Parametric X-ray Radiation Research Articles

Effects of dynamic diffraction in coherent X-ray radiation from a beam of relativistic electrons in a periodic layered medium with three layers in a period

The possibilities of manifestation of dynamic diffraction effects in coherent X-ray radiation (CXR) of relativistic electrons in a periodic layered medium (PLM) with three different layers in a period are being investigated. A dynamic theory of CXR of relativistic electrons has been developed for the case of such a target. The corresponding formulas for the spectral-angular characteristics of parametric X-ray radiation (PXR) and diffracted transition radiation (DTR) are obtained and studied. The influence of asymmetric diffraction on the spectral-angular densities of PXR and DTR is shown. A striking effect of anomalous photoabsorption in PXR in a layered target, similar to the Bormann effect in a single crystal, is predicted.

The Difference between the Spectra of Parametric X-Ray Radiation from Powder Targets and Textured Polycrystals

The Difference between the Spectra of Parametric X-Ray Radiation from Powder Targets and Textured Polycrystals

Breaking the barriers of electron-driven x-ray radiation in crystals

Parametric x-ray radiation (PXR) is a prospective mechanism for producing directional, tunable, and quasi-coherent x-rays in laboratory-scale dimensions, yet it is limited by heat dissipation and self-absorption. Resolving these limits, we show the PXR source flux is suitable for medical imaging and x-ray spectroscopy. We discuss the experimental feasibility of these findings for a compact commercial PXR source.

Coherent X-ray radiation excited by a relativistic electron in a periodic layered medium

The work is devoted to the development of the theory of coherent X-ray radiation near the direction of Bragg scattering of a beam of relativistic electrons crossing a target with a periodic layered structure with three different amorphous layers per period. The coherent X-ray radiation is considered within the two-wave approximation of the dynamic theory of diffraction as a sum of parametric X-ray radiation and diffracted transition radiation excited in the target by a relativistic electron. Expressions describing the spectral-angular and angular densities of PXR and DTR have been obtained and studied.

Use of parametric X-ray radiation of electrons in crystals to determine the parameters of imaging-plates

Good agreement between experimental data and the results of calculations of the yield and angular distributions of parametric X-ray radiation of electrons in crystals within the framework of kinematic theory enables us to use the measurement results to determine the photon energy dependence of the sensitivity of X-ray plates. The results of measurements of the angular distributions of parametric X-ray radiation of electrons with an energy of 255 MeV in a Si crystal using imaging plates of several types were compared with the results of calculations that take into account all currently known experimental factors that influence the measurement results. The spectral dependence of the sensitivity of the studied X-ray plates was determined for photon energies of two orders of reflection and for the (111), (1̅1̅1), and (110) reflection planes.

Phase-matching in high-energy photon sources by relativistic laser–plasma interactions

In this article we briefly review three generation mechanisms of high-energy photons related, directly or otherwise, to high-intensity laser–matter interactions. The three mechanisms are the “Betatron Radiation” (BR), the “Parametric X-ray Radiation” (PXR) from Laser-WakeField Accelerated Electrons (LWFA), and the all-optical “Compton BackScattering” (CBS). The aim is to highlight similarities and differences, in particular regarding the phase-matching conditions for the high-energy photons to be emitted.

Observation of Parametric X-Ray Radiation from Highly Oriented Pyrolytic Graphite in Backscattering Geometry

Observation of Parametric X-Ray Radiation from Highly Oriented Pyrolytic Graphite in Backscattering Geometry

FREQUENCY OF PARAMETRIC X-RAY RADIATION

The exact solution of the equation for the frequency of parametric X-ray radiation (PXR) of relativistic charged particles moving in a crystal is obtained and compared to the approximate solution. It is found that the exact solution is in good agreement with the approximate one and that the approximate PXR frequency solution is practically correct for a comparison to experimental data.

Superradiant parametric x-ray emission

We compute a spectrum of parametric x-ray radiation inside a crystal from a bunch of electrons, which is periodically modulated in density. We consider that the bunch of electrons is exiting from an x-ray free-electron laser (XFEL) channel. We demonstrate that in the case of a resonance between the frequency of parametric x-ray radiation and a frequency of modulation of an electron bunch, the sequence of strong quasimonochromatic x-ray pulses is formed---superradiant parametric x-ray emission (SPXE) with frequencies that are multiples of the modulation frequency. The number of photons in the pulse of SPXE in the case of extremely asymmetric diffraction is comparable with the photon number in the pulse of an XFEL. Moreover, the SPXE is directed under the large angle to the electron velocity and every harmonic in the spectrum is emitted under its own angle.

