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.     

Observation of parametric X-rays produced by 400 GeV/c protons in bent crystals

Spectral maxima of parametric X-ray radiation (PXR) produced by 400 GeV/c protons in bent silicon crystals aligned with the beam have been observed in an experiment at the H8 external beam of the CERN SPS. The total yield of PXR photons was about 10⁻⁶ per proton. Agreement between calculations and the experimental data shows that the PXR kinematic theory is valid for bent crystals with sufficiently small curvature as used in the experiment. The intensity of PXR emitted from halo protons in a bent crystal used as a primary collimator in a circular accelerator may be considered as a possible tool to control its crystal structure, which is slowly damaged because of irradiation. The intensity distribution of PXR peaks depends on the crystal thickness intersected by the beam, which changes for different orientations of a crystal collimator. This dependence may be used to control crystal collimator alignment by analyzing PXR spectra produced by halo protons.     

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.     

Spectrum broadening effect in coherent x-ray radiation of a relativistic electron crossing a single-crystal plate

Based on the dynamic diffraction theory [1], coherent x-ray radiation of a relativistic electron crossing a single-crystal plate at a constant velocity is considered in the Bragg geometry. In the general case of asymmetric reflection of the radiation from the target, expressions are derived for the spectral-angular distribution of parametric x-ray radiation (PXR) and diffraction transition radiation (DTR). For a fixed angle between the electron trajectory and the system of parallel atomic planes of the crystal (Bragg’s angle) it is shown that a decrease in the angle of electron incidence on the crystal plate gives rise to a significant increase in the PXR and DTR spectra, and the causes for spectral broadening for each of these radiation mechanisms are different. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 74–83, March, 2009.     

Parametric x-ray radiation of a relativistic electron under conditions of asymmetric reflection

Coherent x-ray radiation of a relativistic electron crossing a single-crystal plate in the Laue scattering geometry is considered in a two-wave approximation of the dynamic diffraction theory [1]. Analytical expressions describing the spectral-angular distribution of parametric x-ray radiation (PXR) and diffraction transition radiation (DTR) formed on the atomic planes located at an angle δ to the crystal plate surface (asymmetric scattering) are derived. Dependence of the spectral-angular density of PXR, DTR, and their interference term on the angle δ is investigated. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 80–89, August, 2008.     

Manifestation of dynamic effects in coherent X-radiation from relativistic electron in Bragg scattering geometry

Analytic expressions describing the spectral-angular distribution of parametric X-radiation (PXR) and diffracted transient radiation (DTR) of a relativistic electron traversing a single-crystal plate in the Bragg scattering geometry are derived on the basis of the two-wave approximation of the dynamic diffraction theory. The expressions are obtained in the general case of asymmetric reflection of a particle’s field relative to the surface of the crystal plate. It is shown that in the given geometry, a significant increase of the PXR angular density is possible due to the dynamic effect of PXR spectral broadening. The conditions for an increase in the DTR spectrum width, which causes a considerable increase of the DTR angular density, are derived. The possibility of the most pronounced manifestation of the Bormann dynamic effect in PXR is demonstrated for a thick absorptive crystal.     

Enhancement of spectral-angular density of parametric X-rays in Laue geometry due to change in the angle between a target surface and reflecting atomic planes

Coherent X-rays of a relativistic electron crossing a single crystal with a uniform velocity in the Laue scattering geometry are considered in the two-wave approximation of dynamic diffraction theory [1]. Analytical expressions for the spectral-angular distribution of parametric X-rays (PXR) and diffracted transition radiation (DTR) have been obtained. The case when the system of diffracting atomic planes of a crystal is located at an arbitrary angle δ to a crystal surface (asymmetric reflection) is considered. The value δ = π/2 corresponds to the symmetric reflection in the given scattering geometry. The dependence of the PXR and DTR spectral-angular density on the angle δ has been investigated. It has been shown that the PXR spectrum width depends substantially on the given angle, which, in particular, allows one to increase significantly the PXR angular density by decreasing the angle δ.     

