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.     

Bremsstrahlung during electron capture to continuum

Theoretical approaches for radiative ionization (RI) in energetic collisions of highly stripped projectiles with target atoms are reviewed and set into context with related processes. The interrelation between RI and electron-nucleus bremsstrahlung is displayed with the help of inverse kinematics. Particular emphasis is laid on the forward-peak region of the electron spectrum resulting from target electrons which are slowed down to approximately zero velocity in the projectile frame of reference. The forward-peak intensity and shape for RI is contrasted to the one obtained from nonradiative capture to continuum. The ridge in the photon spectrum related to forward-peak electrons can unambiguously be identified in measured doubly differential photon emission cross sections resulting from ion–atom collisions at relativistic impact energies.     

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 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 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 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

     

“Atomic Bremsstrahlung”: Retrospectives,current status and perspectives

We describe here the “Atomic bremsstrahlung” (AB)—emission of continuous spectrum electromagnetic radiation, which is generated in collisions of particles that have internal deformable structure that includes positively and negatively charged constituents. The deformation of one or both colliding partners induces multiple, mainly dipole, time-dependent electrical moments that become a source of radiation. The history of AB invention is presented and its unusual in comparison to ordinary bremsstrahlung (OB) properties, are discussed. As examples, fast electron atom, non-relativistic and relativistic collisions are considered. Attention is given to ion–atom and atom–atom collisions. Specifics of “elastic” and “inelastic” (i.e. radiation accompanied by destruction of collision partners) AB will be mentioned. Attention will be given to possible manifestation of AB in nature and in some exotic systems, for instance scattering of electrons upon muonic hydrogen. Some cooperative effects connected to AB will be considered. New classical schemes similar to AB will be presented.     

General deconvolution of thin-target and thick-target Bremsstrahlung spectra to determine electron energy distributions

Electron energy distributions have been determined from thin-target Bremsstrahlung for many years. For thick-target Bremsstrahlung however, even the simple diagnostic that immediately gives the distribution from the second derivative of the emissivity has been established only recently. For cases when the underlying assumptions are unjustified, further deconvolutions are proposed: a matrix inversion method involving a functions basis, and an iterative method which is more general and stable. Both methods could also be applied to thin-target Bremsstrahlung.     

mj Rainbows in molecule-surface scattering

A quasiclassical trajectory study of N₂ scattering from a rigid corrugated crystal surface was carried out. Rainbow maxima were seen in the m_j distribution. The maximum value of product j at which m_j, rainbows occur is found to increase linearly with the corrugation and decrease linearly with the hardness of the interaction potential.     

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.     

Reactive scattering in hyperspherical coordinates

Preliminary results are presented for transition probabilities in the H + H₂ system derived from an adiabatic representation in terms of surface functions on hyperspheres. Special attention is given to the resonance structure for transition probabilities in the first vibrational level.     

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.     

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.