International Journal of Modern Physics A

Since the end of 1950s the Frascati National Laboratories have started intense research program devoted to studying the coherent phenomena of relativistic electron interactions in crystals. First results were impressive: Frascati's Spectrometer has first detected coherent radiation by electrons in crystals, i.e. coherent bremsstrahlung. The discovery not only proved the Ferretti-Uberall prediction, but also has corrected the theory of coherent bremstrahlung. Carrying out intensive studies on coherent radiation phenomena in crystals a number of innovative techniques to measure fine features of radiation, such as polarization measurents, was developed. The Frascati's group was even close for the first time to observe channeling radiation discovered about two decades later the pioneering work on coherent bremsstrahlung.

2008 accomplished the 100^th anniversary from the birth of L.D. Landau (Nobel Prize 1962). In the paper a short description of Kharkov period of Landau's work from 1932 to 1937 is given. One of areas of his work is connected with investigation of bremsstrahlung by ultra relativistic electrons in matter. In this paper a short review of the results obtained studying both Landau-Pomeranchuk-Migdal effect and coherent radiation by high-energy electrons and positrons in crystals, is given. The main attention is paid to the general properties and difference of radiation processes in amorphous media and in a crystal and to advances in the coherent bremsstrahlung studies.

The spectral distribution of positron created by photon and the spectral distribution of photons radiated from electron in an oriented single crystal of intermediate thickness is calculated at intermediate energies. The energy loss of charged particles as well as photon absorption are taken into account. The used basic probabilities of processes include the action of field of axis as well as the multiple scattering of radiating electron or particles of the created pair (the Landau-Pomeranchuk-Migdal (LPM) effect).

Coherent electron-positron pair production by high-energy photons is investigated in a periodically deformed single crystal with a complex base. The formula for the corresponding differential cross-section is derived for an arbitrary deformation field. The conditions are specified under which the influence of the deformation is considerable. The case is considered in detail when the photon enters into the crystal at small angles with respect to a crystallographic axis. The results of the numerical calculations are presented for SiO₂ single crystal in the case of the deformation field generated by the acoustic wave of the S type. It is shown that, in dependence of the parameters, the presence of deformation can either enhance or reduce the pair creation cross-section.

While information exists on high energy negative particle channeling there has been little study of the challenges of negative particle bending and channeling collimation. Partly this is because negative dechanneling lengths are relatively much shorter. Electrons are not particularly useful for investigating negative particle channeling effects because their material interactions are dominated by channeling radiation. Another important factor is that the current central challenge in channeling collimation is the proton-proton Large Hadron Collider (LHC) where both beams are positive. On the other hand in the future the collimation question might reemerge for electron-positron or muon colliders. Dechanneling lengths increase at higher energies so that part of the negative particle experimental challenge diminishes. In the article different approaches to determining negative dechanneling lengths are reviewed. The more complicated case for axial channeling is also discussed. Muon channeling as a tool to investigate dechanneling is also discussed. While it is now possible to study muon channeling it will probably not illuminate the study of negative dechanneling.

The use of bent crystals for beam manipulations in particle accelerators or in external beam-lines is an old concept, still under scrutiny. In the last three decades, the knowledge on the features of crystal-particle interaction has been continuously growing on the basis of a large number of experimental findings. In all cases, the results of measurements have been found in agreement and, sometimes, have been anticipated by numerical models and simulations. Bent crystals can impart angular deviations to the incoming particles, through channelling or volume reflection mechanisms. The efficiency of the latter mechanism has been found to be intrinsically large, whilst the channelling efficiency has been improved by the increased technological expertise in crystal cutting and bending. In this paper we review the recent milestones of the worldwide effort to propose a routine use of bent crystal in particle accelerators, with a specific attention to the proposals of promoting the use of bent crystals as primary element in a collimation system.

