Hollow electron beams are today highly desirable for many applications, but are still challenging in view of their detection. In this Letter, we focus on the unique character of the electromagnetic radiation that relativistic hollow electron beams can produce when traveling above a metasurface. We investigate theoretically the specific features of the radiation in a coherent mode, which provides the highest intensity, and show that the radiation from a hollow beam can be considerably more intense than that from a conventional solid beam. This solves the problem of distinguishing between hollow and solid beams. Moreover, we consider the two-layer internal structure of a hollow beam and reveal that the radiation characteristics are sensitive to the width and population of each layer. This allows detecting the internal structure of hollow beams. Interestingly, we found that the factor describing the annular beam form is a separated multiplier in a conventional form factor, independent of the properties of periodic structure. Thus, we can conclude that our results will stay correct for different profiles of periodic structures and metasurfaces made of metaatoms of different topologies and forms. The results pave the way towards a variety of newly emerging applications based on hollow electron beams, very diverse in topics, such as manipulation of objects at the nano-level, studies of chiral matter, plasma acceleration in donut wakefields and even applications in huge facilities such as LHC for controlling proton beam halos etc.

We analyze the polarization of radiation arising during the pass of a relativistic electron over a 2D photonic crystal slab consisting of subwavelength particles arranged at the nodes of a flat rectangular lattice. It is shown that the contribution of the parallel and perpendicular polarizations depends on the direction of propagation of the radiation, the bifurcation of the maxima is due to the contribution of the perpendicular polarization, while the asymmetry of the angular distribution is due to the contribution of the parallel polarization.

In this work, using Geant4, we simulate a time-of-flight positron emission tomography detector based on Cherenkov radiation. We consider various materials as a basis for the detector, including a pure PMMA and PMMA used as a matrix containing small resonant particles of copper and gold. For that, in CST studio, we determine the dielectric permittivity for a composite material, which we then use for simulations in Geant4. The resonant nature of the small metal particles allows producing Cherenkov photons by secondary electrons with lower energy, increasing the output of the radiated photons. We consider it as the way to increase temporal resolution as well as efficiency of the detector.

Inverse Compton scattering is a promising x-ray source, very bright, quasi-monochromatic, and compact. In this paper, we present a generalized theory of Compton backscattering in terms of luminosity, suitable for both classical and quantum regimes. We show that the optimal parameters, which require a certain mutual orientation and inclination of the fronts of the laser and electron beams described by 3D Gaussians, correspond to the crab scheme. This scheme is widely used in particle physics but is not yet used for x-ray sources. The constructed theory not only predicts the optimal geometry for laser and electron beams but also describes the luminosity. Our results reveal the opportunity to sharply increase the luminosity of compact x-ray sources based on Compton/Thomson backscattering.

A generalized theory of Smith-Purcell radiation (SPR) from a two-dimensional (2D) array of subwavelength particles is presented. Unlike most theories of SPR, this one is valid in the prewave zone and the near zone. We have obtained a generalized dispersion relation that determines the points of concentration of radiation in the prewave zone rather than in the ordinary directions of propagation of the most intense radiation; in the wave zone, the dispersion relation transforms into the ordinary one. Analytically, we derived that it is the parabolic law that describes how the individual elements should be arranged for the most intense radiation to be achieved. Interestingly, for the ultrarelativistic limit, the parabolic law turns into the hyperbolic one. Therefore, to focus the radiation from ultrarelativistic electrons in the prewave zone, a hyperbolically arranged 2D grating should be used instead of a rectangular one. Our findings suggest that the radiation intensity at a point of focus in the prewave zone can be sensitive to numerically very small changes in the locations of the individual constituent elements of the array determining the topology-the way the array's elements are arranged. The theory is constructed for particles with arbitrary dielectric properties, including metal ones; numerical analysis has been performed for dielectric particles.

The form factor of a hollow electron beam for transition radiation has been calculated. It has been shown that the characteristics of coherent radiation are significantly different for conventional solid and hollow beams. Numerical estimates have been obtained for terahertz radiation frequencies and relativistic energies of el-ectrons.

