Table of contents

Error contour charts are derived from a comparison of the Mie theory for light scattering by spheres with the Fraunhofer diffraction prediction for the forward-scattered lobe. It is concluded that forward lobe particle sizing methods are accurate for quite small particles, particularly if they are absorbing, but that problems may arise with refractive indices close to unity.

Optically pumped NH₃ laser emission at 12.8 mu m with signal strengths approximately 775 kW and efficiencies of 10% are reported. The fine temporal structure of the emission is shown to be fast (<5 ns) and output powers are shown to scale with pressure and pump power.

The Schottky barrier heights have been investigated for Au and Ag on atomically clean and oxidized cleaved silicon (111) surfaces by the conventional C-V and I-V techniques. A novel ultrahigh-vacuum scratch technique enables the existence of impurity layers to be established at the interfaces after the establishment of contact. The invariance of the measured barrier height for various interfaces is discussed in terms of existing theories.

The tilt angles of eutectic mixtures of cyano-biphenyl nematics on 5 degrees SiO films have been measured using a conoscopic optical technique. The results show that the tilt angle of a mixture may be calculated from the known tilt angle of the constituents and the composition. It is also shown that the nematic layers adjacent to the aligning surface retain the bulk composition.

The energy spectrum of neutrons emitted when deuterons impinge on a thick beryllium target has been measured using an NE213 scintillation detector and the time-of-flight technique. Spectra have been measured at angles of 0, 30, 45, 60, 90, 120 and 150 degrees and deuteron energies of 1.4, 1.8, 2.3 and 2.8 MeV. Results presented are angular yields and angle-integrated energy spectra. Tabulated angle-dependent neutron energy spectra are available elsewhere. There are significant differences between the present results and those reported by Inada et al. (1968) for a similar experiment.

Perturbation theory is used to determine precisely the asymptotic neutron flux distribution (or equivalently the 'dipole' response coefficient of the distribution) in the medium surrounding a homogeneous neutron-absorbing rod when, in the absence of the rod, the flux distribution in the medium is of linear form. The rod height is infinite and the precise determination is made in the limiting cases of rods of large or small radii. The medium is isotropically neutron-scattering, non-absorbing and of infinite extent. The one-velocity approximation is made.

Lead silicate glass is widely employed for the manufacture of channel electron multipliers. An experimental study of the backscattering of electrons from reduced lead silicate glass in the energy range 0.5-10 keV has been conducted. The measured values of eta vary from 0.27 at 0.5 keV to 0.37 at 10 keV. Moreover, the backscattered yield from a surface is found to depend upon its lead content. The experimental results are in fair agreement with Everhart's theory of backscattering of electrons.

It is shown that a magnetoresistor connected in series with the winding of an electromagnet can behave as a rectifier when a bias field is applied. A current rectification ratio of up lo 11.3 may be reached with the device. Experiments have been carried out using a sample of bismuth at 4.2K, with the field provided by a ferrite-cored magnet with superconducting windings. The frequency limit for the low temperature rectifier is shown to be about a kilohertz while that for a room temperature rectifier based on InSb-NiSb is likely to be one or two hundred hertz. A most important feature of the device is that it can be operated successfully from very low voltage sources and it may find application in the production of large direct currents.

Existing analogue methods for the solution of two-dimensional flow problems have been extended to axially symmetrical problems by use of a scale transformation method. The technique, although approximate, has been found to give results for flow in axially symmetrical filter beds in good agreement with results by computer and by means of flow models. It is also shown how the method may be used in the optimum design of axial flow filter beds.

High-speed time-resolved Schlieren photographs are taken between impulse voltage application and breakdown in transformer oil using an image convertor camera at various applied voltages and framing speeds. Needle-to-plate electrodes are used and the oil is under vacuum. The pre-breakdown disturbance accompanied by shock waves emanates from the needle electrode for both polarities of the needle point. The disturbance from the negative point propagates step by step like a lightning discharge in air, while that from the positive point moves radially and reaches the plate electrode in one step. The pre-breakdown disturbance seems to be a gaseous phase which is generated in the liquid.

The direct breakdown voltage of degassed and oxygen-saturated oil has been measured as a function of gap length (28-200 mu m) using different combinations of etched aluminium, anodized aluminium and stainless steel electrodes. Polarity reversal tests revealed the presence of a crossover point in the breakdown characteristics analogous to that obtained for non-uniform field geometries. It is therefore inferred that the breakdown mechanism is initiated at local asperities on one of the electrode surfaces. The shapes of the characteristics were found to be either concave or convex with respect to the gap axis, depending on the material of the electrodes used. A concave shape could lead to a minimum in the average breakdown strength against gap length characteristic.