This report overviews studies accomplished in the U70 proton synchrotron of IHEP-Protvino during the recent two decades. Major attention is paid to a routine application of bent crystals for beam extraction from the machine. It has been confirmed experimentally that efficiency of beam extraction with a crystal deflector of around 85% is well feasible for a proton beam with intensity up to 10¹² protons per cycle. Another trend is to use bent crystals for halo collimation in a high energy collider. New promising options emerge for, say, LHC and ILC based on the "volume reflection" effect, which has been discovered recently in machine study runs at U70 of IHEP (50 GeV) and SPS of CERN (400 GeV). Perspectives to use bent crystals for extraction of light ions from the U70 accelerator are also disclosed.

Bent-crystal channeling is a technique with a potential to increase beam-halo collimation efficiency in high-energy colliders. First measurements at the Tevatron in 2005 have shown that using a thin silicon crystal to deflect the 1-TeV proton beam halo onto a secondary collimator improves the system performance by reducing the machine impedance, beam losses in the collider detectors and irradiation of the superconducting magnets, all in agreement with simulations. Recent results, obtained with an improved goniometer and enhanced beam diagnostics, are reported here for dedicated beam studies and first full collider stores along with simulation results and plans for substantial enhancement of the T-980 experimental setup.

An alternative way to the conventional positron source using intense electron beams, of some GeV, impinging on thick amorphous targets is presented. This source is using two successive targets: the first one is a tungsten crystal, which <111> axis is aligned with the beam direction, and the second is an amorphous target put at some distance from the crystal. The enhanced radiation due to electron channelling produces a large amount of photons which, consecutively, create high quantity of e⁺-e^- pairs in the amorphous target. Between the two targets a sweeping magnet takes off all or part of the charged particles coming out from the crystal. An optimization procedure is carried out based on two important parameters: a) the distance between the two targets; b) the minimum energy above which the charged particles coming out from the crystal are allowed to hit the amorphous target. This optimization, which first step is presented here, aims to obtain the highest accepted yield (> 1e+/e-) after the capture system and the minimum Peak Energy Density Distribution (PEDD) (PEDD < 35 J/g) in order to avoid thermal gradients, which are destructive for the target.

In the work the mechanism of generation of hypothetical magneto-charged particles, which were observed in experiments on supercompression of condensed target in Electrodynamics Laboratory "Proton-21", is discussed. It was shown that these particles are most probably the hypothetical light magnetic monopoles, that were introduced by George Lochak [1] as magneto-excited state of neutrinos. Parameters of these particles (including mass of monopole and both size and binding energy of monopole-antimonopole pair) and the method of their generation in "Proton-21" Lab are discussed and calculated.

Radiations from a crystal irradiated with relativistic charged particles are reviewed with the emphasis on their fundamental processes. As their "compound processes" diffraction of both channeling radiation and coherent bremsstrahlung are considered. Coherent polarization radiation from neutral particles with magnetic moment is discussed within the framework of classical electrodynamics.

The dechanneling process of electrons in silicon single crystals has been studied at the Mainz Microtron MAMI for (110)-planar channeling of electrons at beam energies between 195 and 855 MeV. Dechanneling lengths were derived from a high and a low energy loss signal of the electrons which were recorded as function of the crystal orientation with respect to the beam direction for various crystal thicknesses in the range between 14.7 µm and 467 µm. The high energy loss signal corresponds to an energy loss of about 75 % of the total electron energy by emission of a bremsstrahlung photon, while the low energy signal to an energy loss of 0.7-1.7 % by emission of channelling radiation. While the high energy signal saturates as function of the crystal thickness, the low energy signal does not. The nearly constant dechanneling length of about 35 µm, as extracted from the high energy signal, is interpreted to originate from a small fraction of electron which initially occupy deeply bound quantum states.

A quantum mechanical treatment of the effects of dislocations on planar channeling of positrons is presented. The effects of anharmonic terms on the positron planar potential are considered in these calculations. The wavefunction of channeled positron in the perfect channel and the two regions of dislocation affected channel are found and compared with harmonic case. The corresponding effects on bound states and the transitions among these states are calculated. The transition probabilities and the resulting dechanneling probabilities are found for initially channelled particles in states |0〉 and |1〉. The calculations are carried out with varying channel distance from the dislocation core. Applications of anharmonic interactions on channeling radiation are also investigated.

