Table of contents

The applicability of various analytical functions and semi-empirical formulae proposed for representing the full energy peak efficiency (FEPE) curves of Ge(Li) and HPGe detectors has been studied for 114 cm³ and 157 cm³ HPGe coaxial detectors for gamma ray energies ranging from 244.66 to 1408.03 keV and from 80.997 to 1408.03 keV respectively. One of the functions proposed provides by far the best description of the present data. The most recent function, employed above approximately=110 keV, also gives more or less the same quality of fit. The low-energy (below approximately=110 keV) version of this function reproduces the trend of the 157 cm³ detector data very well below 122.06 keV. This particular function also describes the efficiency of a 38 cm³ Ge(Li) coaxial detector from 319.80 to 2598.80 keV exceedingly well. The semi-empirical formulae of Freeman and Jenkin (1966), Mowatt (1969) and of Vano and co-workers (1975) provide (above 244.66 keV) a very good representation of the present data. In general, the analytical functions describe the data better than the semi-empirical formulae. The present investigation shows that the analytical functions and semi-empirical formulae which represent the FEPE of moderate volume Ge(Li) detectors quite satisfactorily, can also be used for large volume HPGe detectors quite fruitfully, particularly in the energy range 244.66 to 1408.03 keV. This is also true the other way round for a 38 cm³ Ge(Li) detector from 319.80 to 2598.80 keV.

The dispersion relations for low-frequency TE waves guided by the interface between semi-infinite non-linear and linear frequency-dependent gyromagnetic media have been investigated theoretically and numerically. It is shown that TE waves can propagate, with or without linear limits. The power dependence of the non-reciprocal behaviour of these waves is quite dramatic and, in principle, can lead to some interesting experimental applications. The variation of the wave index with the power flow, and the consequent interface nonlinearity, are found to be very frequency dependent.

An application of liquid crystals to astronomical adaptive optics which would produce sharpened images from optical telescopes is proposed. To measure the appropriate optical properties a method is described for determining the dispersion and thickness of a thin film based on a spectroscopic technique using Edser-Butler fringes. The application to birefringent media, in particular liquid crystals, and a method for assessing the fine structure is discussed. Results of measurements of the dispersion of liquid crystal BDH-E44 are given.

The maximum entropy method (MEM) is a signal processing technique for reconstructing high quality, all positive images from noisy data. The paper describes an extension to the MEM that allows consideration of images having both positive and negative parts. The necessary alterations to the algorithm of Skilling and Bryan (1984) are described. Simulations using positive and bipolar images have been carried out with the aim of observing the effect of the enhancement on the performance of the MEM. Three areas are discussed in detail: the frequency extension property; the comparison of spike and rectangular bipolar images; and the reliability of the restoration of peak heights when using bipolar images. Where appropriate, the MEM results are compared with those from the Wiener-Hopf filter. The modification now extends the application area of the MEM in ultrasonic signal processing, allowing its use in situations where images contain both positive and negative images.

The attenuation and dispersion of elastic waves in multi-phase materials such as granite, cement, mortar and concrete have been evaluated by the point-source/point-receiver technique with glass capillary breaking as an ideal source and a calibrated pin-type transducer as an ideal detector. The magnitude and phase spectrum from the Fourier analysis of the velocity signal gave attenuation and dispersion of the materials respectively, and the frequency dependence was also obtained in a single experiment. The sequence for attenuation was mortar, cement, granite and concrete; however, that for dispersion was cement, mortar, granite and concrete in increasing order. The difference in resonant frequency spectra of mortar and concrete blocks was explained in terms of the measured attenuation and dispersion.

Two-phase materials have been assumed to contain spherical particles arranged in a three-dimensional cubic geometry. The arrangement has been divided into unit cells each of which contains a sphere. The resistor model has been applied to determine the effective thermal conductivity (ETC) of the unit cell. A sphericity term has been introduced to account for the ETC of materials with non-spherical particles. A porosity correction term has also been applied to the present model and other models cited in the literature. Reported results have been discussed in the light of the proposed model and suggested modifications.

