摘要 :
Measurements of (110) planar channeling radiation have been performed at the radiation source ELBE at electron energies of 14.6, 17, 30, and 34 MeV using diamond crystals of thickness 42.5, 102, 168, and 500 μm. The influence of ...
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Measurements of (110) planar channeling radiation have been performed at the radiation source ELBE at electron energies of 14.6, 17, 30, and 34 MeV using diamond crystals of thickness 42.5, 102, 168, and 500 μm. The influence of different line-broadening mechanisms on the spectral shape of radiation from the 1-0 transition has been investigated. The analysis bases on fitting a convolution of the intrinsic Lorentz-like line shape with a Gaussian-like multiple-scattering distribution to the measured spectra. The asymmetry parameter involved relates to the standard deviation of the effective multiple-scattering angle. Its dependence on the crystal thickness was found to be weaker than for nonchanneled particles. The deduced coherence lengths show no significant dependence on the electron energy.
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Using the theory of crystal rainbows, we prove that the well-known doughnut patterns observed in ion channeling in thin crystal membranes are manifestations of the rainbow effect. This is done by a detailed morphological study of ...
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Using the theory of crystal rainbows, we prove that the well-known doughnut patterns observed in ion channeling in thin crystal membranes are manifestations of the rainbow effect. This is done by a detailed morphological study of the high-resolution experimental angular distributions of 2 MeV protons channeled in a 55-nm-thick (001) silicon crystal tilted away from the [001] direction. The inner side of the doughnut is the dark side of the rainbow, analogous to the Alexander's dark band, occurring between the primary and secondary meteorological rainbows.
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We report channeling patterns where clearly resolved effects of the narrow {111} planes are observed in axial and planar alignments for 2 MeV protons passing through a 55 nm [001] silicon membrane. At certain axes, such as <213> and <314>, ...
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We report channeling patterns where clearly resolved effects of the narrow {111} planes are observed in axial and planar alignments for 2 MeV protons passing through a 55 nm [001] silicon membrane. At certain axes, such as <213> and <314>, the offset in atomic rows forming the narrow {111} planes results in shielding from the large potential at the wide {111} planes, producing a region of shallow, asymmetric potential from which axial channeling patterns have no plane of symmetry. At small tilts from such axes, different behavior is observed from the wide and narrow {111} planes. At planar alignment, distinctive channeling effects due to the narrow planes are observed. As a consequence of the shallow potential well at the narrow planes, incident protons suffer dechanneled trajectories which are excluded from channeling within the wide planes, resulting in an anomalously large scattered beam at {111} alignment.
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The results of two channeling experiments show that highly charged heavy ions at moderate velocities (v « Zv0) may lose more energy in the traversal of a thin crystal when they are injected along a major crystallographic directio...
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The results of two channeling experiments show that highly charged heavy ions at moderate velocities (v « Zv0) may lose more energy in the traversal of a thin crystal when they are injected along a major crystallographic direction than when they traverse the crystal in random conditions. This is due to the fact that the large reduction of electron capture probabilities allows them to keep their high electronic charge throughout the crystal, which is not the case for projectiles traveling in random conditions. Although channeled projectiles experience reduced electron densities, their energy loss rate, that is, at first order, proportional to the square of the ions charge, is then strongly enhanced. This feature could be used as a step for decelerating highly charged ions from the high energies that are needed to produce them, and also to improve our understanding of the slowing down of very highly charged projectiles at low velocities, for which the current perturbative models are not well suited.
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Beam deflection due to axial channeling in a silicon crystal bent along the <111> axis was observed with 400 GeV/c protons at the CERN Super Proton Synchrotron. The condition for doughnut scattering of protons by the atomic strings of ...
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Beam deflection due to axial channeling in a silicon crystal bent along the <111> axis was observed with 400 GeV/c protons at the CERN Super Proton Synchrotron. The condition for doughnut scattering of protons by the atomic strings of the crystal was attained. Such a condition allowed one to observe a beam deflection of 50 μrad with about 30% efficiency. The contribution of hyperchanneled states of protons to the observed beam deflection was less than 2% according to simulation results.
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We consider the elastic scattering of a fast incident electron both prior and subsequent to its involvement in an atomic inner-shell ionization (core-loss) event in a crystal. By using numerical simulations, it is shown that elast...