The ionic conductivity of polycrystalline beta "-alumina has been measured over a range of temperatures extending from 25 degrees C to 800 degrees C. The measurements have been performed upon material which has been well characterized structurally in terms of it being a polycrystalline aggregate of solely beta "-alumina, free from any detectable traces of the beta phase. Two different materials have been studied, one containing solely Li₂O additives, the other an additive mixture of Li₂O and MgO. These additives are commonly employed in achieving a high phase purity of beta "-alumina during the sintering procedure employed in the fabrication of the solid. The conductivity measurements have been obtained with various different electrode-alumina systems.

Carrier multiplication measurements have been made on a silicon avalanche photodiode bombarded with electrons at energies between 3.5 and 30 keV in a scanning electron microscope. The observed dependence of the multiplication characteristics on the incident electron energy is interpreted by a simple one-sided step junction model, using literature data for Si ionization rates and for the electron generation function. Evidence has been obtained that kilovolt electrons can be an alternative excitation source to light in multiplication measurements; possible applications of this injection method are discussed.

A transmission electron beam spectrometer has been constructed with which the angular distributions of electron energy losses in thin films ( approximately 150 nm) of polyethylene, polystyrene and evaporated carbon ( approximately 50 nm) have been studied. The results indicate that the broad energy loss around 20 eV, which is common to the three materials studied, may be explained in terms of inelastic collisions of beam electrons with the bound electrons of the valence state and generation of plasma oscillations.

A surface electrostatic charge may be generated on rubber by contact with a dissimilar material and it has been suggested that resulting electrostatic attraction may contribute significantly to the adhesion. Measurements made to determine what proportion of the work of adhesion is due to electrostatic attraction indicate a contribution of less than 10%. The investigation demonstrates, in particular, how important it is to consider the details of interface events. For smooth rubber it becomes clear that the work of adhesion in the contact region is magnified by a viscoelastic factor characteristic of the particular rubber under test and that variation in this factor swamps any increase in adhesion or rolling friction brought about by electrostatic forces.

The stability of electric currents in multifilamentary superconducting composites against flux jumps is discussed in detail. The normal procedure for such a quantitative stability investigation is carried out. In some particular cases the stability criteria are found and analysed.

An analysis is given of the scattering of light from an asymmetric slab waveguide into the substrate when this is caused by a surface acoustic wave of the Rayleigh type, both the effects of the photoelastic index fluctuations and the elastic displacements are considered. Design criteria for efficient scattering are established and numerical results are given. An experimental acousto-optic scanner consisting of a sputtered glass film on a quartz substrate is demonstrated with a resolution of 46 spots obtained with only a 10 MHz bandwidth. A possible implementation as a microscanning device is discussed.

The thermally modulated reflectance spectrum of gold has been measured over the range 1.8-3.2 eV, at 107 and 323K. A shift of +0.050+or-0.004 eV in the position of the principal peak at 2.5 eV in the Delta _{epsilon i} spectrum has been observed as the temperature decreases, which is in agreement with the shift observed in wavelength modulated spectra. A corresponding shift of -0.075 eV in the subsidiary peak at 2.95 eV has also been observed. The results are discussed in the light of Christensen and Seraphin's (1971) band calculations and a possible alternative transition assignment suggested for the peak at 2.95 eV.

When the optical constants n and k of a thin film are calculated from the measured normal incidence reflectance and transmittance (R and T) by a successive approximation method, the results are complicated by multiple solutions and critical points of the equations R(n,k)=0 and T(n,k)=0 in the n,k plane. A method is given which enables the physical solution to be determined and which provides solutions in regions near the critical points where the successive approximation method is very inaccurate. The method, which combines Kramers-Kronig analysis of the transmittance data with the successive approximation method, is illustrated by results of measurements on an amorphous Mg-Sb alloy.

Visible and infrared reflectance of various electrodeposited chrome-black films is unusual. A theoretical model explaining this behaviour is presented. It appears that a graded volume density of Cr metal across the film is required to produce the observed effects. The substrate's role is examined. Its direct influence is minor, because of the film's effective skin depth even at moderate metallic densities and in the infrared (for all films over about 0.15 mu m thick). The substrate must have a major indirect influence, though, by determining film structure. These systems make excellent solar selective absorbers.

A process for producing In₂O₃ layers from indium is described. The preparation technique involves the thermal decomposition of InCl₃ in an oxidizing atmosphere in a suitable furnace. A differential technique has been used to investigate the fundamental absorption edge in this material. The absorption data are interpreted as direct allowed (3.55 eV) and indirect forbidden (2.4 eV) transitions.

The image of a transparent crystal viewed in the cathodoluminescent mode of the scanning electron microscope is constructed from light emitted from a thin layer of material below the surface. Examples are given of how the image of a diamond surface is modulated by cracks in this layer. The use of this method, with suitable luminescent specimens, readily reveals damage by the pattern created in the image. Brief reference is made to other artifacts which may be present in the image, and which are less informative.