Table of contents

The maximum-entropy approach is used to calculate some problems in radiative transfer and reactor physics such as the escape probability, the emergent and transmitted intensities for a finite slab as well as the emergent intensity for a semi-infinite medium. Also, it is employed to solve problems involving spherical geometry, such as luminosity (the total energy emitted by a sphere), neutron capture probability and the albedo problem. The technique is also employed in the kinetic theory of gases to calculate the Poiseuille flow and thermal creep of a rarefied gas between two plates. Numerical calculations are achieved and compared with the published data. The comparisons demonstrate that the maximum-entropy results are good in agreement with the exact ones.

We present a simple analytic expression for the generalized diagonal matrix element <nl∣r^β∣nl> for arbitrary β and fixed l, using the fully relativistic hydrogen wavefunctions. On the basis of this expression, exact matrix elements corresponding to the value of l in the range n - 5 ⩽ l ⩽ n - 1 are obtained and shown to reduce to their nonrelativistic counterparts when the appropriate limit is taken.

We present two different approaches to evaluate the nonlocal square root operator that usually appears in the relativistic equation. We also introduce a new strategy to solve coupled equation systems such as Dirac coupled equations and Bethe-Salpeter coupled equations and apply this strategy to the solution of Dirac coupled equations with vector and scalar potentials.

The coupling of bound states in a single-particle effective Hamiltonian through the addition of a negative definite imaginary potential is studied, both in a stationary and in a time-dependent formalism. Corresponding physical cases where this coupling is relevant are exhibited. Properties of the imaginary coupling matrix in the representation of the eigenstates of the real Hamiltonian are investigated. Consequences of these properties on the level widths and level shifts are exhibited on the two level case. Non trivial results, like forbidden values of the widths and level attraction are underlined. The conditions for the validity of the weak coupling approximation are examined. In the time-dependent problem, the typical time evolution patterns are illustrated both in the two level case and in the more realistic case of charmonium decay. When the initial state is an eigenstate of the real part of the Hamiltonian, it is shown that mixing of decay modes and quantum interference arise as consequences of the non diagonal imaginary coupling. Finally, the non locality of the imaginary potential corresponding to a diagonal imaginary coupling matrix in the state representation is also briefly studied and illustrated.

By employing the turbulent theoretical approach of Alber, Davidson and Roy and Bhakta the dynamical evolution of the wave spectrum of nonlinear wavetrains near marginal stability is derived, starting from a derivative nonlinear Schrödinger equation, extended with a quintic nonlinearity, called the EDNLS-equation. The stability of three homogeneous background spectral densities (the delta-, the normal- and the uniform distribution), is investigated in the linear regime. The most important findings in that respect as as follows: First, the delta-distribution instability criterion deviates by a numerical factor from the one presented in Flå and Wyller for the modulational instability in the EDNLS-equation. Secondly, the growth rate of this instability result in the limit of small, but finite spectral width limit is equivalent with the turbulence theory result based on the derivative nonlinear Schrödinger equation up to modulo translation in the central wave number.

The existence of particle outflow in the decay of many-body system leads to a special description with two sets of state eigenvectors and corresponding complex eigenvalues. Using such eigenvectors the general time behavior of the decaying system is obtained. As an example, we consider the problem of a particle decay of the many-body system to demonstrate the difference between this problem and the problem of a single particle in potential well. We also consider the case of particle scattering by an optical potential and show that the existence of outflow also leads to two sets of state eigenvectors, but with real energy eigenvalues.

This paper summarizes the LEP results on beauty physics obtained with more than one million hadronic Z° decays registered per experiment. They mainly concern measurements of the inclusive b lifetime, the semi-leptonic branching ratio, exclusive lifetimes and production rates for the different B hadrons, B°-° oscillations and measurements of the partial decay width and asymmetry of the Z° to b quarks. These results are compared with similar results obtained at different e⁺e⁻ machines and p colliders.

We study the single electron capture process by impact of bare ions on hydrogen-like atoms, at intermediate and high collision energies. The theoretical analysis is carried out within the impact parameter approximation, by using distorted wave models. We present a prior version of the scattering amplitude: the Continuum Distorted Wave-Eikonal Final State approximation. Post-prior discrepancies are studied, by comparison of total cross sections calculated with this model and with the Continuum Distorted Wave-Eikonal Initial State approximation. We introduce a single prescription to select the model that should be employed, according to the asymmetric character of the collision system. This prescription also considers the case of symmetric collisions. Capture to selective final states and to any final state, by impact of H⁺, He²⁺ and Li³⁺ projectiles on H(1s) targets is analyzed. Comparisons with experimental data and with Continuum Distorted Wave and Symmetric Eikonal theoretical predictions are given.

The spectra of selenium and bromine were photographed in the 400 Å-1200 Å wavelength region on a variety of spectrographs using a triggered spark source. The spectra of Se V and Br VI were investigated. Two previously reported levels in Se V viz. 4p^{2 1}S₀ and 4s5s¹S₀ and two previously reported levels in Br VI viz. 4p²¹S₀ and 4s5p³P₀ were revised. All levels of the 4s4f, 4p4d and 4p5s configurations in Se V, and all levels of the 4s4f and 4p4d configurations in Br VI have been established. Least-squares-fitting (LSF) parametric calculations involving configurations interaction have been carried out to interpret the spectra satisfactorily. 57 new lines in Se V and 54 new lines in Br VI have been classified.

We study the formation of positronium through electron capture in collisions of positrons impacting on mono and dielectronic atoms (ions). We obtain the scattering amplitude of the charge exchange process by using the Coulomb Born Approximation (CBA). We apply the CBA approximation to the system e⁺ + H(2s) and e⁺ + He⁺(2s) with formation of positronium in the ground state. We also study the process e⁺ + He(1s²) with formation of positronium in 1s, 2s and 2p states analysing the influence of the non-captured electron on the reaction. We compute differential and total cross sections for these reactions. The energy range varies from some eV to hundreds of eV. We compare our results with other theories and recent experimental data.