Creating primary defects in calcium fluoride crystals with various prehistories, using pulsed electron irradiation

Optical spectroscopy with nanosecond resolution has been used to study how the prehistory (the presence of impurities, heat treatment) of CaF₂ crystals affects the processes of creating autolocalized excitons (AEs) under the influence of a pulse of accelerated electrons. The breakdown of the dispersivity condition for the creation of AEs is detected in a nominally pure, nonheat-treated crystal. It is shown that the initial structural defects play a defining role in creating the initial defect density. A model of the radiation disordering of an actual fluorite crystal is constructed. An analogy is traced to the processes of impurity and thermal disordering. Fiz. Tverd. Tela (St. Petersburg) 41, 442–450 (March 1999)     

Collinear interaction of bichromatic radiation with one acoustic wave in biaxial crystals

The regime of collinear interaction of bichromatic optical radiation in biaxial crystals is considered. It is shown that the possibility of the realization of such an interaction is due to dispersion of the optical axes in a biaxial crystal. Zh. Tekh. Fiz. 69, 131–132 (January 1999)     

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.     

Emission of single gamma rays by electrons with energies of hundreds of GeV in oriented crystals

Cross sections are calculated for the emission of single hard photons by electrons with energies of 150–1000 GeV as they pass through oriented crystals at small angles to the crystallographic axes. The contribution of incoherent emission at isolated atoms of the crystal is taken into account in the calculations, along with the emission in the continuum potential. The calculations are compared with the customary Bethe-Heitler spectrum for a thick amorphous target with allowance for photon absorption due to electron-positron pair production. It is shown that, in this range of energies, an oriented crystal can be more efficient than a thick amorphous target for creating a larger number of hard gamma rays with energies comparable to the energies of the emitting electrons. Zh. Tekh. Fiz. 68, 37–41 (September 1998)     

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.     

Nonlinear frequency conversion in 2D χ (2) photonic crystals and novel nonlinear double-circle construction

The analytic solution to the wave equation for small-signal sum-frequency process is derived in 2D χ ⁽²⁾ photonic crystals with use of the Green function method. It is predicted that the sum-frequency electrical field at quasi-phase matching (QPM) resonance is proportional to the angle-dependent effective crystal length. This implies that multiple wavelength QPM frequency conversion with controllable intensity output can be realized in a single 2D χ ⁽²⁾ photonic crystal. It is revealed that efficient frequency conversion requires both the QPM and the proper structure matching. A novel double-circle construction, different from the conventional Ewald construction, is presented to reflect important QPM processes. It is also shown that the QPM resonance tuning of second-harmonic generation can operate over the whole transparent wavelength range of crystals. Received 19 April 2001     

Studies of parity and time reversal symmetries in neutron scattering from165Ho

We describe searches for parity and time reversal violations in the scattering of polarized neutrons from polarized and aligned¹⁶⁵Ho targets. We have completed a search with 7.1 and 11.0 MeV neutrons for P_oddT_odd terms in the elastic scattering forward amplitude of the form s. (I×K), wheres is the neutron spin,I is the target spin andk is the neutron momentum vector. The target was a single crystal of holmium, polarized horizontally along itsb axis by a 1 Tesla magnetic field. The neutrons were polarized vertically. Differences in the neutron transmission were measured for neutrons with spins parallel (antiparallel) toI×k. The P,T violating analyzing powers were found to be consistent with zero at the few 10⁻³ level: ρ_P,T(7.1 MeV)=−0.88 (±2.02) x 10⁻³, ρ_P,T(11.0 MeV)=−0.4 (±2.88) x 10⁻³. We have also attempted to find enhancements with MeV neutrons in P-violation due to the term s\k. We are preparing an aligned target cryostat for investigations of P_evenT_odd terms {bd(I\k)(I×k)\s} in neutron scattering. The target will be a single crystal cylinder of¹⁶⁵Ho cooled to 100 mK in a bath of liquid helium and rotated by a shaft from a room temperature stepping motor. The cylinder will be oriented vertically and the alignment (c) axis oriented horizontally. Warming or rotation of the sample allows one to separate effects that mimic the sought-after time reversal violating term.     