This study aims to create a new tool for fast computer simulations allowing one to design advanced electromagnetic calorimeters with the required properties. The application must calculate the calorimeter efficiency and measure the particles' energies, momenta and interaction time to detect the particles. This application should become the basis for a new technology of positron emission tomography. To solve the problem, a new C++ application based on Geant4 simulation toolkit has been developed. To monitor the response of calorimeters to different types of primary particles, we used different auxiliary Geant4 classes. In addition, we compare the simulation results for the detectors of three different setups, taking into account the detection of both electrons and gamma-quanta, and analyze their efficiency. To evaluate the capability of calorimeters to work under radiation load, we use an experimentally measured transmission function of radiation-damaged PbF2. Three calorimeter setups exploiting PbF2 were simulated with a new C++ application based on Geant4. We showed that such type of calorimeter has an energy resolution of 4.1%/ . -0pt√(E_e^ + [GeV]) and good linearity of response for GeV positrons measurements. The efficiency of such structures is found to be approximately 20

In this work we theoretically investigate the radiation from electrons passing along a periodic chain of dimers---the particles consisting of two coupled subwavelength parts. High-quality resonances can be excited in dimers due to interaction between the constituent particles, making quasibound states in the continuum. We show that at the resonant frequencies, the intensity of Smith-Purcell radiation increases sharply. Although numerical analysis is performed for the dimers consisting of two equal-coupled particles, the theory constructed is valid for more general binary objects consisting of two different subwavelength-coupled particles of arbitrary form and dielectric properties, and we have formulated the analytical conditions describing this effect. We prove that there is a wide range of the parameters for which Smith-Purcell radiation can be boosted by up to two orders of magnitude. Apparently, the effect can be even stronger with an appropriate choice of parameters. A resonant mechanism of enhancement of Smith-Purcell radiation paves the way to the novel and very effective radiation source based on metasurfaces made of resonant elements.

Smith–Purcell radiation is well known as a source of quasi-monochromatic electromagnetic radiation that occurs when fast electrons move above a diffraction grating. In this paper, we calculated the Smith–Purcell radiation generation from a flat surface along which there is a field of a standing laser wave. A periodically changing laser field induces a periodic inhomogeneity in the distribution of electrons in the near-surface layer. This periodicity, being an analogue of a diffraction grating, leads to the possibility of generating Smith–Purcell radiation. It is shown that the properties of Smith–Purcell radiation from such an unusual “light” grating are also unusual: the dispersion relation, unlike the standard for Smith–Purcell radiation, does not contain diffraction orders, so that all radiation is concentrated in one peak. The calculated effect makes it possible to control the radiation frequency or angle by changing the laser frequency and may be of interest for the development of new compact radiation sources with tunable characteristics and for non-invasive diagnostics of relativistic electron beams.

In the theory of radiation emitted by bunches of charged particles, the effects of coherence are commonly taken into account by multiplying the intensity of radiation generated by a single particle by the form factor of the bunch, which depends on its size, shape, and particle distribution. Here, it is demonstrated that this approach is, generally speaking, incorrect for polarization radiation from a wide class of structures like photonic crystals and metasurfaces. The theory of coherent Smith–Purcell radiation from such structures has been developed. It is shown that the commonly accepted approach is applicable only under two conditions: (i) the observation point lies in the plane containing the trajectory of the bunch and the normal to the surface of the target, and (ii) the radius of the bunch is much smaller than the effective range of the Coulomb field of the moving electrons.

