Evolution of the characteristics of Parametric X-ray Radiation from textured polycrystals under different observation angles

The Parametric X-Ray radiation (PXR) spectra and yield dependencies on the orientation angle are measured during the interaction of 7 MeV electrons with a tungsten textured polycrystalline foil for different observation angles. The effects of PXR spectral density increase and PXR yield orientation dependence broadening in the backward direction is shown experimentally for the first time. The experimental results are compared with PXR kinematical theories for both mosaic crystals and polycrystals.     

Parametric X-ray radiation from powders

Parametric X-ray Radiation (PXR) produced in powders has been observed for the first time. PXR spectra were measured under observation angles of 150° and 180° during the interaction of relativistic 7 MeV electrons with a tungsten powder. All the PXR peaks that theoretically can be produced in the studied energy region were registered. The performed absolute comparison of the experiment with the PXR kinematical theory from randomly oriented crystallites showed a good accordance.     

Design of a PXR experiment based on the NCEL

Parametric X-ray Radiation （PXR） can be used as a novel, quasi-monochromatic energy-tunable and high-yield X-ray source. It is produced at the Bragg angle by a relativistic electron beam passing through the periodic structure of crystal materials. This article concerns the PXR experiment using low energy electrons （10 MeV） from NCEL （Novel Compact Electron-Linac）. The difficulty of the experiment is to distinguish the PXR photons form the background. The design of the experiment relies mainly on the yield of PXR, the Bremsstrahlung background of the X-rays and the capability of the detector.     

Design of a PXR experiment based on the NCEL

Parametric X-ray Radiation (PXR) can be used as a novel, quasi-monochromatic energy-tunable and high-yield X-ray source. It is produced at the Bragg angle by a relativistic electron beam passing through the periodic structure of crystal materials. This article concerns the PXR experiment using low energy electrons (10 MeV) from NCEL (Novel Compact Electron-Linac). The difficulty of the experiment is to distinguish the PXR photons form the background. The design of the experiment relies mainly on the yield of PXR, the Bremsstrahlung background of the X-rays and the capability of the detector.     

Parametric X Radiation from Electrons in Mosaic Crystals

A simple model for calculating the contribution of diffraction of actual photons to the measured spectrum of parametric x radiation (PXR) from relativistic electrons in mosaic crystals is suggested. It is shown that the diffraction of real photons of bremsstrahlung and transition radiation in mosaic crystals whose thickness t is about 0.01 of the radiation length makes a significant contribution to the measurable emission spectrum. The spectra and angular distributions of PXR from electrons with energies 500 and 900 MeV have been measured in mosaic pyrolytic graphite crystals. It is demonstrated that with consideration of diffraction of actual photons in the mosaic crystal, including diffraction of PXR photons, all experimental data presently available agree well with the results of calculations for the suggested model.     

On the possibility of using parametric X-ray radiation to study anisotropy of a crystal mosaic structure

Parametric X-ray radiation (PXR) of relativistic electrons moving in a mosaic crystal is considered. A strong dependence of the PXR angular spectral distribution on the mosaic structure is shown. The effect of mosaic structure on the PXR spectrum strongly increases in the longitudinal plane specified by the radiating electron velocity vector and the averaged reciprocal lattice vector and decreases in the transverse plane with a decrease in the angle of incidence of the radiating electron onto a crystal plane, which allows one to study the anisotropy of mosaic structure of crystal planes.     

Investigation of the production mechanism of parametric X-ray radiation

Parametric x-ray Radiation (PXR) in the energy range from 5 to 20 keV produced by 855 MeV electrons using a single Si crystal was studied at the Mainz Microtron MAMI. The production mechanism was investigated by measuring the angular distributions for several orders of PXR and its dependence from the crystal thickness. A silicon crystal shaped in steps with thicknesses ranging from 100 μm to 600 μm was used. The experiments were carried out using LN₂-cooled PIN photodiodes with an energy resolution of 950 eV. The absolute photon flux was measured for the (111)-, (220)- and (224)-reflection planes and are in very good agreement with a kinematical model. The angular distributions are well reproduced by an ansatz, which incoherently adds to the PXR a portion of diffracted bremsstrahlung and diffracted transition radiation.     

Quantum effects for parametric X-ray radiation during channeling: Theory and first experimental observation

The theory of X-ray radiation from relativistic channeled electrons at the Bragg angles—parametric X-ray radiation (PXR) during channeling (PXRC)—is developed while accounting for two quantum effects: the initial population of bound states of transverse motion and the transverse “form-factor” of channeled electrons. An experiment was conducted using a 255 MeV electron beam from a linac at the SAGA Light Source. We have identified a difference in the angular distributions of PXR and PXRC and obtained a fairly good agreement between the theoretical and experimental results.     

Tunable source of parametric X-ray radiation

A new design of an X-ray source with a radiation energy controllably varying from 2 to 130 keV and a radiation intensity exceeding >10^?5 photon/e^?/sr is suggested. The source is based on parametric X-ray radiation (PXR) generated by moderately relativistic electrons in an original geometry. In this geometry, kinematic bunching of PXR reflections is used, which makes it possible to increase the intensity of the source. A model to calculate the PXR characteristics that takes into account absorption of radiation, beam parameters, multiple scattering, and energy loss is developed. The monochromaticity of the source lines is estimated.     