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.     

Influence of uniaxial compression at 80 K on the formation of radiation defects in KCl, KBr, and KI crystals

The influence of uniaxial compression at 80 K on the efficiency of formation of stable radiation defects in KCl, KBr and KI crystals is investigated by absorption spectroscopy. It is found that compression along the 〈100〉 directions does not alter the efficiency of radiation defect formation in KCl and KBr crystals, but in KI the efficiency drops by more than an order of magnitude. It is concluded from a semiquantitative analysis that the observed difference is attributable to the impossibility of H centers fitting into the compression-reduced interstitial voids in KI, whereas several multiples of ten-percent compression is required to produce the analogous effect in KCl and KBr. Fiz. Tverd. Tela (St. Petersburg) 40, 73–78 (January 1998)     

Transmission spectra of thin CdS and CdSe crystals near higher-order isotropic points

Summary For photon energies below the absorption edge in CdS, CdSe and other II–VI crystals the polariton dispersion curves forE⊥c andE‖c (c-axis in a wurtzite crystal) corss at some points called isotropic points (IP). The occurrence of isotropic points provides the possibility of mode coupling between ordinary and extraordinary waves. Since the consequences of mode coupling on the optical properties for photon energies near the lowest IP lying much below the first excitonic state were widely discussed in recent years, more attention is now paid to isotropic points lying near the band gap and related to then=2,3, … excitonic states (?higher isotropic points?). Making use of Stahl's real density matrix approach we derive the polariton dispersion relationsk _⊥ (ω), andk _‖ (ω), for CdS and CdSe bulk crystals and determine the positions of IP's due to the crossing of theB-polariton with higherA-excitonic resonances. By the method of multiple internal reflection we calculate the transmission spectra for various crystal thicknesses (between 3 μm and 0.5 mm) and coupling mechanisms. The calculated transmission shows sharply peaked structures centred at the isotropic points.     

Interference between spontaneous two-photon radiation from two macroscopic regions

Interference is observed between the spontaneous parametric radiation from two nonlinear crystals separated by a macroscopic air gap and excited in series by a common pump beam. The phase of the interference depends on the phase shifts at three frequencies. A simple quantum model agrees well with the experimental results and makes it possible to interpret the effect in terms of general vacuum fluctuations which give rise to the spontaneous emission of mutually coherent radiation from the two crystals. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 20–25 (10 January 1997)     

Time-resolved spectroscopy of natural and synthetic BeO crystals

The results of comparative luminescence investigation of natural and synthetic BeO crystals are presented. Time-resolved luminescence (2.5–8 eV) and luminescence excitation spectra, and the kinetics of glow decay were measured using ultraviolet-vacuum-ultraviolet (VUV) synchrotron radiation (5–22 eV) or x-radiation (50–620 eV or 3–62 keV) ranges. X-ray and thermostimulated luminescence of natural BeO crystals were compared to the glow of additively colored synthetic crystals. The characteristic luminescence of F and F ⁺ centers was found in natural crystals. In synthetic crystals similar luminescence is observed only after additive or radiation coloration by virtue of the creation of F ⁻ and F ⁺ centers on anion vacancies. The defects found in the crystal lattice of a natural BeO crystal testify to the degree of mineral metamictization of the given deposit.     

Experimental observation of parametric X-ray radiation directed along the propagation velocity of relativistic electrons in a tungsten crystal

Parametric X-ray radiation (PXR) due to dynamic diffraction of relativistic electrons is experimentally observed at small angles to the propagation velocity of electrons in a tungsten crystal. The specific features of the experimental method are described, and forward PXR reflections from two crystallographic planes of tungsten are reliably measured.