Parametric X-ray radiation from nanopowders

The first observation of Parametric X-ray Radiation (PXR) produced in a crystalline powder constituted of crystallites whose largest dimensions are of the order of some nanometers is reported. PXR was registered during the interaction of a 7 MeV electron beam with a platinum powder. The experimental results were compared with the kinematical theory showing a good agreement concerning the position, form, and intensity of all observed PXR spectral peaks. Finally, an analysis of the influence of crystallite dimensions on the PXR peak full width hall maximum (FWHM) and PXR peak intensity was performed.

Observation of tunable parametric x-ray radiation emitted by laser-plasma electron beams interacting with crystalline structures

Parametric x-ray radiation (PXR) is the quantum process analog to Laue diffraction where a pseudophoton is scattered out of a crystalline structure as a radiation photon. In this work, PXR emitted by electron beams generated in a compact laser-plasma accelerator and interacting afterward with a Si 220 crystal is observed in single-shot operations. The combination of laser wakefield acceleration and PXR is an efficient and table-top way to obtain monochromatic, pulsed, ultrashort, and stable emission of high-energy photons: to our knowledge such a complete set of performance parameters is new. Unlike other relevant radiation mechanisms, such as betatron radiation or incoherent bremsstrahlung, PXR is insensitive to the beam energy spread. With a long crystal, we demonstrate the stability of the PXR bandwidth toward shot-to-shot variations of the beam energy and divergence.

Effect of Reflection Asymmetry on the Output of the Parametric X-Ray Radiation of Electrons in Crystals

Effect of Reflection Asymmetry on the Output of the Parametric X-Ray Radiation of Electrons in Crystals

A proof-of-principle experiment on a new diagnostic tool for determining beam sizes from angular distributions of parametric X-ray radiation

A previously proposed technique for measuring the transverse sizes of an electron beam was verified experimentally by comparing the angular distributions of parametric X-ray radiation in a thin crystal for two distances between the crystal and the detector. The measurements were performed on a 255-MeV electron beam incident on a 20 μm-thick silicon crystal. The angular distributions of parametric X-ray radiation were measured using an imaging plate as a two-dimensional position-sensitive detector. The obtained horizontal and vertical beam sizes were in good agreement with the results of measurements by optical transition radiation. The proposed method would be useful for electron beam diagnostics for recently advanced linear accelerators where optical transition radiation cannot be used as a beam profile monitor owing to the coherent effect.

Temporal and spatial design of x-ray pulses based on free-electron–crystal interaction

Tunable x-ray radiation sources are of wide importance for imaging and spectroscopy in fundamental science, medicine, and industry. The growing demand for highly tunable, high-brightness lab-scale x-ray sources motivates research of new mechanisms of x-ray generation. Parametric x-ray radiation (PXR) is a mechanism for tunable x-ray radiation from free electrons traversing crystalline materials. Although PXR has been investigated over decades, it remained limited in usages due to the low flux and strict dependence on fixed crystal properties. Here, we find new effects hiding in the PXR mechanisms, which provide control over the radiation polarization and spatial and temporal distribution. The radiation can form ultrashort pulses and delta-pulse trains, which makes the new effects fundamentally different from all conventional mechanisms of x-ray generation. We show how these new effects can be created from free-electron interactions with van der Waals materials. Furthermore, we consider free electrons traversing near material edges, which provides an additional degree of tunability in angular distribution and polarization of PXR. Our findings enable us to utilize recent breakthroughs in the atomic-scale design of 2D material heterostructures to provide platforms for creating tunable x-ray pulses.

Observation of parametric X-ray radiation for the asymmetric Laue geometry

Angular distributions of parametric X-ray radiation from 255-MeV electrons in a thin Si crystal have been investigated for asymmetric Laue geometries using an imaging plate as a two-dimensional position-sensitive X-ray detector. The peak intensities of parametric X-ray radiation observed for (111) and (1¯1¯ 1) reflections, normalized by taking into account the effective target thickness and the transmission fraction of X-rays in the crystal, coincided within several percent. A large intensity difference depending on the asymmetry parameter, which is predicted by a dynamical theory dealing with the asymmetric Laue geometry, was not observed under the present experimental conditions. This indicates that the dependence of angular distributions of parametric X-ray radiation on the asymmetry parameter is negligibly weak, i.e., that it can be well explained by the well-established kinematical theory and dynamical theory as well as those for symmetric Laue and Bragg geometries.

