E. Uggerhøj

Detailed investigation of the channeling phenomena involved in bending of high-energy beams by means of crystals

Abstract Strong bending effects for 12 GeV/ c positive particles traversing 2–3 cm long silicon crystals are reported, and results for crystal bendings of 4, 20, and 52 mrad are presented. Bending efficiencies up to 20% are seen. A combination of axial and planar channeling is responsible for the effects observed. Computer simulations, based on binary collisions, can explain in detail the experimental findings, and also elucidate important channeling phenomena in the transition region between axial and planar channeling. For negative particles a weak bending effect, caused by doughnut scattering, is demonstrated. Interesting results for planar feed-in and dechanneling are found in an experiment where three detectors are placed along the crystal.

A crystalline undulator based on graded composition strained layers in a superlattice

Abstract We propose a new method to produce a crystalline undulator which works without active modifications to the crystal lattice. The individual sections of the superlattice are deformed by growing Ge x Si 1− x -mixtures on a Si substrate, where x is varied along the crystal length to generate the desired shape of the undulator. With this method, it is possible to generate, e.g., 0.3 MeV photons of high intensity by use of a 2 GeV positron beam in a 1 mm thick crystal – a compact undulator with ⩾20 periods. The advantages and drawbacks compared with the acoustically bent crystal approach (AIR) are discussed.

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.

Ionization of helium and molecular hydrogen by slow antiprotons

Measurements of the cross sections for single and double ionization of helium as well as for the creation of H2+ and H+ ions from H2 for impact of antiprotons in the energy range 13-500 keV are presented. The results are compared with our earlier, less accurate data and with data for equivelocity proton impact. The single ionization cross section of helium agrees remarkably well with a continuum distorted wave (CDW-EIS) calculation. The ratio between the double and the single ionization cross sections of helium increases dramatically with decreasing projectile energy. New theoretical calculations are called for.

Measurement of the stopping power of silicon for antiprotons between 0.2 and 3 MeV

Our previous measurement of the stopping power of silicon for antiprotons has been extended down to 200 keV. The antiproton stopping power is found to be more than 30% lower than that for equivelocity protons at 200 keV. The “Z13 contribution” to the stopping power (the Barkas effect) is deduced by comparing the stopping power for protons and antiprotons. Comparisons to theoretical estimates are made.

High efficiency multi-pass proton beam extraction with a bent crystal at the SPS

Recent measurements of 120 GeV proton extraction by means of a bent silicon crystal at the CERN-SPS accelerator are summarized. The existence of multi-pass extraction has been proven by blocking first-pass extraction: using a crystal covered with an amorphous layer, extracted beam with high efficiency was observed, which provides a direct proof for the importance of the multi-pass mechanism. This opens new possibilities in the design and optimization of a bent crystal extraction scheme.

Measurement of the antiproton stopping power of gold - the Barkas effect

Abstract The stopping power of gold has been measured for antiprotons in the energy range 0.2–3 MeV using a novel time-of-flight technique. The antiproton stopping power is found to be less than half the equivalent proton stopping power near the electronic stopping power maximum. In the high-energy limit the two stopping powers merge.

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.

Hard photon yields from (70–240) GeV electrons incident near axial directions on Si, Ge and W single crystals with a large thickness variation

Abstract The dramatic peak found in photon spectra from 150 GeV channelled electrons has for the first time been investigated for 70, 150, and 240 GeV electrons incident on crystals with thickness from 100μ (Si). Very pronounced variations for the high energy part of the photon spectra are found. In Si the photon peak is not found for lower energies and thin crystals. For well-aligned electrons in Si the yield is more than 160 times the Bethe-Heitler one. In general the peak in the photon spectra disaapears for incident angles larger than half the critical angle for channeling. Dramatic radiative energy losses are found along axial directions - in 0.6 mm 〈110〉 Ge a 150 GeV electron loses more than 60% of its total energy.

Non-dissociative and dissociative ionization of N2, CO, CO2, and CH4 by impact of 50-6000 keV protons and antiprotons

Measurements of the cross section for non-dissociative single ionization and the cross sections for the creation of charged fragments have been performed for 50-6000 keV antiproton and proton impact on N2, CO, CO2, and CH4. The results support the understanding of the ionization phenomenon that has been achieved via measurements with fundamental charged particles on atoms. The present high-energy antiproton fragmentation data supply a stringent test of the validity of the published electron-impact fragmentation data which, unfortunately, most of them fail.