Table of contents

It is shown that the density, compressibility and absorption profiles of a plane stratified acoustic continuum can be determined from the reflection coefficient measured at two angles of incidence and two set incident frequencies for all values of the free-space wave number. After transforming the acoustic wave equation into an equivalent one-dimensional Schrodinger equation, approximate solutions to the inverse scattering problem are considered and then recent exact solutions which have been developed via the Gel'fand-Levitan equation.

The decay of the metastable states has been investigated in a DC afterglow discharge in helium, argon, krypton and xenon at room temperature. This investigation was done by measurement of the electrical breakdown time delay (t_d) as a function of passive time ( tau ) ('memory curves'). It is shown that the concentration of these states during this time decays according to an approximately exponential law. It is shown that the nonradiative lifetime of metastable states in helium is much longer than that in argon, krypton and xenon.

The authors explain the absence of significant open-circuit voltage in illuminated n-i-n structures of amorphous hydrogenated silicon, and the presence of significant open-circuit voltage in p-i-p structures. Studies of the latter are proposed as a method of investigating the hole diffusion length in this material.

The electric strength of poly(vinylidene-fluoride) (PVDF) with different types of crystalline structure is studied at high temperatures between 50 and 150 degrees C. On the basis of experimental results, the relation between the solid structure and electrical breakdown of PVDF is discussed assuming the impulse thermal breakdown mechanism. It is indicated that the ionic conduction parameters estimated from the breakdown data vary with the crystalline structure.

Shows that isothermal TL decay data presented by Liley and McKeever in a recent Letter (see ibid., vol.16, p.139, 1983) do not prove conclusively that the TL process in LiF TLD-100 follows first-order kinetics.

The behaviour of phase conjugated waves produced by degenerate four-wave mixing (DFWM) at 488.0 and 514.5 nm of laser beams in ruby crystals has been examined at large pulsed pump beam intensities and enhanced reflectivities observed.

The velocity distribution of gas atoms in a centrifuge may to a first approximation be represented by a plane Couette flow problem with a force term linearly dependent on position. The authors have reformulated this problem as a pure integral equation for the axial flow velocity and have shown how it reduces, for special values of certain parameters, to a number of well known limiting cases. An approximation scheme based upon successive collision is outlined. The main advantage of the integral equation lies in the reduction of the number of independent variables. Some numerical results illustrating the theory are presented for the axial velocity profile and the stress as a function of plate separation.

By comparison of experimental K_e,0 factors, determined in reactor positions with greatly different thermal-to-epithermal neutron flux ratios, with theoretically calculated K_e,0 factors, it is possible in some cases to select the best values from literature data for (n, gamma ) activation cross-sections and for absolute gamma intensities. Twenty different isotopes were considered.

Results are reported of an investigation on the performance of small scale TE discharge excited HgBr and HgI lasers. From a 10 cm³ active volume, the HgBr laser produced 6.5 mJ output pulses of 65 ns duration under the optimum excitation conditions. Laser emission has been found to be due to a large number of vibronic lines of the B-X electronic transition spanning the 495 to 507 nm spectral region. The HgI laser output energy was 2.9 mJ and pulse duration was 78 ns under optimum conditions. Laser emission has been observed over the 441 to 445.5 nm spectral region. The dependence of HgX (X=Br or I) laser performance on the operational conditions has been investigated and results are presented. Laser output spectra were found to be independent of the operational conditions.

A novel type of electrostatic state selector for molecules has been devised which has a high transparency to rejected molecules and permits focusing of molecules with mu _eff<0. The electrode geometry is that of a single-wire helix placed concentrically within a coaxial outer electrode, which may be the vacuum chamber itself. Theoretical analysis of the electrostatic potential within the helix reveals that the device has the characteristics of a 'leaky' potential well for a molecular beam passing through it. An optimal electrode geometry of a helix pitch three to four times the helix radius is calculated. Experimental results are obtained for the unifilar helix focuser in a molecular beam maser oscillator and spectrometer, the latter being used as a beam detector. Whilst the state selector in its present form appears to have state selection properties somewhat inferior to existing electrostatic focusers, it is unique in respect of its simplicity of construction.

Proposes the use of an optical waveguide comprising a glass fibre in three concentric layers, the middle one having a rather higher refractive index than either of the others. The 'tubular' structure thus formed provides, by total internal reflection, for the support of the HE₁₁ mode and by suitably limiting the thickness of the wall of the 'tube' all other modes can be cut off. The overall diameter of the fibre may be quite large, 150 mu m or more, while the wall thickness of the 'tube' would by only about 5 mu m. Calculations are made for a practical case and the results are compared with those of the equivalent standard 'core' type of fibre. The calculations also show that negative power flow can sometimes occur in multi-layer fibres.

The authors discuss the experimental problems associated with measurement of molecular impact rates inside cylindrical tubes under rarefied gas flow conditions. These are reviewed with particular reference to the recent paper by Davies and Lucas (1983) who found a discrepancy between their calculations and earlier measurements of Kurepa and Lucas (1981). The authors show that the impact probe sampling orientation in these earlier experiments could well be the cause of the discrepancy and would qualitatively explain their results. They suggest improvements in the sampling probe arrangement which should give better agreement with the theory removing the need for a modification of the usual well founded assumptions regarding surface interactions under free molecular flow (e.g. cosine distribution).