Transport properties of combustion plasma seeded with an aqueous solution of potassium carbonate are investigated. An atomized seed solution is added to the combustion products of oxygen and liquefied petroleum gas to obtain a weakly ionized plasma at atmospheric pressure. Seed droplets obeying the Nukiyama-Tanasawa distribution are considered to pass through seven stages until eventual evaporation. The electrical conductivity of the plasma has been observed to increase with decreasing mean drop size for a given seeding ratio, especially away from the burner where the seed has completely evaporated. The experimentally measured conductivity is in reasonable agreement with the analytical predictions.

Hollow aluminium microcylinders were irradiated internally through a circular aperture located in one wall using a pulsed ruby laser beam. Plasma formation and ion flows were monitored using a Nomarski interferometer and an array of Faraday cups. A collimation of ion energy along the cylinder axis was observed.

Experimental results concerning the propagation of a spark in a 16.7 m rod-plane gap are presented. Positive switching impulses are used. The parameters of the discharge are measured and the trajectory of the leader-leader corona system is studied as a function of time (withstand and breakdown events). Three main types of propagation are found: continuous, oscillatory and restrike. For each type, the propagation mechanisms of the leader-leader corona system are discussed from a physical point of view.

The vibrational distribution of the singlet metastable state N₂(a¹ Pi _g) was measured in a DC glow discharge by optical emission spectroscopy of the Lyman-Birge-Hopfield bands in the VUV range. The pressure ranged from 4 mTorr to 2 Torr and the current from 5 to 50 mA. The observed distribution is strongly non-Boltzmann. A population inversion similar to that in aurora was observed at low pressure and low current but not at higher pressures. The experimental results are compared with a kinetic model based on solutions to the electron Boltzmann equation and the rate balance equations for the N₂(a¹ Pi _g) vibrational levels.

The authors propose and put to use a new approach to studying magnetised plasmas sustained by a high frequency (HF) field, particularly aiming at examining the case of discharges achieved at electron cyclotron resonance. This approach considers the methodology and the formalism of the modeling of cylindrical plasma columns produced by electromagnetic surface waves and extends them to the case where these discharges are submitted to an axial, static magnetic field B₀. It leads to a variety of waves that are guided by the plasma column, these waves differing in particular by the spatial distribution of their electric field intensity. This distribution plays on the power transfer from the HF field to the plasma and it influences the spatial density distribution of excited atoms. This led them to analyse, as a function of B₀, the respective effects of the wave attenuation coefficient, wave polarization and HF power required to maintain an electron-ion pair in the discharge upon the plasma density and upon the electric field for the gas breakdown.

Luminous, sharply defined and nearly spherical objects ('fireballs'), typically several centimetres in diameter, have been observed in Ar, Kr and Xe plasmas. The 'fireballs' are anchored on a disk anode biased at 40-50 V above ground potential. They have been analysed in terms of a spherical double-layer model, in which ionization of the gas atoms by electrons energized at the double layer plays an essential role. The relevance is discussed of the results to the so-called 'plasma contactors' to be used in space, especially in view of applications to electrodynamic tethers.

The filamentation of high-frequency excited CO₂ laser discharges with dielectric electrodes is investigated theoretically and experimentally. A self-consistent two-dimensional discharge model including equations for the electron density, the electric field and the neutral gas density is used to calculate the spatio-temporal development of filaments in a high-frequency discharge. The space charge field is described implicitly by the complex dielectric constant of the discharge plasma. The side-on light emission of high-frequency excited discharges is recorded, time-integrated by a camera and time-resolved by using a framing camera. Filaments develop above a critical value of the power density which depends on the pressure. The development of the filaments is partly suppressed by the dielectric electrodes.

A time-dependent two electron group model 2EGM, based on a bi-Maxwellian electron energy distribution function (EEDF), has been developed to simulate the discharge kinetics in a high-current, high-repetition frequency, pulsed He-Sr recombination laser. A comparison is made between the results predicted by 2EGM and 1EGM (Maxwellian) simulations and experimental data corresponding to typical operating conditions of the laser. Results from the 2EGM indicate that during the current pulse, the high-energy tail region of the EEDF is severely depleted due to both inelastic collisions between electrons and ground state helium atoms and incomplete thermalization via Coulomb collisions. This leads to a highly non-Maxwellian EEDF, a feature which cannot be accommodated in the 1EGM simulation. In addition, the He*(2³S), Sr, and Sr⁺ population densities predicted by the 2EGM are shown to be in close agreement with the experimental data, whereas the 1EGM predicts a partitioning of energy between helium and strontium states which is inconsistent with the observed population densities.