LambdaN spin-orbit splittings in (9)(Lambda)Be and (13)(Lambda)C studied with one-boson-exchange LambdaN interactions

Parametric x rays (PXR) produced by bombarding silicon and diamond crystals with electrons of 30 to 87 MeV were detected at 180 degrees relative to the direction of the electron beam. It was found that the dependence of the intensity on the orientation of the crystal agrees with the predictions of the kinematical theory of PXR. The absolute intensity is twice as large as predicted. These findings can be explained considering dynamical effects that govern the x-ray crystal interaction. Additionally, x rays caused by self-diffracted transition radiation have been observed.     

Observation of coherent X-ray production by 800-MeV electrons in a periodic triple-crystal target

The production of coherent x radiation by 800-MeV electrons in a target consisting of three 16-μm silicon crystals is investigated at the Tomsk synchrotron. The target structure makes it possible to observe from each crystal in turn, as the target is rotated, the radiation due to the summation of parametric x radiation (PXR) and the diffracted resonance transition radiation (DRTR) produced at the surfaces of the preceding crystals. The orientational dependence obtained shows that the contribution of the DRTR increases with the number of the crystal in the series, so that the angular density of the DRTR from the third crystal is approximately 1.7 times higher than the density of the PXR. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 594–599 (25 April 1997)     

Excitation cross sections of low-lying levels of the tungsten atom

The excitation of transitions from low-lying levels of the tungsten atom by a beam of slow electrons with an energy of 50 eV is studied. The results obtained are used to calculate the total excitation cross sections of the WI energy levels taking into account the branching ratios.     

Theory of secondary electron emission

The fine structure in angle-resolved secondary electron spectra is shown to be related to the total reflectivity in low-energy electron diffraction (LEED). Theoretical results for tungsten are compared with experimental data. For non-normal emission, spin-orbit coupling is predicted to produce spin polarization of the emitted electrons.     

Effect of surface potential on field-emission energy distributions and on the surface density of states

We present numerical results for the total energy distribution of electrons field emitted from the (100) plane of tungsten for two different surface barriers and two different values of the applied field. Numerical results are also presented for the surface density of states.     

Estimation of the parameters of the cascade curve of electromagnetic shower development in a spectrometer with oriented crystal converter

Based on the experimental results on the development of electromagnetic showers from 26-GeV electrons in misoriented and oriented along the 〈111〉 axis tungsten crystals 2.7, 5.8, 8.4mm thick and then in a composite Cherenkov shower spectrometer, the parameters of the cascade curve of shower development in a spectrometer with 1-mm tungsten crystal converter were estimated.     

Asymmetric wave reflection in kinematic and dynamic approaches to description of parametric X-radiation of a relativistic electron in the crystal

Parametric X-radiation (PXR) of a relativistic electron travelling through a monocrystalline plate is examined in the Laue scattering geometry under the assumption of asymmetric and symmetric reflections of the particle field with respect to a target surface. The formulas of the kinematic and dynamic approaches to PXR characterization are analyzed and compared. The results of the experiment on relativistic electron PXR implemented at the Mainz microtron are interpreted using an expression derived for its angular distribution. It has been revealed for the first time that the dynamic effect of anomalous photoabsorption (the Bormann effect) of PXR has taken place in this experiment.     

Energy distributions of electrons emitted from tungsten tips covered by diamond-like films

The energy distribution of electrons emitted from the surface of diamond-like pointed cathodes under the action of a high electric field is reported. Diamond-like coatings are applied on thin tungsten tips by ion-beam evaporation in an ultrahigh vacuum. The structure of the carboniferous films covering the tungsten tips is examined by field-emission microscopy. The stability of the field-emission cathode current is considered, and the Fowler-Nordheim I-V characteristics are presented. Based on the results obtained, a model of field-emission cathode covered by a thin diamond-like coating that explains the energy distributions is suggested.     

Energy loss of electron-induced showers in oriented tungsten crystals

The development of the shower induced by 26- and 28-GeV electrons in the oriented tungsten crystal results in strong absorption of the shower energy by the crystal and an increase in the energy fraction of shower particles scattered between the crystal and detector. The ratios of total shower energy losses in oriented tungsten crystals to total losses in misoriented crystals 1, 2.7, 5.8, and 8.4 mm thick were 3.9, 2.6, 2.1, and 1.9, respectively.     

Electrochromic colour centres in amorphous tungsten trioxide thin films

Amorphous tungsten trioxide films, investigated by the Raman scattering method, are shown to be composed of a spatial network of tightly bound (WO₆)_n·mH₂O clusters with a large number of terminal oxygen W=O and W-O-W bonds between clusters. The injected electrons in an amorphous tungsten trioxide film are localized in the tungsten 5d orbitals in an axially distorted octahedron, as is shown by ESR analysis. The optical absorption of a coloured amorphous tungsten trioxide film, as has previously been proposed, can be satisfactorily described by an intervalence charge-transfer transition between localized W⁵⁺ and W⁶⁺ states.     

Cascade multiplication of strongly non-equilibrium charge carriers in metals by low energy electron excitation

The steady-state distribution function of the energies of non-equilibrium electrons and holes formed as a result of a cascade of electron-electron collisions in the presence of a primary electron flux is found by solving the linearized transport equation in the isotropic scattering approximation. The distributions obtained in this way include dependences on the energy of primary electrons and on the characteristics of the medium. Near the Fermi level they have a singularity and are close to a power law far from this level. A study is made of the possibility of deriving distributions for crystals with a complex energy band structure (in particular, of tungsten).