Table of contents

The thermal neutron diffusion parameters for Plexiglass have been determined with high accuracy. The classic pulsed neutron experiment based on the concept of variable geometric buckling has been performed (the buckling B² being from 0.036 to 1.234 cm^-2). Details on the treatment of data are given. The following values of the parameters have been obtained at the temperature of 20 degrees C: v Sigma _a=(4120+or-30) s^-1, D₀=(37312+or-100) cm² s^-1, C=(6192+or-457) cm⁴ s^-1, F=(1203+or-400) cm⁶ s^-1. They are compared with the values reported by other authors. The influence of experimental conditions on the final results as well as some theoretical problems concerning diffusion cooling are discussed.

An external electrostatic deflector system for a cyclotron beam is reported. The various design considerations, of significance, are dealt with in fair detail. The maximum and minimum pulse amplitudes are derived in terms of the mechanical parameters of the deflector, the beam emittance and also the charge to energy ratio of the accelerated particle. The system has been tested and found to work satisfactorily for variable pulse widths in the mu s range.

The equilibrium fraction of an ion beam in various charge states (-1, 0, +1, +2) has been measured for the passage of C⁺, N⁺, O⁺, Fe⁺, Cu⁺, Mo⁺ and W⁺ through hydrogen gas in the energy range 10-100 keV. At 30 and 80 keV the fraction in various charge states has been recorded as a function of target thickness. The evolution of ion fragments arising from dissociation of CH_n⁺ (n=1-4), OH⁺ and H₂O⁺ has been investigated as a function of target thickness at 30 and 80 keV.

A theoretical model of a partially ionised rotating plasma is presented which treats the steady state in a low-power plasma centrifuge. Perturbation analysis is employed to derive the properties of the ionisation instability in this low-density environment, in particular, allowing for a finite difference in velocity between the ions and the neutral particles. The ionisation wave speed is predicted to increase from close to the ion speed towards the electron speed as the degree of ion-neutral slip increases. Theoretical predictions of wave speed and angle are in reasonable agreement with experimental measurements.

The flow characteristics of a pulsed plasma jet have been studied using streak and conventional (time-resolved) photography of the luminous plasma jet. The nature and placement of the shock structure observed in these photographs is that expected of under-expanded jet flow. Direct measurements of the gas pressures at the nozzle of the cavity (made using an optical technique) are in good agreement with those calculated from the observed shock structure. A second set of pressure measurements, of the stagnation pressure throughout the jet, shows good agreement with the calculated pressure behind the bow shock which is located at the front of the expanding jet. On the basis of these results, a simple model of jet penetration is proposed in which the jet 'tunnels' its way into the dense surrounding air. The implications of this model, in view of its similarities to other well studied phenomena such as astrophysical jets, are discussed in terms of the turbulence created by the jet, a factor which could be most significant when such jets are used as igniters.

Electron motion in the cathode sheath region of a glow discharge in helium has been simulated using a one-dimensional computer model. Distribution functions for the electron flux across the cathode sheath have been calculated for fourteen values of the cathode fall between 150 and 1700 V. Macroscopic variables including the Townsend ionisation coefficient, multiplication coefficient and mean electron energy have been determined for the cathode fall range 150-1000 V. It is shown that the proportion of electron flux crossing the cathode sheath without collisions increases as the cathode fall is raised. For a cathode fall between 400 and 1400 V, the electron flux at the cathode sheath/negative glow boundary possesses beam-like characteristics. Beyond 1400 V, the largest component, in the electron flux distribution, to reach the sheath boundary is the collision-free group. These electrons enter the negative glow with energies corresponding to the cathode fall potential, and are practically mono-energetic. The authors propose that this is a suitable criterion to refer to the entire cathode sheath/negative glow region as an 'electron-beam' glow discharge.

