Table of contents

The combination of strongly forward and backward scattering with usual linearly anisotropic scattering has been studied for one-speed neutrons in infinite slabs with vacuum boundary conditions. For critical systems the criticality factor is given for two slab thicknesses and different degrees of the anisotropic scattering. The special case when the forward and backward scattering completely dominate over the 'ordinary' scattering is discussed in detail. The time decay of a neutron field in a moderator has also been studied. It is shown that the decay constant for thin slabs cannot exceed a certain limit determined by the degree of strongly forward and backward scattering.

By utilizing the polarizabilities of a small chiral sphere embedded in a chiral medium, the Bruggeman model of a composite formed by randomly dispersing identical, electrically small, isotropic chiral spheres in an isotropic achiral host medium has been formulated. Numerical results show that the Bruggeman estimates of the effective properties of the composite differ significantly from the Maxwell-Garnett estimates for a sufficiently large chirality parameter of the inclusion spheres.

The authors consider a two-dimensional model of an aperture excited homogeneous half-space whose admittivity (or complex conductivity) is different in the horizontal and vertical directions. The criterion for focusing the aperture, of finite width, to a point in the half-space is developed. It is demonstrated that, for an electrical field perpendicular to the axis of the aperture, the anisotropy can be allowed for by a suitable scaling of the isotropic model. The numerical results show that there is a significant enhancement of the fields (and the power dissipation) at the focus point provided that the conduction currents are relatively small compared with the displacement currents in the half-space. Also the width of the aperture should be about 3 or 4 wavelengths for effective focusing.

Samples of different materials were repeatedly contacted with metal plates in high vacuum. The transferred charge after contact and separation was measured. This resulted in individual contact electrification curves. Different behaviour of the materials was observed. High-conductivity samples such as oxidized metals or graphite show charge saturation after few contacts. Low-conductivity samples like quartz, calcite and Teflon show charge accumulation up to a large number of contacts. Reversal of the net charge polarity with increasing number of contacts was observed for calcite and quartz samples.

A method for obtaining an intense source of transversally excited helium atoms in the 2³S₁ and 2¹S₀ metastable states (He*) is described. As a consequences of the particular kinematics of electron-He atom collisions, the metastable beam is deflected 12 degrees from the ground state beam, with a suitable choice of experimental parameters such as electron beam energy and geometry of the excitation apparatus. The maximum measured value of the flux intensity is 10¹⁶ atoms sr^-1 s^-1. Due to a fairly narrow electron energy distribution of the electron beam, the velocity distribution of metastable atoms remains almost as sharp as that of the ground state atoms. As a consequence, the average velocities of the metastable beam and of the ground state atoms, scattered along the same direction, are separated from each other by more than 2 FWHM. The excitation apparatus shows a bistable behaviour, due to space charge effects, which strongly influences the He* formation efficiency.

Effects on diamond-diamond friction of ultra-high vacuum (UHV) ( approximately 4*10^-9 Torr) and low pressure ( approximately 1*10^-5 Torr) gases including hydrogen, oxygen and nitrogen in their molecular and atomic states have been investigated. The friction coefficient for diamond sliding on diamond in UHV was found to be between 0.6 and 1.0, which is about ten times that measured in air. Among molecular gases studied at pressures of approximately 1*10^-5 Torr, only oxygen caused a significant reduction of the friction coefficient. When a heated filament was used to dissociate the gases in the vacuum chamber, atomic hydrogen produced the largest decrease in friction coefficient. The reduction in friction coefficient increased with increasing exposure time before sliding started. These results suggest that passivation of diamond surfaces by atomic hydrogen and oxygen as well as molecular oxygen is effective in reducing the carbon-carbon bond formation during sliding and consequently the friction coefficient.

The motion of an electrically conducting fluid film squeezed between two parallel disks with heat transfer in the presence of a magnetic field applied perpendicular to the disks is studied. Attention is given to the case where a similarity solution can be obtained. Approximate analytic solutions are given and a numerical solution to the resulting nonlinear ordinary differential equations for a wide range of values of the three independent parameters which influence the motion is presented. Special attention is given to the effects of squeezing and magnetic forces on heat transfer. The results establish that while squeezing has considerable effects on heat transfer, the effects of magnetic forces are negligible.

The boundary layer problem of a weakly ionized collision-dominated plasma in front of a negative absorbing wall is considered. The electrons are assumed to be Boltzmann distributed; the ions are governed by symmetric charge exchange. The Debye length is small compared with the mean free path. A former kinetic analysis of the Knudsen layer is matched with an adapted macroscopic theory of the plasma body accounting for ionization, nonplanar geometry and filed-dependent ion mobility. The consistent analysis of plasma and boundary layer is illustrated with the examples of a cylindrical plasma column, a spherical probe in an infinitely extended plasma without ion generation, and a semi-infinite plasma with Saha ionization equilibrium distorted by wall losses.

