Ultraviolet Bremsstrahlung spectroscopy

Recent experimental progress has introduced Bremsstrahlung spectroscopy especially in the isochromat mode as a new surface analytical tool. Bremsstrahlung production may be regarded as radiative capture by solids and is therefore the inverse process to the well known photoemission. In contrast to photoemission which probes occupied electronic states at solid surfaces, Bremsstrahlung spectroscopy opens up the possibility to investigate unoccupied electronic states including the important region between Fermi- and vacuum energy. Measurements on polycrystalline materials will be discussed on the basis of an isotropic three step model for Bremsstrahlung emission. More recent experiments on single crystals with electrons of well defined energy and initial momentum $\text{k}\text{}$ have demostrated that $\text{k}\text{}$-resolved spectroscopy provides energy versus momentum dispersion relations for unoccupied bands. Application of spin polarized electron beams has advanced $\text{k}\text{}$-resolved Bremsstrahlung spectroscopy to a state of maturity comparable to spin polarized angle resolved photoemission. This paper attempts to review the development and summarize the present state of ultraviolet Bremsstrahlung isochromat spectroscopy.     

Bremsstrahlung from few-electron ions

We have developed the new approach which can be used for accurate evaluations of the bremsstrahlung cross-sections in the case of few-electron atoms and ions. Our approach is based on the explicit formula for the electron density distribution in such systems. We derive the closed analytical formula for the matrix elements which are needed for highly accurate computations of atomic form-factors of two-electron atoms and ions. Multiple bremsstrahlung from few-electron ions is considered. We also discuss the energy loss due to bremsstrahlung in a plasma which contains multi-charged ions and free electrons. Bremsstrahlung from a high-temperature plasma is considered as well as its role in the high-temperature burn-up of deuterium plasma.     

Bremsstrahlung from a shielded beta irradiator

A beta irradiation unit is described which contains a 4 GBq⁹⁰Sr source. The shielding, comprising perspex, aluminium and lead is designed to provide an acceptably low doserate to the user without exceeding a manageable mass as it is necessary to move the shielded source while in use for thermoluminescence dating. The cylindrical shield is 192 mm in diameter and 192 mm high and has a mass of 33 kg. Measurmeents of bremsstrahlung spectra from the system are reported and interesting features noted. A comparison of the bremsstrahlung dose-rates as determined by three different portable dose-rate monitors is also presented.     

Study of low-energy internal Bremsstrahlung radiation from Pm

The absolute intensity of the internal Bremsstrahlung radiation following the beta decay of ¹⁴⁷Pm has been measured using a high-resolution X-ray detector in the low-energy region from 3 to 33 keV. Magnetic-deflector method has been used to avoid the betas from reaching the detector. The corrected spectral intensity has been compared with the Nilsson-corrected KUB theory. Good agreement between experiment and theory is seen in this energy region.     

Bremsstrahlung from gas atom targets: Study of background processes

Recent absolute bremsstrahlung cross section experiments on gas targets of Ne, Ar, Kr and Xe at 28 and 50 keV have shown a significant polarization bremsstrahlung (PB) contribution, in contrast with previous thin-film experiments where no PB has been seen. Recently, Obolensky and Pratt have considered ways to improve the PB model, but the theory is still about 20% below the data. While a more complete theoretical calculation is certainly needed, we consider two additional background processes, not corrected in the experiment, that depend on the background photon spectrum in the beam line produced by electron interaction with collimators in the beam. We compare an estimate of both backgrounds with that from beam electrons elastically scattered by the gas into the cell window or wall and discuss initial efforts to measure these backgrounds.     

