon are seen for large bending angles, but significantly smaller than expected for such a crystal with higher atomic number. Fourth, deflection efficiencies using a strongly irradiated silicon crystal have been measured for the first time, and a small reduction in efficiency is seen in the irradiated region. Finally, deflection of positive particles using axial alignment of a bent silicon crystal has been investigated at 450 GeV/c. Qualitatively similar behaviour as in previous experiments at 12 GeV/c is seen; the beam splits into several beams corresponding to the different planes, and even weak planes are observed.