The authors have measured the sheath potential around a probe in a range of different plasma conditions in the UMIST quadrupole GOLUX and in a related experiment in which the plasma expands freely to supersonic velocity. In the latter case, the sheath potential agrees well with an appropriately modified form of the usual expression for a field-free plasma, for both hydrogen and argon plasmas. In GOLUX, however, the sheath potential is found to be significantly less than the accepted value, even when the magnetic field is taken into account. For the slow-moving plasma in the outer part of the quadrupole confining field, they present both theoretical and experimental results showing that the reduction is due to truncation of the electron velocity distribution as the probe drains electrons from a closed flux-tube faster than they can be replaced. In the central hot plasma, however, this explanation cannot apply. Here the plasma is moving at about sonic speed and magnetic effects are weak. Nevertheless the results are significantly different from those in the field-free experiment.

Experimental and theoretical investigations of a transverse HF discharge are carried out. A physical model of the discharge is proposed which accounts for the entire range of experimentally observed properties of such a discharge. The features of electron density and temperature radial distributions, spontaneous radiation intensity, laser radiation gain factor in the discharge plasma are covered. Part II covers the stability and uniformity of the discharge plasma column at high values of the discharge current and the conditions of formation and shape of the spatially nonuniform structures arising in the plasma as a result of decreasing discharge current.

For pt.I see ibid., vol.25, no.10, p.1454-60 (1992). On the the basis of the physical model of transverse high-frequency (HF) discharge presented in part I, the stability of a uniform discharge at high values of the HF discharge current is examined. Conditions for the discharge plasma column to break up into plasma filaments are determined and the physics of contraction with decreasing discharge current is considered. Numerical simulation of spatially nonuniform structures in transverse HF discharge is carried out. The results of this simulation describe adequately the experimentally observed plasma distributions in such a discharge.

The radiation efficiency of Hg-Ar low-pressure surface wave (SW) discharges has been measured in a broad range of Ar pressures (40 mTorr-12 Torr), Hg partial pressures ( approximately 1-100mTorr) and excitation frequencies (200-1000 MHz). Gas filling and tube geometry (26 mm outer diameter) were chosen to allow a comparison with the conventionally excited Hg-Ar low-pressure discharge used for general lighting purposes in the well known fluorescent lamp. The study focuses on the quantitative measurement of the two Hg UV resonance lines at 185 and 254 nm. The RF power input into the plasma column extending outside the surfatron coupling structure was determined quantitatively. This was done by measuring the power dissipated in the part of the plasma column hidden within the surfatron by a calorimetric method. The measurements of the Hg UV radiation efficiency are supplemented by radial scan experiments of a visible Hg line, and by the study of phenomena occurring at a higher power density such as saturation and standing wave formation.

The position-dependent swarm parameters and the energy distribution F( epsilon ,x) of electrons in argon gas have been analysed using a propagator method in which a proper relation between the trajectories of electrons and the cells in an energy epsilon and position x space were considered. The results showed fine structures of damped fluctuation in the swarm parameters, which has been seen from the results of a Monte Carlo method. When initial electrons with zero energy were ejected from the cathode, F( epsilon ,x) at E/N=283 Td could not reach the equilibrium condition even at the distance at which the equilibrium values of mean energy, drift velocity, etc. of electrons seemed to have already been obtained.

X-ray diffraction, transmission electron microscopy and transmission electron diffraction studies on cadmium selenide thin films deposited on glass substrates have been carried out. X-ray line profile analysis of the diffraction patterns has been performed to study the microstructural parameters. The variation of different microstructural parameters, such as crystallite size, RMS strain, dislocation density and stacking fault probability with film thickness as well as substrate temperature is studied. The optimum growth condition is fixed to a film thickness of the order of 1.0 mu m. The change of preferred orientation is observed for films deposited at and above 473 K.

A radial hydrodynamic flow in cylindrical freely suspended films of HOAB, OOBA, DOBA, PAA and BOBT around the nematic-isotropic phase transition is observed. The flow is explained by the existence of a nonlinear temperature dependence of the surface tension. The infusion of a warmer isotropic liquid into the nematic layer due to the flow and vice versa generates bubble domains around the N-I interface in HOAB, OOBA and DOBA.

The synthesis of stable and adherent TiSi₂ layers by one-step laser irradiation of Ti deposited on Si wafers is reported. A new thermal self-aligned effect is observed. By multipulse excimer laser irradiation, a regime was obtained whereby the Ti covering the Si substrate was completely reacted while the Ti covering the SiO₂ layer was completely expelled by vaporization.

