[en] Published in summary form only

[en] The collisionless Vlasov-Poisson system in the drift approximation is examined for the existence of maximum-entropy nonlinear coherent solutions in the steady state. Two major nonlinear effects are taken into account. The first is the velocity-space trapping of particles, leading to closed trajectories in phase space. The second is the physical-space trapping of particles, leading to closed trajectories in the plane perpendicular to the magnetic field. The regions of validity of these nonlinearities are discussed and their relative importance demonstrated. Numerical solutions of the equations describing the nonlinear stationary states in one and two dimensions are presented. (author)

[en] The laser-induced chemical etching mechanism of Cr-O doped GaAs (1 0 0) substrate in 40% HF-H₂O solution is explained by the generation of e-h pairs through defect states in the presence of sub-bandgap photon illumination by using a Nd:YAG Laser (λ∼1.06 μm). The central feature of the laser etching technique is pits initiation by surface defects. The etched GaAs samples are characterised by photoluminescence (PL) spectroscopy. The PL spectrum shows the formation of GaAs nanostructures and chemical reaction products. The shape of the PL band (1.3-1.8 eV) obtained from the top surface depends on the penetration depth of excitation wavelength. An estimate of the nanocrystallites sizes present in the laser etched layer is obtained by quantum confinement model

[en] The performance of a nuclear radiation detector fabricated from Vertical Gradient Freeze (VGF) semi-insulating Fe-doped InP was investigated. Pulse height spectra were acquired when the detector was irradiated with alpha particles from ²⁴¹Am, as a function of temperature and detector bias voltage. The spectroscopic performance of the detector was limited at room temperature due to the presence of a high leakage current. At a bias of -150 V, a room temperature leakage current density of 2.4x10^-6 A/mm² was observed which reduced to 7.1x10^-8 A/mm² at a temperature of -21 degree sign C. By biasing the irradiated detector contact at either a negative or positive potential, the charge collection efficiency (CCE) was measured separately for pulses produced predominantly by electron transport and for pulses produced predominantly by hole transport, respectively. At -21 degree sign C a maximum CCE of 72% was obtained for the electron signal and 44% for the hole signal. As a function of bias the CCE of the electrons remained constant in the temperature range -21 degree sign C to +19 degree sign C, whilst that of the holes exhibited a significant variation. By comparison with the Hecht relationship estimates of the carrier mobility-lifetime (μτ) products are deduced, which are similar for both holes and electrons and in the range 5x10^-7-8x10^-7 cm²/V. A reduction in μτ is observed at lower temperature for holes, whereas the value for electrons remains constant over the temperature range studied

[en] The complete equations governing fluid drift waves in a uniformly magnetised inhomogeneous plasma are reexamined. It is shown that in realistic parameter regimes the ion parallel motion nonlinearity gives rise to new monopole vortex solutions. These continue to exist even in the presence of a weak temperature gradient. (orig.)

[en] A novel method has been proposed for indefinite nonstochastic acceleration of particles by electrostatic waves propagating normal to a magnetic field. Use is made of a magnetic field inhomogeneity to prevent the particle from detrapping from the electrostatic wave. Numerical plots of particle trajectories are presented

[en] Ultrathin films of organic photochromic material (fulgides) are prepared via UHV-deposition on helium-cooled substrates. Controlled annealing results in two different types of nanostructures: amorphous dots of tunable size and nanocrystals. Both forms are stable at room temperature. Interconversion between the two photochromic isomers, the colorless E- and the colored C-isomer, can be achieved by irradiation with UV or visible light, respectively. Reversible optical switching of single dots is monitored by confocal fluorescence microscopy. For nanocrystals the isomeric state is monitored by optical-absorption spectroscopy and by X-ray Bragg diffraction

[en] We have characterised the radiation response of thick epitaxial GaAs layers that have been grown by water vapour-based chemical reaction, a new growth technique that has the potential to fabricate high quality thick epitaxial GaAs at low cost and with fast rates. The uniformity of these thick epitaxial layers has been assessed using room temperature photoluminescence mapping. The material shows very low EL2 concentrations and excellent whole-wafer uniformity, in contrast to conventional semi-insulating LEC or VGF grown bulk GaAs. Test detectors have been fabricated from the material and their response has been measured to alpha particle and low-energy gamma ray irradiation as a function of temperature. Pulse shape analysis of the signals produced by alpha particles show a fast and slow component, corresponding to charge transport in the depleted and under-depleted regions of the device. This indicates a shallow depletion layer due to non-intentional doping of the material from residual impurities. However, even outside the depletion region, good charge transport is observed in the material, which is consistent with long carrier lifetimes and a low EL2 concentration. CV measurements have been carried out in the temperature range +20 deg. C to -55 deg. C, which indicate a doping level of N_D∼1x10¹⁴ cm^-3. The thickness of the depletion region was measured using CV with an applied bias of 20 V, and was found to be approximately 10 μm at room temperature, increasing to >30 μm at -55 deg. C

[en] Extracerebral tumors represent 29-30% of the intracranial tumors. Meningiomas rank first in incidence with 13-19%, followed by neurinomas and pituitary adenomas, 6-8% each. Due to their slow growth rate, meningiomas in the initial stages do not produce symptoms. Complete recovery can be achieved by complete removal of the tumor, which in general is possible in case of tumor sites at the convexity. Meningiomas at the basis, or meningiomas spreading into the sinus sagittalis superior often cannot be totally removed, so that for treatment of remaining or recurrent tumors and anaplastic meningiomas, post-operative radiation therapy is recommended. Neurinomas of the nervus acusticus originate at the N. vestibularis and may spread from the Meatus acusticus internus into the brain stem. With the modern methods of microsurgery combined with intraoperative monitoring it is possible to preserve the Nn. facialis and cochlearis and their functions. As an alternative to surgery, radiosurgery techniques have been on trial in the last few years. Via transsphenoidal approach, intra- and suprasellar pituitary adenomas can be completely destroyed. Parasellar tumors can be resected by transcranial approach only. Remaining tumors in the Sinus cavernosus, or recurrent adenomas, should receive post-surgery radiation treatment. (orig./CB)

[en] The material uniformity and charge transport properties in CdZnTe radiation detectors are investigated by various non-destructive room temperature techniques. High-resolution secondary electron microscopy shows evidence of Te-enriched regions near the metal contacts. Photoluminescence (PL) mapping is used to image material defects and variations in Zn composition. Close to the electrodes PL spectra show strong defect-activated emission which is not observed in the material bulk. Ion-beam-induced-charge (IBIC) microscopy using a 2 μm resolution focussed proton beam is used to investigate the spatial variation in charge transport across detectors. Localized regions of poor charge collection efficiency seen in the IBIC images correlate with material defects observed in PL such as tellurium precipitates. The effects of these material defects in terms of the spectroscopic performance of the detectors to X-ray and γ-ray irradiation is discussed