Table of contents

The authors describe the operation and properties of a novel, simple waveguide carbon dioxide laser. The device is energized by a Blumlein-type pulser and employs a simple photoionization stabilization technique. Output powers of 5 kW in 400 mu J pulses have been obtained. Calculated values for the losses introduced by the waveguide used are in agreement with the experimental results.

Using direct voltages the EHD head due to co-field motion in transformer oil has been measured for degassed oil and oil saturated with oxygen. It was found that the presence of oxygen produced a great reduction in the measured head compared with degassed oil. The effect of gap length on the head was also investigated.

For pt.I see ibid., vol.7, p.2174 (1974). The authors consider the diffusion of neutrons in an infinite non-absorbing medium, in which is embedded a black sphere. A numerical study of the extrapolation distance for the sphere has been carried out using the variational method. The effects of energy-dependent cross-sections and linear anisotropy of scattering have been included. It is found that, when scaled in units of the Maxwellian averaged mean free path, the effect of energy dependence leads to differences of about 1% in the value of the extrapolation distance when compared with the one-speed values. The effect of linear anisotropy of scattering is included and evaluated numerically in the one-speed case by means of a novel, non-variational method.

Measurements have been made of the propagation of intense neutron pulses in a graphite assembly of transverse dimensions 137.2*127.6 cm². It was found that, by scattering the pulsed beam and by proper positioning of the detector, the dispersion parameters could be measured over a relatively wide frequency range without significant contamination by higher space and energy modes. The results showed that the Haywood-Page phonon model gives an adequate description of neutron wave interaction with graphite over a fairly wide frequency range. From the observed dispersion, parameter values of (2.092+or-0.006)*10⁵ cm² s^-1 for the diffusion coefficient and (34.1+or-0.7)*10⁵ cm⁴ s^-1 for the diffusion cooling coefficient were inferred at a graphite density of 1.685 g cm^-3.

Measurements have been made of neutron pulse propagation in three graphite assemblies of B_{perpendicular to} ²=(1.07, 1.78 and 3.55)*10^-3 cm^-2. It has been found that a pseudo-discrete mode can be observed over a substantial range of frequencies and throughout a considerable volume of each assembly. For a given frequency, the volume associated with an observable dominant pseudo-mode decreases as the transverse buckling increases. Measurements taken in this region led to values of diffusion and thermalization parameters which were independent of transverse buckling. The dispersion relations have shown that asymptotic reactor theory can be used with confidence for frequencies up to 500 Hz. The existence of interference between the cold and thermal neutron waves has been broadly confirmed.

A pulsed neutron beam from the thermal column of a reactor has been used to study neutron pulse propagation in graphite. A peculiar irregularity (an anomalous peak occurring rather earlier than the main peak) has been observed in the shape of the propagating pulse. The anomalous peak first appears at along distances from the source and its relative magnitude increases quickly with distance. Observations of the effect on the anomaly of changes in source conditions, source separation, and transverse buckling suggest that it is attributable to interference between sub-Bragg and thermal neutron waves in the frequency range around 2500 rad s^-1, and not to direct penetration by cold neutrons.

The authors present a study of the effects of partial coherence (via the effect of condenser aperture) on the resolution of two pinholes by a semitransparent and pi -phase annular aperture under Gaussian illumination. The response of the resolution has been studied for different values of eta , which is inversely related to the size of the Gaussian spot. It has been seen that by making the central region of the pi -phase annular aperture fully transparent, the best resolution is obtained in a good-quality image even for very high values of eta compared with the clear circular objective case with uniform illumination. The oscillatory behaviour of the resolution has also been displayed graphically.

It is shown that the transverse thermomagnetic (Nernst) effect can, in principle, be used in an amplifier that incorporates its own power source. As in the Hall-effect amplifier, the input signal is applied to the coil of an electromagnet. The output EMF appears across a sample of semiconductor or semi-metal. It would seem to be very difficult to make a practical amplifier to operate at room temperature, but it has been demonstrated that gain can be achieved, with bismuth as the active element, when the heat sink and magnet winding are cooled to liquid nitrogen temperature.

The precipitate phase, 6 LiF.MgF₂, has been identified in slowly cooled single crystals of TLD-100. Ionic conductivity measurements indicate a solubility limit of 250 degrees C and that substantial undercooling is necessary to cause precipitation. These effects are important in the practical heat treatment of such dosimeter crystals.

A study of the influence of holding TLD-100 crystals at different solution temperatures, between 210 degrees C and 470 degrees C, on the thermoluminescent glow curve has revealed no effect. The ageing of TLD-100 crystals at 177 degrees C, 150 degrees C, 131 degrees C and 102 degrees C as followed by ionic conductivity measurements has shown that 6LiF.MgF₂ precipitates in all cases. This strongly reduces the thermoluminescent response.

Methods to improve the band-pass characteristics of CuO-doped glasses are described. Shifts in the transmission maxima caused by the introduction of 1 mole.% CuO into eight different glass compositions based on silicate, borate, phosphate and tellurite systems are reported. Further shifts in the transmission peaks as well as decreased half-widths were achieved by means of the additional oxides CeO₂, TiO₂, V₂O₅, NiO or CoO, each of which on its own would result in cut-on, band-pass or band-reject characteristics in the base glasses. Thus, the possibilities available in the development of new filter glasses with band-pass characteristics are demonstrated.

The theory of the polarization and propagation of an electromagnetic wave in a birefringent medium is briefly reviewed. Particular attention is given to those aspects of the theory which relate to the radio echo sounding of ice sheets. Methods of determining the polarization state of a radar wave are discussed, and it is shown that in certain circumstances this may be achieved by observing the variation of the signal strength in a pair of rotating receiving antennae. The results of experiments performed on the Greenland ice sheet are presented. The radio echo is shown to be elliptically polarized, confirming the suggestion that the ice sheet is birefringent at radio frequencies. A figure is obtained for the anisotropy of the dielectric constant of the ice sheet.