Reflection and transmission energy coefficients at the fusion face between base and weld metal are calculated in the case of austenitic weld examination. The material considered is the austenitic stainless steel X6 CrNi 1811. A plane wave is allowed to travel from acoustically isotropic base metal to anisotropic weld metal and vice versa. Six different cases are analysed for three different wave types, namely, vertically polarized shear (SV) waves, horizontally polarized shear (SH) waves and longitudinal (L) waves. Energy ratios seem to be more meaningful and give a better insight and understanding of reflection and transmission phenomena than amplitude ratios, in view of the fact that ray (energy flow) direction deviates from the wavevector direction.

The authors have studied the contact charging of nylon samples coated with fluoropolymer layers of varying thickness. The observations indicate that for both simple and repeated contacts the charge is confined to a layer about 3 nm thick or perhaps even thinner.

Step-voltage current measurements carried out with commercial poly(vinylidene fluoride) (PVDF) under vacuum showed a prominent peak in the charging current when a sample had previously been in contact with a humid environment. This effect tends to disappear in subsequent measurements performed under the same experimental conditions, giving evidence that it is connected with extrinsic carriers which are swept out of the sample by the external electric field. A model based on a field-dependent ionic dissociation of water molecules absorbed by the bulk of the sample was developed to explain the phenomenon.

The method of molecular dynamics was used to study the structure in a two-dimensional system of parallel magnetic dipoles oriented in the plane of the system. The results predict the build-up of a 'wave-like' ordering showing periodicity in the direction perpendicular to the field.

A method for separately determining the longitudinal, T₁, and transverse, T₂, relaxation times from the saturation curve of an inhomogeneously broadened slow-passage ESR absorption line is presented. It imposes no restriction on the relative values of the spin packet (Lorentzian) and Gaussian widths conforming with the overall inhomogeneous width. It gives a simple way of calculating T₂=2/(spin-packet width) through the deconvolution of a non-saturated experimental spectrum and then, in a direct and elegant way, the T₁ is calculated once the amplitude of the microwave magnetic field, H₁, for which the saturation curve of the absorption line reaches its maximum, is known. In addition, algebraic relations for T₁ and T₂ are given for the case of inhomogeneous broadening which, in the limit of Gaussian distribution width to 0, reduce to the homogeneous case. These algebraic relations give the spectroscopist the possibility of a direct calculation of T₁ once a saturation curve up to its maximum has been obtained.

Recently, it has been pointed out that the use of time-dependent transition rates to explain the non-exponential time-dependence of the dielectric relaxation that is often observed does not lead to the relationship between the responses in the frequency and time domains that is predicted by the superposition principle and linear response theory and which is observed experimentally. In this paper the authors show that such a correspondence is obtained if the transition rates of the entities involved depend intrinsically on the ages of the states rather than on the absolute time.

The authors report the first study of laser processing of GaAs under stoichiometry control. A series of experiments was performed to investigate the influence of laser processing on the electronic and structural properties of liquid encapsulated Czochralski-grown GaAs. A thermal treatment was used to convert samples from semi-insulating to conducting behaviour by severely reducing the EL2 concentration in the material. These samples were subjected to both laser processing and standard thermal annealing under stoichiometry control. Laser-assisted processing resulted in much stronger EL2 recovery rates than for standard thermal annealing and the tendency was more pronounced for arsenic-rich stoichiometry than with gallium-rich stoichiometry. Indeed, selected laser processing conditions were found to yield significantly higher sample resistivities than those of the starting material. Etch pit densities were studied as a function of the laser processing conditions and were found to be a minimum for near-stoichiometric conditions. A simple defect model is proposed and used to explain these experimental results.

A series of measurements of nuclear stimulated desorption was performed for compounds containing the isotopes ⁴⁷Ca and ⁹⁹Mo. A persistent charged component was observed in the desorbed flux of ⁴⁷Sc resulting from the decay of ⁴⁷Ca. A transient charged component in the flux of ^99mTc was detected for some chemical forms of the ⁹⁹Mo compound.

This communication aims at the analysis of a unipolar ionized field. An efficient iterative method that uses the finite element technique (FET) as a numerical tool is proposed. The method seeks a solution to only one second-order partial differential equation (PDE) rather than a solution to a nonlinear third-order PDE or a solution to two simultaneous second-order PDES. The effectiveness of the proposed method has been tested through application to the coaxial cylindrical geometry where the ionized field has been evaluated analytically. The voltage value at the stressed conductor as well as the constancy of the field at the onset value around the conductor represent the boundary conditions of the proposed method, which is characterized by quick convergence to achieve the accuracy and simplicity required for computer programming.

Langmuir-Blodgett films are commonly subject to specific defects called striations which are associated with meniscus instability during deposition. In the present paper these have been characterized by observing their effect on the coupling of evanescent optical modes into the surface modes associated with metallic silver. The reflectivity as a function of incidence angle has been measured and fitted to a theoretical model and the spatial distribution of the defects has been directly imaged. The striations are shown to be exactly a bilayer thicker than the rest of the film.