Table of contents

Helium at pressures ranging from 0.5 to 8 bar is excited by an intense e-beam. The absolute population densities of the He(2 ¹P), He(2 ³P) and He₂^*(a,v), v=0, 1, 2 states are determined by measuring the transient absorption of probe light delivered by a dye laser. The experimental data are readily described by a kinetic model taking into account the excitation and the de-excitation processes. The recombination of He₂⁺ by a termolecular reaction with e+He to He(2 ³P) as proposed by Bates (1984) is confirmed quantitatively. The rate constants for the termolecular reactions of He(2 ¹P) and He(2 ³P) were deduced to be (1.8+or-0.4)*10^-31 cm⁶ s^-1 and (1.6+or-0.4)*10^-32 cm⁶ s^-1 respectively. The rotational and vibrational relaxation of He₂(a) by helium is investigated.

Laser-induced increases of discharge current were observed in the discharge media containing various halogen compounds (F₂, HF, HCl, Cl₂, HBr, CH₃BR, CH₃I and CH₂I₂) in N₂. The increases of transient current were attributed to the photodetachment of negative ions in the discharge media. On the basis of general considerations, the negative ions present in the discharge are assumed to be the atomic halogen negative ions. Photodetachment cross sections were determined from the current increases as a function of laser flux. Photodetachment cross-sections of F^-, Cl^-, Br^- and I^- are (0.75, 2.5, 3.3 and 7.0)*10^-17 cm² at 193 nm and (0.6, 1.0, 1.5 and 3.0)*10^-17 cm² at 248 nm, respectively. These data are compared with the earlier results of negative ion beam experiments and theoretical calculations.

The direct etching of GaAs substrates by excimer laser light has been studied at wavelengths of 193 nm (ArF). 248 nm (KrF) and 351 nm (XeF). For absorbed laser fluences greater than the direct etching fluence thresholds of 23 mJ cm^-2 for ArF, 33 mJ cm^-2 for KrF and 80 mJ cm^-2 for XeF, the authors observe direct etching in air to depths of order one nanometre per pulse. For low-fluence exposures in either vacuum (10^-2 Torr) or N₂ (1 atm) environments the etch depth per pulse is less than a quarter of that observed when exposure takes place in air. For higher fluence exposures ( approximately 250 mJ cm^-2) the etched regions show signs of melting. Using a line-narrowed KrF excimer laser and holographic interference techniques gratings of period 1.5 mu m were directly etched in GaAs substrates.

Exact relations for radiation heat flux at the boundaries of a slab with reflecting boundary conditions and internal source are obtained in terms of the reflection and the transmission coefficients (albedos) of a source-free slab with isotropic boundary conditions. Available exact values of the albedos give exact values of radiation heat flux. Configuration factors are calculated for comparison.

Electron energy distribution functions (EEDFS) in RF collision dominated N₂ discharges have been calculated by solving the time-dependent Boltzmann equation including elastic, inelastic and superelastic collisions. The results have been obtained at different field amplitudes E₀/ rho ₀ (10<or=E₀/ rho ₀<or=20 V cm^-1 Torr^-1), at different frequencies omega / rho ₀ ( pi *10⁶<or= omega / rho ₀<or= pi *10⁸ s^-1 Torr ^-1) and different concentrations of excited (vibrational and electronic) states. It has been found that the presence of excited states strongly modified the modulation of the tail of EEDFS with strong consequences on the collision frequencies of processes having high energy thresholds. The role of field frequency is studied for the understanding of both the temporal behaviour of EEDFS and the averaged (over the period) values of EEDFS and of macroscopic quantities. The authors obtain that at fixed E₀/ rho ₀ the increase of frequency decreases the tail of EEDFS, strongly reducing the collision frequency of the ionisation process. Finally the importance of the time-dependent solution of the Boltzmann equation is tested by comparing the present results with the corresponding ones obtained by using the effective field approximation. Differences up to several orders of magnitude are obtained in this comparison, the differences being reduced at high E₀/ rho ₀, high omega / rho ₀ and large concentrations of excited states.

The physical process of X-ray emission from the anode region in a plasma focus device is explained by the interaction of a powerful electron beam with the metal anode and with ionised metallic vapour from the anode, with the influence of the magnetic field of the beam taken into consideration. A MC-PIC model (Monte Carlo-particle in cell) is proposed for the process, in which an electron-photon collision cascade is simulated by the MC approach and the time-dependent state of metallic vapour is determined by PIC computation. The time-resolved energy spectra and angular distributions of X-ray emission from the extending anode region are calculated. The time-integrated characteristics of the X-ray emission can be compared with the results of experiments as far as they are available.