It has been found previously that for both uniform and non-uniform fields, the direct current breakdown voltages, V_s, of H₂ and NH₃ exceed those of D₂ and ND₃, respectively, for positive polarity. In contrast, it was found that while for uniform fields CH₄ and CD₄ behave in a similar manner, the direct current breakdown voltage, V_s, of CD₄ exceeds that of CH₄ in non-uniform fields for positive polarity. The authors have now found that the CH₄/CD₄ non-uniform field behaviour is polarity dependent (i.e. the V_s of CD₄ is lower than the V_s of CH₄ for negative polarity). The origin of these observed isotope effects is related to changes in the effective ionisation coefficient, alpha /N, and to changes in the value of the secondary coefficient, gamma . Since the alpha /N changes are due to changes in either or both the ionisation, alpha /N, and the attachment, eta /N, coefficients, they have classified the isotopic pairs of molecules in three groups: non-electronegative (where only changes in alpha /N are important), weakly and moderately electronegative (where changes in alpha /N and/or eta /N are important), and strongly electronegative gases (where no isotopic dependence of V_s is expected). alpha /N depends on mass, especially when electron impact dissociation processes are significant and its isotopic dependence arises from the competition between dissociation into neutrals and auto-ionisation. eta /N depends on mass, especially when dissociative attachment proceeds via a negative-ion state which is attractive, and dissociative attachment is then strongly influenced by the time it takes for the dissociation fragments to separate as opposed to the time of auto-detachment. The latter led them to predict that the V_s of H₂S should exceed the V_s of D₂S since the dissociative attachment cross section is much larger for H₂S than for D₂S. This prediction has been confirmed by experiment. Basic data and V_s measurements in quasi-uniform and highly non-uniform fields for both positive and negative polarities suggest that the isotopic dependences of the V_s of gases are apparently of more general occurrence than hitherto recognised.

Current pulse shapes have been measured in negative point-to-plane corona discharges in oxygen, nitrogen-oxygen and argon-oxygen mixtures, with particular reference to the plateau on the trailing edge of the pulses. It was found that argon behaves as a passive third body in argon-oxygen mixtures, while nitrogen plays an active role in the physical processes which determine the current pulse shape and size. The implications of these results on theories of current pulse formation are discussed.

The application of the gas cell and microphone photo-acoustic technique to detect first- and near-second-order phase transitions in 8CB liquid crystal is shown. A relaxation process in the smectic A phase attributed to defect migration is reported. The adopted geometry allows an interpretation of the results according to available models. Microcalorimetric application of the technique can therefore be expected, even close to the phase transitions where critical thermal conductivity phenomena occur.

The utilisation of complex radioactive decay chains for surface studies is proposed. The recoil momentum imparted to the nucleus in beta-decay or electron capture causes the atom to desorb from the surface onto which it has been adsorbed. The measurement of the total probability of desorption and its angular dependence for a variety of isotopes is investigated as a potential source of information about surface structure. Various aspects of this technique are discussed, along with some of its variants and experimental options.

Space charge in the ethylene-vinyl acetate (EVA) copolymer was directly probed by a thermal-pulse-current (TPC) technique. The mechanism of electrical conduction is discussed by using the results of TPC and conduction measurements. The conduction is explained as follows. In the low-field region, positive-carrier injection dominates the conduction. Positive homo space charge due to the injected carriers is formed and controls the conduction as in the case of space-charge-limited current. The conduction is explained quantitatively by the space-charge-perturbed current theory in which the anode field is not reduced to zero. In the high-field region, the carrier injection rate cannot cope with the sweepout of injected carriers and the conduction is injection-limited.

Space-charge-limited current in the Au/CdSe/Ni structure has been studied to determine the trap depth and density in electro-deposited n-CdSe films. A high density of shallow traps was observed. Donor concentration, band bending and minority-carrier diffusion length measurements were carried out by studying the photocurrent-voltage characteristics of an n-CdSe film/electrolyte rectifying barrier using a 5 mW He-Ne laser for illumination. These parameters have been correlated with the observed performance of solar cells. The influence of annealing on the physical properties is also discussed.

Investigations of current transport mechanisms in the high- and low-impedance states of threshold-switched thermally oxidised titanium oxide films have been carried out. It has been ascertained experimentally that the high-impedance branch is dominated by a Poole-Frenkel type conduction. In the low-impedance branch a trap-free space charge limited type of mechanism is operative. The activation energy of donorlike centres responsible for Poole-Frenkel type conduction is also determined as 0.04 eV.

