M. Clément

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.

Observation of high deflection efficiency and narrow energy loss distributions for 450 GeV protons channeled in a bent silicon crystal

Abstract A 450 GeV proton beam has been deflected by various angles from 1 to 11 mrad using planar channeling in a (111) silicon crystal which was mechanically bent to achieve the desired beam deflection. High deflection efficiencies of up to 50% have been measured, in good agreement with present theoretical estimates. It is shown that bent crystals are also a unique tool for measurements of energy loss and straggling of channeled particles, without any influence from random particles: Selecting protons which are deflected by increasing angles corresponds to decreasing the transverse energy at the crystal entrance. With this technique energy loss and straggling was measured for protons channeled in the wide and narrow (111) planes in silicon for the first time.

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.

High efficiency bending of 450 GeV protons using channeling

Abstract For the first time more than 10% of a 450 GeV proton beam has bent using planar channeling in a 5 cm long single crystal of silicon. The bent beam is very narrow in angle (±5 μ rad), symmetrical and has suffered an energy loss of ∼60% of random energy loss. Some of the unbent particles, however, lose more energy than random particles and the straggling is 50% larger than random. From the unbent fraction the bending dechanneling is inferred and found to be in agreement with calculations. The bending efficiency is in good agreement with theoretical dechanneling estimates. The results present very interesting perspectives for extracting beams from future TeV accelerators.

Deflection of 200 GeV/c and 450 GeV/c positively charged particles in a bent germanium crystal

Abstract Experimental results on high-energy beam deflection by means of a bent germanium crystal are presented. Record deflection efficiencies of 60% have been found for 450 GeV/c protons at small angles. The results obtained with 200 and 450 GeV/c positively charged particles are well described by a classical model, giving confidence in predictions for other crystals and different beam momenta.

Deflection of 450 GeV protons by planar channeling in a bent silicon crystal

Abstract A 450 GeV proton beam has been bent by various angles from 4 to 14 mrad using planar channeling in a (111) silicon crystal. Detailed investigations of the deflected beam as well as the unbent and scattered particles have been performed. The incident beam had a divergence of about 35 μrad (FWHM). 20% of the protons hitting the crystal front face were found to be initially channeled. The measured bending efficiencies range from 5 to 2% (for increasing deflection angles) are compared to theoretical estimates including surface acceptance and dechanneling in bent silicon crystals.

On the reduced interaction probability for fully stripped 33 TeV/c Pb ions channeled in a bent Si crystal

We compare experimental results and computer simulations on the reduction of inelastic interactions for 33 TeV Pb82+ ions channeled in a bent silicon crystal. The comparison shows that a very small fraction of the initially channeled ions suffer nuclear interactions while traversing the 60 mm long crystal under perfect alignment, even though its thickness would correspond to about 1.2 nuclear interaction lengths for an amorphous material. This result indicates that a bent crystal approach to extraction of high energy, fully stripped ions from e.g. RHIC or LHC might be feasible.

Deflection of 32.8 TeV/c fully stripped Pb ions by means of a bent Si crystal

Abstract New results on the deflection of fully stripped 32.8 TeV/c Pb ions in a bent Si crystal at the CERN-SPS are reported. Deflection efficiencies above 10% have been measured for deflection angles in the range 4–9 mrad. The effect of particle losses due to interaction in the crystal and other systematic errors have been carefully investigated. The experimental results are in agreement with theoretical calculations.

First observation of the deflection of a 33 TeV Pb ion beam in a bent silicon crystal

For the first time, the deflection of an ultra-relativistic, fully stripped Pb/sup 82+/ ion beam in a bent silicon crystal has been observed. The ions were provided by the CERN-SPS in the H4 beam at a momentum of 400 GeV/c per unit of charge. A 60 mm long silicon crystal, bent over 50 mm to give a 4 mrad deflection angle, was used in this experiment. The measured Pb ion deflection efficiency is comparable to the one obtained with protons at an equivalent ratio of momentum per charge, and is found to be about 15% for a beam with a divergence of 35 microradians (FWHM). The interaction rate observed in a background counter is found to drop when the crystal is well aligned with the beam. This corroborates further the channeling model, which predicts that channeled ions are steered away from regions of high electron densities as well as the nuclei in the crystal.

High efficiency beam deflection by planar channeling in bent silicon crystals

Experimental results on the deflection of a 450 GeV proton beam by means of (111) planar channeling in a bent silicon crystal are presented. The H8 microbeam in the CERN SPS North Area was tuned to be highly parallel in the horizontal plane, i.e. to a divergence smaller than the critical angle for planar channeling at this proton energy, and focused to less than 1 mm in the vertical plane. The Si crystal was bent to deflect the beam horizontally in a classical 3-point bender. Unprecedented deflection efficiencies of up to 50% have been observed. Since channeling of positive particles is a well-understood phenomenon over many orders of magnitude in particle energy, the present data can be extrapolated to the TeV range. This opens exciting possibilities for the application of bent crystals, e.g. as small and tunable beam splitters or as extraction devices, in the future multi-TeV proton accelerators.<<ETX>>