Bremsstrahlung cross-section with screening and Coulomb corrections at high energies

Using analytical formulae which are exact in Born approximation, the doubly differential bremsstrahlung cross-section with form-factor screening is calculated. For the atomic form factor parameters are applied which approximate self-consistent Dirac–Hartree–Fock–Slater calculations. The evaluation of the bremsstrahlung spectrum requires a single numerical integration. The results are superior to the customary Bethe approximation, in particular at the high-energy part of the spectrum. At high energies the screening correction can be added to any Coulomb-corrected cross-section without screening. In the present work, we are using a cross-section calculated by means of Sommerfeld–Maue functions with additional higher-order terms.     

Applicability study on existing dosimetry systems to high-power Bremsstrahlung irradiation

Applicability of the existing dosimetry systems to high-power Bremsstrahlung irradiation was investigated through a dose intercomparison study, where several dosimeters were irradiated in the dose range 4–12 kGy in identical polyethylene phantoms in a Bremsstrahlung beam obtained from a 5-MeV electron accelerator. Included in the study were alanine dosimeters molded by three different binders, three types of liquid dosimeters—ceric-cerous, dichromate and ethanol-chlorobenzen (ECB), and glutamine powder. The dosimeter responses for Bremsstrahlung radiation were analyzed at the issuing laboratories, and the dose values determined using calibration based on cobalt-60 gamma-ray irradiation. Dose values for all the three dose levels for all dosimetry systems were in good agreement—better than 3%. The results of the study demonstrate that these existing dosimetry systems have a potential for application to high-power Bremsstrahlung irradiation.     

Bremsstrahlung emitted during (n;t)-reactions in light atomic systems

Bremsstrahlung emitted during (n;t)-nuclear reactions in light atoms and ions is discussed by using both the classical and quantum-mechanical approaches. In particular, the spatial and angular distributions of the emitted radiation and its spectrum are considered in detail. It is shown that the total radiation I(t) rapidly decreases with time I(t)∼t⁻⁴ at large t. The spectral density of the emitted bremsstrahlung is also studied. In general, bremsstrahlung emitted during (n;t)-nuclear reactions in the fast ions and atoms can be used to study the electron–electron position correlations in these systems. The approach developed in our present study can also be applied to describe the emission of bremsstrahlung during (n;p)- and (n;α)-nuclear reactions in light atoms, spontaneous and neutron-stimulated nuclear fission in heavy atoms.     

Bremsstrahlung spectra from lead and tantalum at 6 MeV electron energy

Bremsstrahlung spectral distributions have been experimentally measured for thick targets of lead and tantalum at 6 MeV electron energy, using Compton scattering technique. The experimental results are in agreement with the theoretical results.     

Bremsstrahlung induced Auger electron spectra (BAES) of transition metals

Summary Bremsstrahlung induced high energetic Auger transitions of Nb(L₂₃M₄₅M₄₅) and the 5d-transition metals (M₄₅N₆₇N₆₇) have been measured, some of them for the first time. The fine structure splitting as well as the lineshapes were calculated in the frame of the mixed coupling scheme (jj-coupling in the initial state and intermediate coupling in the final state).The background, induced by inelastically scattered electrons was taken into account. Good agreement between experiment and theory was then obtained in all cases. From the Auger spectra of a LiNbO₃-sample the oxidation state of Nb could be determined.It was demonstrated that the Nb oxidation state could be changed by ion-bombardment and that the relative amount of the different oxidation states could be determined with high precision.

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Bremsstrahlungsinduzierte Augerelektronenspektroskopie (BAES) an Übergangsmetallen, angewandt auf die Analyse der Oxidationsstufen von LiNbO₃-Oberflächen

     

Small-angle scattering in materials science

The use of small-angle scattering techniques in the study of inhomogeneities and microstructural changes in inorganic materials is reviewed with emphasis on X-ray scattering. Recent applications of anomalous scattering with synchrotron radiation and kinetic studies in amorphous and crystalline systems are stressed.     