It is shown that the band structure of transverse energy levels for (111) planar channeled electrons in the Si and LiF crystals qualitatively changes the angular distributions of X-rays emitted at the Bragg angles compared to calculations, which do not take into account this effect [5-7].

Recently using the theory of Afanasev-Aginian (1977-1978) it has been reconsidered theoretically the properties of the contributions of PXR, DTR and their interference into the radiation intensity produced by particles in single crystals in Laue geometry, essentially differing from those of Bragg geometry. After a short review of theoretical and experimental works devoted to DTR in this work we discuss the properties of Laue geometry and make comparison of the predictions of the above theory with the results of the second experiment performed by Avakian et al in 1987 with the help of 4.5 GeV electrons.

The experiments of the PXR performance carried out for the target crystals with different cutting planes have shown significant difference in the PXR property, which suggests another way to increase the PXR intensity than by increasing the electron beam current. In order to investigate the effect of the geometrical condition of the crystal surface on the PXR property, the experiments have been carried out for the target crystal with a knife-edge-shaped cut surface. For the case with symmetric Bragg geometry on the front surface and asymmetric condition on the rear surface, the rather low intensity X-ray beam has shown considerably good spatial coherence. The X-ray beam with narrow line width has made it possible to obtain X-ray absorption spectra with a high resolution. In contrast, relatively high intensity, which enabled taking an absorption image with the exposure for several tens of seconds, has been obtained for the geometry with asymmetric front surface and symmetric rear surface. This configuration, however, has raised a problem of degradation in the spatial coherence of the X-rays due to the superposition of two different X-ray beams.

The characterization of the transverse phase space for high charge density and high energy electron beams is demanding for the successful development of the next generation light sources and linear colliders.

Due to its non-invasive and non-intercepting features, Optical Diffraction Radiation (ODR) is considered as one of the most promising candidates to measure the transverse beam size and angular divergence.

The recent results of our experiment, based on the detection of the ODR angular distribution to measure the electron beam transverse parameters and set up at FLASH (DESY), are presented. A thin stainless steel mask has been installed at 45° with respect to the DR target and normally to the beam propagation to reduce the contribution of synchrotron radiation (SR) background. In addition, interference between the ODR emitted on the shielding mask in the forward direction and the radiation from the DR target in the backward direction is observed. This is what we call Optical Diffraction Interferometry (ODRI). The contribution of this interference effect to the ODR angular distribution pattern and, consequently, its impact on the beam transverse parameters is discussed.

The processes of interaction of accelerated protons with the surface of insulating capillaries have been investigated to determine the character of motion of particles during such interaction. The angular distributions of 240-keV protons transmitted through glass capillaries have been measured, and splitting of the distribution into a series of lines equally spaced from each other is revealed.

There is considerable interest in the development of inexpensive lithography techniques for applications in the area of nanoscale electronics. The semiconductor industry is pursuing the development of photolithography techniques such as extreme UV and x-ray. However these techniques are extremely expensive and not suitable for smaller scale applications. In this paper we describe research on the feasibility of exploiting x-ray propagation within carbon nanotubes (CNTs) for the fabrication and characterisation of nanoscale devices. A description is given of a test structure designed to explore experimentally the possibility of x-ray propagation in carbon nanotubes. As x-ray propagation requires a grazing angle of incidence the nanotubes need to be straight and reproducible. In order to alleviate this problem the possibility of using Bragg reflection is investigated. This approach to the problem is stimulated by the inherent Bragg structure of multiwall carbon nanotubes. It is further encouraged by the recent development of coatings using materials such as WS₂. Results from simulations presented in this paper show that although Bragg reflection in as-grown multiwall carbon nanotubes is weak the potential for exploitation of this phenomenon in suitably coated nanotubes exists.

The Smith–Purcell radiation generated by relativistic electrons has been studied experimentally. To investigate the posibility of radiation focusing in the pre-wave zone two types of concave gratings focusing radiation in azimuthal and polar planes have been used. Comparison of radiation characteristics of such gratings with the flat grating ones in the same conditions has showed that application of concave gratings may significantly increase coherent Smith–Purcell radiation spatial density.