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We consider the elastic scattering of a fast incident electron both prior and subsequent to its involvement in an atomic inner-shell ionization (core-loss) event in a crystal. By using numerical simulations, it is shown that elastic scattering subsequent to ionization can strongly affect the qualitative features of atomic resolution core-loss images of crystals recorded in the scanning transmission electron microscope (STEM). This conclusion holds even for a thin crystal foil and for a relatively large detector, which is matched to the probe-forming aperture in an aberration-corrected STEM. Such a conclusion is potentially very important for the interpretation of experimental core-loss images. We also introduce an approximate model that incorporates the effects of elastic scattering subsequent to ionization in the case of a small detector.
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Crystal blocking lifetime measurements have been made for highly excited Th nuclei with neutron number well removed from the stability line. Thin W crystals were bombarded with ~(32)S ions in the energy range 170-180 MeV and the y...
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Crystal blocking lifetime measurements have been made for highly excited Th nuclei with neutron number well removed from the stability line. Thin W crystals were bombarded with ~(32)S ions in the energy range 170-180 MeV and the yield of fission fragments was measured for emission close to a <111> axis. The fission blocking dips are compared to the appropriately scaled ones for elastic scattering of the ~(32)S beam ions and no significant difference is seen between the dips. This implies that the fraction of nuclei fissioning with lifetimes longer than 10 as is less than 2%. Fission lifetimes are increased by viscosity in the nuclear mass flow and comparison with a statistical model calculation indicates that the viscosity parameter, η, must be lower than for Th and U nuclei near η-stability. The effect of the N = 126 magic number is discussed.
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We report on an experiment performing channeling and volume reflection of a high-energy electron beam using a quasimosaic, bent silicon (111) crystal at the End Station A Test Beam at SLAC. The experiment uses beams of 3.35 and 6....
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We report on an experiment performing channeling and volume reflection of a high-energy electron beam using a quasimosaic, bent silicon (111) crystal at the End Station A Test Beam at SLAC. The experiment uses beams of 3.35 and 6.3 GeV. In the channeling orientation, deflections of the beam of 400 μrad for both energies with about 22% efficiency are observed, while in the volume-reflection orientation, deflection of the beam by 120 μrad at 3.35 GeV and by 80 μrad at 6.3 GeV is observed with 86%-95% efficiency. Quantitative measurements of the channeling efficiency, surface transmission, and dechanneling length are taken. These are the first quantitative measurements of channeling and volume reflection using a primary beam of multi-GeV electrons.
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We report the observation of efficient steering of a 855 MeV electron beam at MAMI (MAinzer MIkrotron) facilities by means of planar channeling and volume reflection in a bent silicon crystal. A 30.5 μm thick plate of (211) orien...
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We report the observation of efficient steering of a 855 MeV electron beam at MAMI (MAinzer MIkrotron) facilities by means of planar channeling and volume reflection in a bent silicon crystal. A 30.5 μm thick plate of (211) oriented Si was bent to cause quasimosaic deformation of the (111) crystallographic planes, which were used for coherent interaction with the electron beam. The experimental results are analogous to those recorded some years ago at energy higher than 100 GeV, which is the only comparable study to date. Monte Carlo simulations demonstrated that rechanneling plays a considerable role in a particle's dynamics and hinders the spoiling of channeled particles. These results allow a better understanding of the dynamics of electrons subject to coherent interactions in a bent silicon crystal in the sub-GeV energy range, which is relevant for realization of innovative x-ray sources based on channeling in periodically bent crystals.
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A graded Si_(1-x)Ge_x crystal has been manufactured for operation with high-energy protons to excite coherent interactions of the particles with the crystal such as channeling and volume reflection. The crystal had the shape of a ...
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A graded Si_(1-x)Ge_x crystal has been manufactured for operation with high-energy protons to excite coherent interactions of the particles with the crystal such as channeling and volume reflection. The crystal had the shape of a parallelepiped though its (111) atomic planes were curved at a radius of 25.6 m because of the graded Ge content. The crystal was exposed to a 400 GeV/c proton beam at the external lines of CERN Super Proton Synchrotron to probe its capability to steer high-energy particles. Measured deflection efficiency was 62.0% under planar channeling and 96.0% under volume reflection. Such values are critically compared to their counterparts for a standard bent Si crystal under peer conditions. A Monte Carlo simulation of the dynamics of channeled and volume reflected particles in a graded crystal including the effect of Ge impurities and of lattice dislocations has been carried out. We found that the effect of crystal imperfections spoiled the efficiency of channeling while it negligibly affected the performance of volume reflection. We finally propose the usage of the graded crystal as a primary scatterer to aid halo collimation for the new generation of hadronic machines. As a unique feature, a properly cut graded crystal circumvents the problem of the miscut angle, which is currently a severe limitation for implementation of crystal-assisted collimation.
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