A numerical model has been developed to study the thermal behaviour of salt gradient solar ponds under different operating conditions. A cylindrical solar pond system with a thermal sink below and around the pond is also modelled to determine the heat losses to the earth. In these calculations, the solar pond is considered as a steady-state flat-plate collector. The effects of the thickness of three different zones and insulating materials below and around the pond on the overall thermal performance of the pond are studied. There is an optimum thickness of the non-convective zone for which the heat storage is maximum. The pond bottom and side heat losses are reduced using some low-cost insulating building materials such as dry dune sand, dry mica powder and dry marble dust below and around the pond. The results of a comparative study indicate that heat losses are considerably reduced with marble dust insulation and the solar thermal energy retaining capability of the pond is improved.

A recent communication by Graneau (see ibid., vol.20, p.391, 1987) on railgun recoil and relativity is refuted.

A sequence of colour photographs of an emissive probe moving through a luminous anode double layer is presented. The double layer is seen 'following' the probe for as much as 10 cm. This effect may result in erroneous measurements of both the double-layer width and its position.

The lattice parameters of the MgCu₂-type rare-earth intermetallics PrAl₂ and NdAl₂ have been measured from room temperature to 943 K using a Unicam 19-cm high-temperature powder camera. The data have been used to evaluate the coefficients of thermal expansion at various temperatures. It was found that the lattice parameter increases while the coefficient of thermal expansion decreases with increasing temperature in both compounds. An attempt is made to explain this behaviour in terms of crystal-field effects present in these compounds.

The tensile strength of water measured dynamically is less than that measured statically. It is claimed that this anomaly, which has been known for a long time, can be accounted for if one assumes that the transition layer at the free surface of a liquid contains a region for which the effective velocity of sound is very small. This region should be a source of cavitation nuclei, thus lowering the tensile strength. The hydrodynamics of reflection of pressure pulses by such a region are investigated and the imperfect reflection of pressure pulses by the free surface observed by Sedgewick and Trevena (1976) can be explained on the same hypothesis.

The onset of the reverse transition in phase-transforming solids, under planar loading, is discussed. It is shown that the earlier interpretation, which correlates this onset with the stress level just prior to the rarefaction shock, is more adequate than a different interpretation which was suggested recently. According to this interpretation, it is impossible to determine the exact pressure at the reverse transition with in-material gauges. The author brings three arguments which oppose this new interpretation based on the expected shapes of in-material piezoresistance gauges. If the new interpretation is right then measured stress histories in shock-loaded Armco iron specimens, with manganin gauges, should look differently than they do, as recorded by three different groups.

A calculation method is proposed for estimating a momentum transfer cross section or a total cross section (summation of the elastic momentum and inelastic), from electron drift velocity versus E/N data, by a Boltzmann equation method. Here, E is the electric field and N is the gas number density. In this method, the calculation is performed knowing that the electron energy distribution is shifted towards higher energy with increasing electric field. The validity of this method is examined by applying the analysis to a model gas, which consists of elastic momentum transfer and electronic excitation as collisions between an electron and a gas atom. The results show that a good approximation, a correct cross section can be obtained automatically from the electron drift velocity data by this method.

The coercivity of epitaxial magnetic garnet films has been measured on both stripe and bubble domain structures by the domain wall oscillation method. The bubble coercivity versus bubble array density relation has been determined experimentally. It was found that on the array reaching a certain density the apparent coercivity was strongly influenced by the neighbouring domains interaction. Also, the coercivity of bubbles was found to be a function of domain wall structure (domain wall energy); the presence of Bloch lines increased the experimentally measured coercivity value.

X-ray photo-emission spectroscopy (XPS) and X-ray-induced Auger spectroscopy have been used to study changes induced by Ar⁺ ions at silica surfaces. No evidence of appreciable surface damage was found for ion energies up to 2 keV. Results of the 6 keV exposed sample showed occurrence of both SiO₂ and SiO_x phases. Quantitative XPS analysis was used to monitor modifications in the silica surface stoichiometry as a function of the etching treatment. Whereas Ar⁺ ions of energy up to 2 keV leave the surface composition virtually unchanged, the nO/nSi atomic ratio drops to 1.40 in the 6 keV exposed sample.