In order to evaluate the possibility of tunable F₂/F₃⁺ CW colour centre laser operation in LiF crystals at room temperature, the authors performed an investigation of the emission characteristics of gamma -irradiated LiF crystals under CW argon ion laser excitation at room temperature. They measured a strong quenching of the F₃⁺ emission intensity that was dependent on the pumping laser power. The characteristic decay times of the F₃⁺ luminescence bands were also investigated with different pumping powers and a model for the recombination kinetics is discussed.

The elastic strain energy density has been determined for a deformation twin in alpha -iron. The twinned region has been assumed to be ellipsoidal in shape and the effect of twin anisotropy has been taken into consideration. The energy has been calculated using the Eshelby method together with its later modification for anisotropic inclusions by Asaro and Barnett (1975). It has been demonstrated that the elastic strain energy density is dependent on the direction of shear within the twinned region and twin geometry. For a twin situated on the (112) plane the strain energy density is smaller when the twin is deformed in the (111) direction than in (111) for the same non-dilatational strain.

Ion beam mixing in cobalt silicide has been studied using silicon, argon and helium ions. Both Co-Si and Co-SiO₂-Si systems have been studied, the latter in the form of coated silicon wafers which have not had their native oxide removed. Dose and temperature dependence and the effects of post-bombardment annealing have been studied. Oxide free samples show both temperature dependent and temperature independent mixing regimes. Mixing is retarded in samples with an oxide layer and this is reflected in the mixing activation energies which for Si⁺ ions are 0.90+or-0.28 eV and 2.17+or-0.79 eV for the two types of sample. There is evidence that implanted Si dilutes the interfacial barrier and enhances migration. The results indicate that the chemical species of the ion is important in determining the degree of mixing. Argon ion dose dependence experiments indicate a minimum dose of 1*10¹⁶ ions cm^-2 for the onset of mixing (at 350 degrees C) in both types of sample. Temperature dependence studies have yielded mixing activation energies for Ar⁺ ions of 0.46+or-0.10 eV from the Co-Si samples and 0.64+or-0.18 eV for the samples with oxide present. These values are intermediate between those obtained for Si and Xe bombarded samples. He⁺ ion bombardment studies indicate that mixing is not associated with ionization spikes. Considerations of ion concentration, bombardment flux, silicide thickness and activation energies favour thermal spike mixing rather than models based on radiation enhanced diffusion.

The diffusion of metal impurity into a doped semiconductor crystal of finite size was studied taking into account dissociative and kick-out mechanisms simultaneously. Equations for the determination of critical sample thicknesses and temperature were obtained, when one diffusion transfer mechanism (regime) is replaced by another. For the Au-Si system the ranges of existence for these regimes are determined. For large time values the equivalence of diffusive-kinetic and purely kinetic descriptions of impurity diffusion in the crystal is shown.

Noise in field-induced electron emission from graphite composite (pencil lead) has been investigated by spectral analysis and autocorrelation measurements in addition to its characterization by recording I-V and I-t curves. The I-V curves for the field emission from pencil lead exhibit several glitches leading to oscillatory behaviour in Fowler-Nordheim plots. The I-t characteristics of field emission current display trains of pulses with characteristic distribution which appears to change unpredictably from one train to another. Sometimes a train will exhibit a totally random noise. Analysis of the behaviour of the power spectrum and autocorrelation of individual trains indicates that the observed noise originates from a variety of processes such as adsorption, flip-flop, diffusion and desorption of residual gases on the heterogeneous surface of submicrometre-sized graphite a planes.

Simultaneous stretching and corona poling (SSCP) of polyvinylidene fluoride (PVDF) films at elevated temperatures in batch mode greatly enhanced the piezo- and pyroelectric constants. When poled under optimum conditions, a piezoelectric constant d₃₁ approximately 60 pC N^-1 and a pyroelectric constant p approximately 6 nC cm^-2 K^-1 at 30 degrees C were obtained. Both these values are about twice the best values found by the authors, and other workers, for films that were first stretched and then corona poled. The time stability of d₃₁ at all temperatures was also greatly improved by SSCP. They postulate that there is a greater contribution of whole beta crystallite rotation in these SSCP films. X-rays show that they have enhanced beta phase over those that are first stretched and then poled.