The literature data on Si XI (Be-like ion) have been compiled with a critical re-analysis for their consistency. High-resolution EUV beam-foil spectra provided new data. With the help of Cowan code ab initio and parametric calculations and of isoelectronic comparisons a revised level scheme was constructed which includes most levels up to configurations with principal quantum number n = 5, as well as some n = 6 levels. About 200 spectral lines in the region 30-613 Å are newly identified with transitions between configurations with n = 2, 3, 4, 5 and 6.

The isotope shift in a platinum hollow cathode discharge was studied for transitions of the type 5d⁹7s → 5d⁹6p, 5d⁹7s → 5d⁸6s6p and 5d⁹6p → 5d⁸6s² through laser optogalvanic spectroscopy and high resolution interferometry. Considering configuration mixing in each of the levels of 5d⁹6s, 5d⁸6s², 5d⁸6s6p and 5d⁹6p of all known and the new evaluated level isotope shifts, we obtain average configuration isotope shift values for the even configurations ΔT(5d⁹6s) = 2425 (90) MHz, ΔT(5d⁸6s²) = 5960 (60) MHz, ΔT(5d⁹7s) = 260 (160) MHz and for the odd configurations ΔT(5d⁹6p) = −230 (380) MHz, ΔT(5d⁸6s6p) = 2740 (250) MHz, ΔT(5d⁷6s²6p) = 9700 (4700) MHz. These experimental values compare well with the theoretically predicted configuration isotope shifts in platinum, based on pseudo-relativistic Hartree-Fock calculations.

The energy spectra and the electric dipole transitions in the boron isoelectronic sequence (Z = 8 - 26) between the levels of the 1s²2s²2p, 1s²2s2p² and 1s²2p³ configurations were considered. The stationary second-order many-body perturbation theory (MBPT) was used to account for the electron correlations. The relativistic corrections were included in the Breit-Pauli approximation.

L_L, L_α, L_β and L_γ X-ray fluorescence cross sections have been measured for the elements Gd, Dy, Ho, Er, Yb, Pt, Au and Pb at the excitation energies 35.86 and 39.90 keV. Measurements have been performed using an X-ray tube with a secondary excitor system as the excitation source instead of radioisotopes. In order to improve the efficiency of the detection system, the excitation source sample and the detector were placed in a vacuum chamber and a pressure of 10⁻² mbar is maintained throughout the measurements. The present system considerably reduced the scattering and background effects and improved the monochromaticity. Experimental results are in excellent agreement with the theoretical values of Chen et al.

The axial distribution of the electron density related to the high-frequency field penetration in gas discharges maintained, in the diffusion controlled regime, by ionizing surface waves is obtained on the basis of the energy balance equations. It is shown that under conditions of comparatively weak electron collisions, the resonance absorption of the surface wave could cause essential deviations of the axial dependence of the plasma density from the linear one at both the end and the beginning of the discharge.

The problem of quasi-flute mode stability in toroidal plasma configuration is reduced to an ordinary differential equation of the second order. This equation has the same form as the "distilled" ballooning equation, however, it is derived in Fourier space of wavenumbers, where the ratio of the radial component of the wavevector to the poloidal one is an independent variable, instead of using ballooning variables. The obtained equation is transformed to the form of a stationary Schrödinger equation. It is shown that the discrete eigenvalue spectrum of this equation corresponds to the unstable modes. Two regimes of quasi-flute mode excitation corresponding to the cases of fulfilment and violation of the Mercier criterion of plasma stability are found. The dispersion relations defining quasi-flute perturbation growth rates are derived for both regimes.

This paper presents an investigation on the conservation of the nonresonant quanta in a new mode-coupling process, namely, the plasma-maser interaction. Considering interaction of ordinary mode electromagnetic wave with Langmuir turbulence in a magnetized plasma, it is shown that the anisotropy induced by the external magnetic field is primarily responsible for the nonconservation of the number of quanta in an open system. In contrast, the number of quanta of the high frequency wave is conserved in a closed system.

The thermal conductivity at 2-200 K, both in-plane and out-of-plane, and heat capacity at 4-80 K of layered vermiculite have been measured. Our experimental data have been analyzed together with the thermal conductivity data for muscovite and phlogopite (from literature), taking into account the peculiarities of the lattice dynamics of the complex layered materials. The low-lying optical modes play an important role both for the heat capacity at T > 20 K and the phonon scattering. The main mechanisms of phonon scattering at the lowest temperatures are found to be the boundaries and the structure defects. The defects give a quadratic frequency dependence of the inverse relaxation time; such a dependence may be explained by the peculiar displacement-distance relation for point defects in layered structures.

A possible explanation for the reported "universal" electron energy exponent Γ ≃ −1.4, which corresponds to an optically thin spectral index α ≃ −0.2(S_v ∝ v²) (Valtaoja E 1990 Variability of Active Galaxies edited by W J Duschl, S J Wagner and M Camenzind (Springer, Berlin)) is discussed. A comparison of Valtaoja's variability data (Valtaoja et al 1998 Astron. Astrophys.203 1) with those of Wardle (1977 Nature269 13) leads to the conclusion that compact, variable radio sources contain much more ultrarelativistic particles than thermal plasma. The shock compression ratio r in a relativistic plasma is 7 (instead of 4 in a nonrelativistic gas). r ≃ 7 results in an optically thin spectral α ≃ −3/(2r − 2) = −0.25, quite close to the value indicated by the observations.