Direct methods in small-angle scattering

Direct methods of small-angle scattering data treatment and interpretation are reviewed. The possibilities of Tikhonov's regularization method for solving ill-posed experimental data processing problems (desmearing, size distribution functions evaluation) are shown; comparison with other data treatment methods is made. Methods for direct reconstruction of the structure of particles in isotropic monodisperse objects are presented. The cases of particles described by one- and two-dimensional density distributions and the possibilities of application of the multipole expansion theory are considered.     

Decoherence effects in reactive scattering

Decoherence effects on quantum and classical dynamics in reactive scattering are examined using a Caldeira-Leggett type model. Through a study of the dynamics of the collinear H + H2 reaction and the transmission over simple one-dimensional barrier potentials, we show that decoherence leads to improved agreement between quantum and classical reactions and transmission probabilities, primarily by increasing the energy dispersion in a well-defined way. Increased potential nonlinearity is seen to require larger decoherence in order to attain comparable quantum-classical agreement.     

Matrix optics in dipolar scattering

Summary P. Soleillet's theory of dipolar scattering has been investigated throughMueller matrix optics. Interesting results are obtained which can be used for testing a dipolar medium as well as for verifyingSolcillet's theory. It is concluded thatSoleillet's theory is non-general in validity.     

Estimation of parameters of the scattering particles in horn keratin by low-angle X-ray scattering

Low-angle x-ray scattering methods have been applied to determine the parameters of the scattering materials in horn keratin. A low-angle camera of Kratky's latest design has been used for the experimental measurements. By treating the substance as a densely packed system, the evaluation of the parameters has been made after theories of Porod and Kratky. Pore analysis of the substance yields a specific inner surface of 4.7 × 10^?1m²/cm³, a heterogeneity distance l_c = 290.2Å and a reduced inhomogeneity length l_r = 99.4Å. The volume of the pores has also been determined approximately.     

Brownian dynamics in strongly scattering porous media - dynamic light scattering with single-mode matching

Brownian motions in porous media represent a challenging problem not only from a theoretical but also from an experimental point of view. A very powerful technique for the measurement of Brownian motions is Dynamic Light Scattering (DLS). For our problem, however, the classic version of DLS is useless because the porous matrix scatters very strongly and the particles to be measured are completely masked. Sometimes it is possible to suppress the background scattering by matching the refractive indices of the matrix and the confined liquid, but this restricts the applicability to a handful of suitable model systems. We have introduced a new technique which overcomes this difficulty: The keyword is single-mode matching and the idea is to select from the complicated random light field generated by the strongly scattering medium only a single mode so that the contribution from the matrix to the selected mode vanishes because of destructive interference. The first experimental results on the dynamics of latex particles in aqueous suspensions confined in a packing of glass beads are very promising. Their quantitative analysis demonstrates clearly the feasibility of measuring the diffusion coefficient within an opaque strongly scattering porous medium.     

Rotational excitation in electron-molecule scattering at intermediate collision energy: A two-centre scattering model

A simple two-centre scattering model is discussed, which leads to compact closed form differential cross sections for rotationally inelastic scattering from diatomic molecules. The model elucidates in a simple way the rotational rainbow structure of the cross sections for both initially rotating and nonrotating molecules. Surprisingly good agreement with extensive computations and experimental measurements fore-Na₂ scattering at 150–300 eV is observed.     

Thomson scattering in strong external fields

In the present paper we shall investigate relativistic Thomson scattering in two external fields. A free classical electron will be embedded in a strong, constant and homogeneous magnetic field and in a powerful electromagnetic field. Both fields will be considered in the Redmond configuration, in which case the electromagnetic wave is circularly polarized and propagates in the direction of the homogeneous magnetic field. The electron will be allowed to have arbitrary initial conditions and the electromagnetic wave will be switched on either suddenly or adiabatically. We shall present the exact solution of the Lorentz equation of motion in the above external field configuration and we shall evaluate the spectrum and cross sections of the scattered radiation. In particular, we shall consider scattering close to resonance and we shall compare our results with the findings of earlier work.