Laser-induced-fluorescence measurements are extended to higher densities in excess of 1*10¹¹ cm^-3 by evaluating the spatial dependence of the fluorescence light. The technique is applied to measure high copper vapour densities after extinction of diffuse vacuum arcs.

The drift velocity and longitudinal diffusion coefficient of electrons in argon were measured over the range of E/N from 0.25 to 50 Td at room temperature by a double-shutter drift tube with variable drift distance. A momentum transfer cross section for the argon atom which was consistent with both of the present electron swarm parameters was derived over the range of electron energy from 2.5 to 15 eV.

The electron swarm behaviour in nitrogen is studied for E/ rho ₀ from 20 to 200 V cm^-1 Torr^-1 by a Boltzmann equation method. A set of electron collision cross sections is determined using newly published data. The modification of these cross sections when necessary is kept to within experimental error. Moreover, the validity of the vibrational excitation cross sections obtained theoretically by Hazi and co-workers (1981) is examined. The results show that the calculated swarm parameters are in close agreement with those obtained by a photon flux experiment of Wedding and co-workers (1985). This suggests that the set of electron collision cross sections determined in the present work is an appropriate one as far as the swarm parameter analysis is concerned. The electron energy distribution and electronic excitation coefficients and frequencies to various excited states are also calculated and discussed.

The basic differences between SF₆ and N₂ arcs have been investigated over a wide range of arcing conditions. It has been found that the thermal size of a SF₆ arc is smaller than that of a N₂ arc when thermal conduction dominates the energy transport, although it is quite the contrary when radiation is the most important energy transport mechanism. Emphasis is given to the revelation of the relationship between the arc behaviour and the properties of the arcing medium. The relative importance of different energy and momentum transport mechanisms is determined and its influence on the characteristics of the arc is discussed. The dynamic behaviour of the arc is studied by using a step change of current. The results indicate that the overall characteristic times of a N₂ arc are shorter than those of a corresponding SF₆ arc, but the electrical conductance of the SF₆ arc decays much faster than that of the N₂ arc under the thermal-conduction-dominated free recovery.

Measurements of total radiation losses from arcs in a commercial SF₆ puffer circuit breaker are analysed to give values of radiated power per unit arc length. Such an analysis is complicated by the need to take account of the optical fibre used for the transmission of the optical signal from within the high pressure tank and highly accelerated nozzle assembly of the circuit breaker. The analysis requires calibration of the system components and its validity is demonstrated through a comparison with the direct measurement of total radiation under laboratory conditions. The investigation makes possible in principle the quantification of radiation power transport during the operation of in-service circuit breakers and is therefore of some significance for condition monitoring of switchgear.

Using a fast pulse counting system with a time resolution of 0.7 ns, the intensity has been measured of the light originating from a negative corona discharge in the Trichel regime in air, resolved in time, in position in the gap and in wavelength. The dominant spectral lines originate from the second positive system of N₂. Both the current and the emission from the plasma carrying the current show a fast increase on a nanosecond timescale and a successive slower decrease. Repetition frequencies are in the MHz range. The maximum of the light pulse intensity corresponds with the maximum of the Trichel pulse. The continuum emission measured originates from the attachment processes during the maximum in the current pulse. The electron density estimated from the continuum measurements is 10¹⁸ m^-3 at maximum for air at a pressure of 100 kPa. From observed Cul line emission the sputter rate for positive ions impinging on the cathode tip has been determined to be 4*10^-5 atoms ion^-1.

Theoretical and experimental works have shown that at high values of E/N(the ratio of electric field strength to gas number density) the reduced mobility of H⁺ ions drifting in helium exhibits a rapidly increasing value, termed the 'runaway mobility'. In the present work, a Monte Carlo simulation technique has been used to investigate the ion motion in the runaway. Comparisons of the dependence of apparent mobility on the number density of helium show agreement with the results obtained by the experiments.

Describes Monte Carlo calculations pertaining to the penetration of electron beams through compounds. The distribution of the range of electrons in solids has been studied quantitatively. New data are presented on the reflection, absorption and transmission coefficients, the most probable range and the range distribution of electrons in a few typical compounds, for incident beam energies up to 0.5 MeV.

Triafol plastic track detectors have been exposed to ²³⁸U ions of energy 17.17 MeV u^-1 at UNILAC, GSI, Darmstadt. The bulk etch rate V_G, the track etch rate V_T and the sensitivity of the track detector V_T/V_G are measured for different etchant temperatures and ion energies. The various activation energies used in fitting the Arrhenius equations are also calculated. The maximum etched track lengths of the ²³⁸ U ions have been experimentally measured. Surface etching and overetching corrections have been applied to the observed track lengths in order to obtain the true track lengths which have been reported for different etching times and incident-ion energies. These results have been compared with the theoretical energy range calculations; they are found to conform to them reasonably well.