Influence of Multiple Scattering on Parametric X-Ray Radiation Excited by a Beam of Relativistic Electrons in a Single Crystal

Parametric X-ray radiation generated by a beam of relativistic electrons in a single-crystal wafer is studied in the Bragg geometry under conditions of multiple electron scattering at target atoms. Expressions are obtained that describe the spectral-angular and angular radiation density under conditions of multiple electron scattering. An increase in the influence of multiple scattering on the spectral-angular radiation density is demonstrated both with an increase in the target thickness and a decrease in the energy of relativistic electrons. It is shown that the asymmetry of the electron-field reflection relative to the target surface considerably affects the spectral-angular and angular densities of parametric X-ray radiation under the condition of strong multiple scattering.

Effect of the Grain Size of Powder Targets on the Spectra of Parametric X-Ray Radiation of Relativistic Electrons

The results of measurements of the dependence of the full width at half-maximum and the relative intensity of spectral peaks of parametric X-ray radiation (PXR) for various crystallographic planes on the grain size of a micrometer diamond powder are presented.

Method of the equivalent photons for modulated electron beam

It is shown in this work that electromagnetic self-field of the periodically density modulated electron bunch can be considered as the beam of the equivalent photons (pseudo-photons), collimated along the electron velocity. Pseudo-photon spectrum includes both the incoherent contribution being proportional to number of electrons in the bunch and the coherent part with the sharp maximum corresponding the modulation frequency. Method of the equivalent photons can be applied for description of the interaction between the electron bunch and a crystal that leads to generation of the coherent parametric X-ray radiation by the modulated bunches exited from the undulator of the X-ray free electron laser. It provides the possibility to obtain the X-ray pulses directed at the large angle to the electron velocity where intensity of the background radiation essentially decreased. It is defined the conditions when the spectral and angular distributions of the coherent pseudo-photons can be compared with the analogous values for the pulses of the X-ray laser.

Coherent X-ray radiation excited by a beam of relativistic electrons in a layered periodic structure

The dynamic theory of coherent X-ray radiation, excited in a periodic layered medium by a divergent beam of relativistic electrons in the Bragg scattering geometry, is developed. In framework of the two-wave approximation of the dynamic theory of diffraction, expressions describing the spectral-angular and angular distributions of parametric X-ray radiation (PXR) and diffracted transition radiation (DTR) are obtained taking into account the multiple scattering of the relativistic electrons by the atoms of the target. Based on the expressions obtained, the possibilities of the manifestation of the effects of dynamical diffraction in coherent X-rays were investigated. The influence of the asymmetry of the electron Coulomb field reflection with respect to the target surface (the reflecting layers situate nonparallel to target surface) on the spectral-angular characteristics of the PXR and the DTR under conditions of the electron multiple scattering is estimated. It is shown that at a fixed Bragg angle the width of the PXR spectrum increases when the angle of incidence of the electron on the target decreases, which leads to an increase in the PXR angular density. In the same conditions the width of the frequency domain of total external reflection and the amplitude of the DTR spectrum also increase, which leads to a significant increase in the angular density of the DTR. The obtained analytical expressions can be used to determine the optimal parameters of the experiment on confirmation of the predicted dynamic effects.

Coherent X-Ray Radiation Generated Near the Axis of the Beam of Relativistic Electrons in an Artificial Periodic Structure

A theory of the coherent X-ray radiation produced by a beam of relativistic electrons in an artificial periodic structure and propagating along the electron-beam axis is developed. Expressions describing the spectral-angular densities of parametric X-ray radiation in the direction close to the electron velocity vector (FPXR) and of transition radiation (TR) and their interference are obtained in the Bragg scattering geometry. An analysis based on the obtained expressions for the spectral-angular density shows possibilities of their use to determine the optimal parameters of the experiment recording the FPXR. The advantages of the experiment recording the FPXR at low electron energies ( $${{E}_{e}} \leqslant $$ 50 MeV) are shown.