The author presents the results of the measurements of the three independent electrostriction coefficients of LiF, NaF, NaCl, KCl, KBr, RbCl, RbBr and RbI. Measurements have been carried out with a double Michelson interferometer to ensure a high reliability. The values of the electrostriction coefficients presented here differ significantly from values reported earlier. The measured differences could be due to the use of an improved measurement set-up.

The dielectric, pyroelectric and thermal properties of ferroelectric Bi₂VO_5.5(Bi₄V₂O₁₁) ceramic have been studied over a temperature range of 300-780 K. The sign of the pyroelectric coefficient is positive at room temperature. The dielectric constant, pyroelectric coefficient and specific heat exhibit anomalies around the Curie temperature, 725 K. The frequency response of the dielectric constant and tan delta has been studied over a frequency range of 1-100 kHz. It is found that both the dielectric constant and the loss tangent decrease with increasing frequency. The pyroelectric figures of merit from the point of view of different applications have been calculated at 320 K by combining pyroelectric, dielectric and thermal properties.

The principles of the high-accuracy universal polarimeter (HAUP) are discussed. The exact formula is derived for the intensity of the light that traverses the polarizer-sample-analyser system. For measurements with nearly crossed polarizers and oriented close to one of the extinction directions of the samples, the exact HAUP formula can be approximated. The approximate formula is used for the interpretation of the experiments. The fitting procedure that is used to derive the birefringence, optical activity, linear and circular dichroism from the data is described. It takes into account the systematic errors in the equipment due to the ellipticities of the polarizers and the so-called delta Y-error, which is caused by the eccentricities of the rotation stages that orient the polarizers. The fitting procedures are illustrated with measurements on a sample of (N(CH₃)₄)₂ZnCl₄.

A new phosphor, K₂Ca₂(SO₄)₃:Eu, which is more sensitive than the widely used CaSO₄:Dy has been recently reported. In this paper the authors present results which demonstrate the effect of various preparation procedures on thermoluminescence sensitivity, including the valence state of the impurity and its possible role in thermoluminescence processes. These studies will be useful in preparing the well characterized phosphor with reproducible properties.

The first experiments in which well defined waveguided radiation has been used to produce infrared multiphoton dissociation are reported. The waveguide used was of the cylindrical dielectric type. Excitation of the EH₁₁ waveguided mode by pulsed CO₂ laser radiation was confirmed by comparison of the diffraction pattern of the radiation emanating from the guide with theoretical calculations. Yields for the isotope selective multiphoton dissociation of deuterated trifluoromethane in the waveguide cell were compared with yields obtained previously using a conventional dissociation cell. Enhancements in yield in the waveguide dissociation cell of up to two orders of magnitude were measured.

The effective temperature of diffuse solar radiation decreases by increasing the number of scatterings. In the case of flat-plate collectors the maximal efficiency of singly or multiply scattered diffuse solar radiation is 0.053. The maximal efficiency can be improved by concentration. Singly scattered radiation has a maximum efficiency of 0.573. The Petela-Landsberg and Jeter formulae give good upper bounds for the maximal efficiency of solar collectors.

CdS/CdTe solar cells were made by depositing CdTe films by an isothermal close-spaced vapour transport method on sintered CdS/glass substrates. The influence of amounts of CdCl₂ ranging from 0 wt.% to 8 wt.% in the CdTe source on the solar cells performance was studied. Increasing the CdCl₂ content enhances the CdTe grain size but degrades the spectral response and increases the reverse saturation current. An optimal CdCl₂ concentration of 1 wt.% was found for a growth temperature of 620 degrees C.

Magnetic measurements using inductive or force methods give the sample magnetic moment. To calculate the magnetization requires an accurate measurement of the sample volume which is not easily possible in recording media. A new method is described for longitudinal recording media which relies on the shift in the transverse-susceptibility anisotropy peak produced by the demagnetizing field when measurements are made perpendicular to the film.

Infrared absorption spectra are presented for freshly evaporated lead phthalocyanine, oxygen doped lead phthalocyanine and material which has been vacuum annealed. The spectra allow characterization of films for utilization in gas sensors and FET structures.