The variation with deposition pressure of birefringence retardation in Langmuir-Blodgett (LB) films of 22-tricosenoic acid is shown to correlate very well with previously reported attenuated-total-reflection studies of their scattering, as expected if the latter is due to orientational fluctuations. It is shown that macroscopic orientational inhomogeneities can account for a significant contribution to the apparent dielectric loss and that microscopic fluctuations of film orientation are probably also involved. The contribution due to macroscopic inhomogeneities varies with the experimental parameters and hence is not characteristic of the film alone.

Electrical resistivity and Hall effect measurements on thin NiSi films were performed in the temperature range 290-900 K. Two types of substrate were used-bare silicon wafers and oxidised ones. The temperature dependence of resistivity and Hall coefficient reveal the metallic character of NiSi films. The resistivity at RT is 14.66 mu Omega cm and about 40 mu Omega cm for samples on bare wafers and oxidised ones, respectively. The sign of the Hall coefficient is negative, indicating that electrons are the dominant species. The electron concentration and Hall mobility at RT are 6.88*10²² cm^-3, 6.2 cm² V^-1 and 3.49*10²² cm^-3, 4.0 cm²V^-1 s^-1 for the bare and the oxidised water respectively.

The influence of temperature and thickness on the electrical resistivity and Seebeck coefficient of polycrystalline SnTe thin films was investigated in detail. The size effect data for the electrical resistivity and Seebeck coefficient fit very well with Tellier's effective mean free path model. Different physical parameters such as Fermi energy (E_F) and effective mass (m*) were evaluated. From this it is concluded that heavy holes are the dominant charge carriers. The Seebeck coefficient data were also fitted into the Jain-Verma expression to evaluate the scattering parameter (b) and the value of b clearly indicates the dominance of lattice scattering in SnTe.

In polymeric, biological and other similar absorbers diffusion contributes significantly to the Mossbauer linewidths. A simple expression has been derived for the optimum thickness for such single-line absorbers. The optimum thickness yields a Mossbauer spectrum having the highest value of the ratio of the depth of spectrum to the statistical errors in background count, in a given experimental time.

Phlogopite samples from different regions were studied by a Mossbauer spectroscopic technique after heat treating them in air at different temperatures. All the samples show a loss of micaceous tetrahedral-octahedral-tetrahedral structure above 1000 degrees C. The oxidation and dehydroxylation processes occurring prior to decomposition were explained with the help of Vedder and Wilkin's propositions. The relaxation patterns observed were the consequence of the superparamagnetic phenomenon due to the presence of fine particles. An attempt was made to calculate particle sizes with the help of Mossbauer parameters.

The chemical composition of oxygen-plasma-anodised (OPA) film on GaAs(100) has been analysed using Auger electron spectroscopy (AES) and X-ray photo-electron spectroscopy (XPS) with argon-ion sputtering. There is xGa₂O₃+yAS₂O₃ in the oxide. Infrared absorption spectra of the OPA film on semi-insulating GaAs(100) were measured by a Nicolet Dx type. Fourier transform infrared spectrometer using the difference spectra method. The experiments show that a strong and broad infrared absorption band is in the range 385-1040 cm^-1. It is mainly a superposition of the characteristic absorption modes of Ga₂O₃ and As₂O₃. In the near-infrared and medium-infrared range, the OPA film has a marked anti-reflection effect to the GaAs substrate.

A microdosimetric track interaction model for heavy charged particles has been developed which is capable of quantitatively predicting the experimentally observed supralinearity of the alpha particle TL dose response of peak 8 in LiF:Mg, Ti (TLD-100, Harshaw/Filtrol) as well as the linear behaviour of the lower temperature glow peaks. The linear behaviour at low dose is due to lack of track interaction arising from the highly localised nature of the alpha particle dose deposition profile (98% of the dose is deposited within 200 AA of the track axis). Track dose overlap in the radiation absorption stage is shown to be incapable of significantly increasing the thermoluminescence efficiency of intersecting tracks. The alpha-induced supralinearity of peak 8 can, therefore, be explained only via greatly increased charge carrier migration lengths in the glow curve heating stage which brings about significant nearest-neighbour track interactions above a fluence of approximately 10⁸ particles cm^-2 (approximately 10 Gy in LiF). The increase in TL efficiency arises from the increased population of luminescence recombination centres available to the migrating charge carriers released from the TL trapping centres. The linear behaviour of the lower temperature peaks (up to a fluence of approximately 10¹⁰ particles cm^-2 followed by exponential saturation) yields a charge carrier average migration distance of approximately 250AA (cf 5000 AA for peak 8) which implies that at low sample temperatures there is little inter-track migration of charge carriers in the luminescence recombination stage.

