L Kocbach

Determination of void fraction and flow regime using a neural network trained on simulated data based on gamma-ray densitometry

This paper describes low-energy gamma-ray densitometry using a 241Am source for the determination of void fraction and flow regime in oil/gas pipes. Due to the reduced shielding requirements of this method compared to traditional gamma-ray densitometers using 137Cs sources, the low-energy source offers a compact design and the advantage of multi-beam configuration. One of the aims of this investigation was to demonstrate the use of a neural network to convert multi-beam gamma-ray spectra into a classification of the flow regime and void fraction, as well as to determine which detector positions best serve this purpose. In addition to spectra obtained from measurements on a set of phantom arrangements, simulated gamma-ray spectra were used. Simulations were performed using the EGS4 software package. Detector responses were simulated for void fractions covering the range from 0 - 100%, and the simulations were performed with homogeneous, annular and stratified flows. Neural networks were trained on the simulated gamma-ray data and then used to analyse the measured spectra. This analysis allowed determination of the void fraction with an error of 3% for all of the flow regimes, and the three types of flow regime were always correctly distinguished. It has thus been shown that multi-beam gamma-ray densitometers with detector responses examined by neural networks can analyse a two-phase flow with high accuracy.

The semiclassical approximation for L- and M-shell Coulomb ionization by heavy charged particles

The semiclassical approximation with straight-line trajectories is applied to the Coulomb ionization of K, L and M shells by heavy charged particles. The calculational aspects are discussed in detail. Scaling relations for the experimentally relevant quantities are derived. The theoretical predictions are compared with experiment. The relation of the present work to the earlier results of the semiclassical approximation and partial-wave Born approximation is discussed in detail.

Interference effects in the ionization of H2 by fast charged projectiles

A theoretical investigation of the experimentally observed (Stolterfoht N et al 2001 Phys. Rev. Lett. 87 023201) interference effects in the double differential cross sections for ionization of the hydrogen molecule by fast ion impact is reported. The H2/2H cross section ratios as a function of the ejected electron velocity show an oscillating pattern, for which Stolterfoht et al propose a formula C + G sin(k D)/(k D), where k is the electron momentum and D the internuclear separation in H2. Our analysis in its simplest form leads instead to a formula C + G sin(k|| D)/(k|| D) where k|| is the component of k parallel to the projectile velocity. The presented theoretical model thus explains why at 90° the interference pattern will be strongly suppressed. In addition to the simplified analysis a numerical evaluation of a more accurate model is presented, confirming the latter qualitative prediction.

Total K–Shell Coulomb Ionization Cross Sections at Very Low Projectile Energies

The first-order semiclassical approximation method with hyperbolic projectile trajectories, velocity symmetrization, relativistic electronic wavefunctions and target recoil amplitudes is used for systematic studies of K-shell Coulomb ionization. The first-order description is modified by the use of a binding correction. In the low projectile velocity region studied the theoretical cross sections are largely consistent with weighted averaged measurements. Two sets of empirical reference cross sections are considered, the first being found to contain an additional superfluous target dependence, the second improved set showing considerably increased agreement with the present SCA results. The inadequacy of the ECPSSR model for the very low projectile energy region is demonstrated. The implications for the construction of model independent empirical scaled cross sections are discussed.

Electronic relativistic effects in the K-shell Coulomb ionization of heavy atoms by massive charged particles

The straight line semiclassical approximation with relativistic Coulomb functions for the electrons is applied to the ionization process. Only the l=O final state electrons are considered. The effects of Coulomb deflection of the projectile are taken into account. The agreement between theory and experiment is considerably improved.

Cross sections for capture, excitation and ionization in proton-oxygen collisions

Cross sections for one-electron capture, excitation and ionization in proton-oxygen collisions are calculated by the two-centre atomic orbital expansion close-coupling method. The transition probabilities are constructed from a one-electron independent-particle approximation. The electronic states in oxygen are represented by orbitals obtained from an analytical model potential with parameters fitted to the Hartree potential of the ground state configuration. Capture cross sections are dominated by the 2p electrons and are in very good agreement with measurements.

Geometrical simplification of the dipole-dipole interaction formula

Many students meet dipole–dipole potential energy quite early on when they are taught electrostatics or magnetostatics and it is also a very popular formula, featured in encyclopedias. We show that by a simple rewriting of the formula it becomes apparent that, for example, by reorienting the two dipoles, their attraction can become exactly twice as large. The physical facts are naturally known, but the transformation presented seems to underline the geometrical features in a rather unexpected way. The consequence of the features discussed is the so-called magic angle which appears in many applications. The present discussion contributes to an easier introduction of this feature. We also discuss the possibility of designing educational toys and try to suggest why this formula has not been written down frequently before this work. A similar transformation is also possible for the field of a single dipole. In this case we found one such formula on the Web, but we could not find any published detailed discussion for this case either.

Proton-induced L-shell ionization at large scattering angles

Abstract The ionization probabilities for proton-induced L-shell ionization have been calculated in the semi-classical approximation with hyperbolic Kepler projectile orbits. The Coulomb deflection of the projectile changes the ionization probabilities considerably at small impact parameters for the L 2 and L 3 shells. Culomb deflection effects on the total cross sections are also briefly discussed.

On the description of electronic final states in K-shell ionization by protons

The choice of free electronic wavefunctions in the description of K-shell ionization by protons is discussed. The previously known discrepancies between PWBA and SCA results are shown to be entirely due to two different choices of electronic wavefunctions. Calculations in the SCA framework with Hartree-Fock-Slater wavefunctions are reported. Some general features of the SCA calculations are discussed.

Electron transport in a double quantum dot controlled by magnetic switching

We investigate the possibility of performing single-electron controlled transport in coupled quantum dots based on magnetic switching. From numerical solution of the time-dependent Schrodinger equation it is shown that certain combinations of static and switched magnetic fields can result in a situation where an initially localized wavefunction can be transferred from one of the dot centres to the other one with unit probability.