Transition radiation (TR) is widely used as a radiation source in a wide spectral range, from terahertz to x rays. Conventional flat surfaces are usually used, but with the development of applications using microscopically structured surfaces, periodic surface structures are beginning to be studied. The periodicity of the surface dramatically changes the characteristics of TR, so this type of radiation received its own name: grating transition radiation (GTR). In this work, we investigate the polarization properties of GTR from a two-dimensional (2D) photonic crystal consisting of small particles arranged in a flat lattice (a 2D photonic crystal slab). We show theoretically that the polarization properties of GTR differ significantly from those of the kindred types of radiation: conventional TR and Smith–Purcell radiation. Since we found that the asymptotic behavior depending on the electron velocity for GTR and classical TR diverges, we performed homogenization and show that the results for GTR after homogenization are in perfect agreement with those for classical TR. This means that different dependence on the electron velocity for TR from a slab and for GTR from a 2D photonic crystal slab is caused by the fundamental difference between a conventional slab and a 2D photonic crystal due to its microscopic structure. The constructed theory contains the coordinates of the particles the photonic crystal consists of, which allows considering structures of finite size, both symmetrical and asymmetric. For asymmetric targets, the polarization of the radiation proves to be very sensitive to the electron’s trajectory. This sensibility of polarization characteristics opens up good opportunities for studying fine fundamental effects connected with the electron trajectory, such as the effect of the quantum nature of free electrons which manifests itself in the properties of radiation generated by free electrons. Also, the obtained results may find application in the design of compact sources of polarized radiation based on microscopically structured surfaces.

We construct the first-principles theory for the scattering of the Coulomb field of a fast electron traveling along an array of nano- or microparticles. The electron's trajectory is arbitrarily oriented in the plane parallel to the surface. We show that Smith-Purcell radiation accompanying this process results in very rich diffraction patterns, which differ dramatically from the ones for conventional diffraction gratings; a numerical analysis was made for THz frequencies. The generalized Smith-Purcell dispersion relation has been derived; it describes the link between frequency, two periods of the structure, the electron's velocity, and the angle of radiation with maximal intensity. The unique spatial distribution of the generated light can form the basis for the generation of structured-light electron-driven photon sources.

In this paper we investigate analytically polarization radiation excited due to interaction of an electron with a cluster of coupled subwavelength particles. The results are valid both for nonrelativistic and for relativistic charged particles. The expression for the Fourier image of radiation field has been obtained, which allows direct calculating spectral and angular density of the radiated energy. In general case, to find the radiation field with taking into account the interaction between particles, it is necessary to solve a system of N! self-consistent tensor equations, where N is the number of particles in the cluster. We suggest the algorithm for reducing the system of N! equations to N equations. The reduction in the number of equations is achieved through the calculation of the fields at the each point in which the dipole moment is, rather than through calculation of the fields from the every source. This facilitates considerably the finding solutions in problems of radiation for the clusters of particles.

Growing energies of particles at modern or planned particle accelerator experiments as well as cosmic ray experiments require particle identification at gamma-factors (γ) of up to ∼ 105. At present there are no detectors capable of identifying charged particles with reliable efficiency in this range of γ. New developments in high granular pixel detectors allow one to perform simultaneous measurements of the energies and the emission angles of generated transition radiation (TR) X-rays and use the maximum available information to identify particles. First results of studies of TR energy-angular distributions using gallium arsenide (GaAs) sensors bonded to Timepix3 chips are presented. The results are compared with those obtained using a silicon (Si) sensor of the same thickness of 500 μm. The analysis techniques used for these experiments are discussed.

The present version of the Transition Radiation (TR) simulation module implemented in the Geant4 toolkit describes very well experimental data for the TR energy distribution; however, it does not allow reproducing the details of angular distribution at small angles. In order to solve this problem, corrections to the existing x-ray TR module in Geant4 are proposed. With these corrections, the results of the simulations are in a good agreement with the angular TR distributions predicted by theory and obtained in the test beam measurements using a 480 um Si pixel detector and Mylar radiator.

An analysis of spectral-angular characteristics of diffraction radiation, both incoherent and coherent, has been performed. It is shown that radiation processes can be interpreted as Cherenkov radiation, which is produced by a region of dynamic polarization moving along a target edge with super-luminal velocity v(SL). Such radiation is generated if the condition v(SL) > c is fulfilled, which is the conventional 'threshold' Cherenkov condition.