A new set of expressions have been derived to evaluate the thermal activation energy, E, of a thermoluminescence (TL) peak. The order of kinetics, b, of the peak may be first order (b=1), second order (b=2) or general order for which the authors have taken b=1.5 as an example. The derived expressions involve temperatures at which the intensity of the peak is ¹/₂, ²/₃, ⁴/₅ of the maximum and/or the peak temperature (T_m). The selection of these points is based on the fact that the upper half of the peak is free from interference of the satellite peaks. The applicability of these expressions has been checked by applying them to some typical experimental TL peaks. The value of E determined by using these new sets of expressions is in good agreement with that found by curve fitting.

Optical absorption of LiF:Mg, Ti (TLD-100) as-received and annealed powders was investigated in the 300-700 nm range by photo-acoustic spectrometry (PAS). Near-logarithmic behaviour was found between 3 kGy and 1 MGy for the observed absorption peaks at 380, 444 and 547 nm. Optical bleaching properties were studied with white light and monochromatic irradiations at 254 and 366 nm. Fading characteristics were determined. It is shown that PAS is an appropriate tool for measuring doses on powders, especially in the high-dose range. Suggestions are made for extending the dose range to lower levels. In this way LiF TLD-100 powder may be used continuously from the mu Gy (by thermoluminescent methods) to the MGy (by PAS) range.

The superconducting state stability in twisted multifilamentary wire is investigated. The case of varying transport current and transverse magnetic field is considered. It is shown that the current-carrying capacity I_m of multifilamentary superconducting composites increases with the decrease of the twist pitch L and I_m attains its maximum value at L less than some critical value L_c.

It is shown that the parametric X-ray spectrum has an anomaly in the case of degenerate two-beam diffraction of the emitted photons. The oscillations of the integrated intensity of the radiation are predicted.

A simple formula is used to compute the hardened depth obtained by scanning steel surfaces with a laser beam. The calculated depth of hardening was found to be in excellent agreement with experimental results for three steels processed with a CO₂ laser at intermediate scanning rates. For either very fast or very slow scanning rates, however, differences between calculated and measured hardened depths support the earlier conclusion that coupling of the laser beam to the absorptive coating applied to the steel surface is inefficient in these regimes.

A stable glow plasma at atmospheric pressure has been achieved for plasma treatment under selected conditions, for example the structure of electrodes, the kind of dilute gas, and the frequency of power. The surface fluorination of PET film and carbon thin-film deposition by such a plasma is described here. The surface energy can be controlled by plasma treatment of various concentrations (O₂/CF₄/He) in the same way as the lower pressure method that has been reported by Kogoma and co-workers (1987). Active species in plasma are identified by emission spectroscopy.

The dark discharge of surface potential on corona or capacitively charged amorphous semiconductors via charge carrier emission from an energy distribution of deep bulk or surface localised states in the mobility gap has been recently studied by a number of authors. The demarcation energy concept has been widely used to extract the necessary theoretical expressions to analyse the surface potential decay experiments. The demarcation energy concept is analytically examined by the author to derive the conditions under which it remains valid. It is shown that for an arbitrary distribution of localised states, the fractional rate of change of the density of states function N(E) with respect to energy at the demarcation energy E'=kTIn vt, over approximately ¹/₂kT, must be less than unity. In the case of an exponentially decaying distribution of mid-gap states the demarcation energy approach predicts the correct functional time dependence of the rate of charge carrier emission. As an example, the decay of surface potential on an amorphous semiconductor due to field-enhanced surface generation from an exponential distribution of surface mid-gap states is considered and a theoretical expression is derived for the time evolution of the surface potential.

The high-T_c Tl-Ba-Ca-Cu oxide superconductor with a critical current density J_c=1630 A cm^-2 at 77 K has been prepared and high-T_c properties have been observed in this system. The zero-resistance transition temperatures T(c₀)=120.3 K and T_c(AC chi )=119 K were observed by resistance and magnetic susceptibility measurements, respectively. The stability and reproduction of the materials are absolutely necessary. Thermal characteristics were investigated.

The slit island method is employed to measure fractal dimensions D_F of fractured surfaces under plane strain conditions with the help of an image analysis technique for two high-strength steels under different heat treatment conditions and at different test temperatures. It is shown that the fractured surfaces are of fractal character. The values of D_F decrease smoothly with an increase of the logarithm of fracture toughness K_1C. This relationship may reflect the changes in the microstructure that occurred during different heat treatments and the changes in the fracture mechanism that occurred at different test temperatures. The fractal dimension D_F can be regarded as a measure of fracture toughness in steel. Finally, the experimental results are explained in terms of the relation of critical crack extension force to the true total area of the irregular fracture surfaces.