Bevir et al (see ibid., vol.14, p.373-88, 1981) have described a method of predicting the response of a given point-electrode electromagnetic flowmeter to any axisymmetric, rectilinear flow. The method is expanded here to embrace all rectilinear flows and also those electrode systems which may comprise either a multiplicity of diametrically opposed point contact electrodes or a pair of large electrodes, of variable size and shape, covered with insulating materials of variable thickness. The mathematical derivations are described and the computing program is outlined. Diagrams are given of both rectilinear and axisymmetric weight functions which have been determined for several different flowmeters.

The authors develop a simple theory of mobility and diffusion of ions of density n in the neutral gas of density N(n<<N) using the Boltzmann transport equation. The interaction between ions and neutral molecules is assumed to be elastic hard-sphere interaction. They calculate the distribution of ions interacting with the neutral molecules which are maintained in thermal equilibrium at a temperature T. The agreement of their results for low and intermediate electric field strengths with the results of the standard method of taking moments of various Sonine polynomials is good. Their method is general enough to include the cases for epsilon =m/M<<1 and epsilon >1 where m is the ion mass and M is the mass of the neutral molecule.

Experimental results for breakdown voltage of sodium vapour measured for the first time using coaxial cylindrical electrodes of fixed gap distance (5 mm) and pressure (corrected to 20 degrees C) in the range 21 to 652 Pa are reported, and are found to follow Paschen's Law. The investigations also reveal that the V-I characteristics are pressure dependent; the current during the breakdown and the build-up of current after a breakdown is either positive or negative, in spite of the central cylinder being always maintained at a positive potential.

The formation of filaments during the transition to spark in a point-to-plane discharge is analysed in air (250 Torr) and SF₆ (60 Torr). Two separate analyses of the phenomenon are effected. First, the evolution of the electronic population is studied by means of a current probe and photomultiplier, and secondly, the Schlieren technique is used to follow the changes occurring in the density of the neutrals. An image converter is employed to record the Schlieren pictures which permit the observation of the expansion of the channel containing the neutrals. The average temperature of the neutrals is deduced from the rate of radial expansion of the channel and hence the resultant pressure rise at the core of the channel can be estimated. Evidence is given of the formation of local constrictions where intense heating of the neutrals occurs. At these points thermal ionisation is no longer negligible and a large fall in the resistivity of the discharge takes place, giving zones where the initiation process of the thermalisation of the discharge can occur.

The electrical DC conductivity, photoconductivity, dark thermopower, and photothermopower of a-As₂Te₃ have been determined in the temperature range 170K to 360K. The photo-equilibrium measurements suggest that although unipolar (holes) action dominates the conduction process throughout the investigated temperature range, the small value of the ratio of hole to electron mobilities will favour ambipolar conduction should the system be subject to slight external disturbance. Photoconductivity and photothermopower measurements confirm this suggestion. The data on photothermopower are the first reported investigations, which reveal that within the temperature range 320K to 200K, the phonon-drag contribution is significant. However, the positive nature of the photothermopower confirms that the existing photoconductivity theory of a-As₂Te₃ assuming the material as p-type is correct.

Transient high-field electrical conduction in evaporated thin potassium iodide films has been studied as a function of the applied voltage at room temperature, covering a time range from microseconds to several hours. The results show that the universal dielectric response is valid separately in three regions. The electrical conduction is achieved through the hopping mechanism. The transition probability, associated with the hopping mechanism in each region, is affected differently by the applied voltage.

An analytical expression allowing one to determine the stationary lifetime of photocarriers via the magnitude of photoconductivity under constant and rectangular-pulse-modulated illumination has been derived in the case of bimolecular (quadratic) recombination. Expressions have also been obtained for the moment photocarriers lifetimes at the beginning and end of the light pulse.

The use of the Maxwell stress tensor method is shown to calculate forces and torques acting in non-homogeneous dielectric systems under the influence of non-uniform electric fields in the case of lossy dielectrics without confining the treatment to the dipole approximation, as usually made. The method is applied to the particular case of a lossy dielectric sphere subject to the field of a point charge and an analytical solution is obtained. The effects of the usual restrictive assumptions are pointed out and the dependence of force upon the electric properties of materials is shown.

Reflectivity spectra in the infrared are calculated for different profile parameters in the case of p-type dopants in silicon. The spectra are found to be sensitive to the surface concentration and the sheet resistivity. The calculations are based on extrapolated data of a preceding Kramers-Kronig analysis, the applicability of these data having been proved by comparison of the calculated and measured reflectivities of boron-diffused samples. It is pointed out that the lack of knowledge of the wavelength and concentration dependence of the optical constants presents the greatest difficulty in evaluating and calculating the reflectivity spectra of diffused and implanted samples.

A method for creating an active medium of annihilation gamma quanta (a medium for the amplification of the flux of annihilation quanta) in solids with defects is suggested. Equations for the linear amplification coefficient alpha and for the minimal bulk density of thermalised positrons N_{0, omega} ^th are calculated. An examination of the dependence of alpha and N_{0, omega} ^th from the frequency of annihilation gamma quanta and the bulk density of defects is made. The character of the amplification coefficient as a function of the bulk density of amplified annihilation gamma quanta is shown.

Inelastic electron scattering is calculated by means of a simplified model of the generalised oscillator strength density distribution of an atomic or solid target. The parameters of the model are a limited set of resonance energies (partial mean ionisation potentials) and oscillator strengths. Methods to estimate these parameters, involving a fit of the partial mean ionisation potentials to the empirical atomic mean ionisation potential, are described. The valence electron partial mean ionisation potential may be determined from the dielectric energy loss function, the local plasma approximation, or empirically. Good agreement with experiment and with recent calculations of stopping power and inelastic mean free path is obtained in the electron energy range 50 eV-50 keV.