Table of contents

A device is described for producing small (ca. 1 mu rad) rigid body rotations of optical elements about an accurately known axis. The usual difficulties of backlash and hysteresis associated with a rotary stage are avoided by mounting the stage on an elastic torsion suspension. Rotation of the stage is accomplished by applying an accurately known torque to the stage by controlled deformation of extensional springs.

A Jouan circular dichroism spectrometer has been modified to measure low temperature magnetic circular dichroism (MCD) and optical absorption spectra. The apparatus has been fitted for the measurement of MCD mainly by adding a superconducting magnet with magnetic fields up to 6.5 T. To obtain higher resolutions, the standard hydrogen and tungsten lamps were replaced by a xenon arc. A device for the measurement of optical absorption spectra in the optical density range 0-2 was developed, which enabled the scanning of MCD and absorption successively under the same circumstances.

A liquid helium freezing technique in combination with the Micromass 602C mass spectrometer fitted with the small volume inlet system is described allowing precise isotopic analyses of inert gases of sample volumes down to 0.02 atm cm³.

An instrument for analysis of wind speed and direction is described. Utilizing the time required for 1/12 mile of travel past the station as its basic measurement, a simple relationship between time and wind speed class is established. Six speed classes are measured within corresponding quadrant wind directions. Low power requirements enable bimonthly data collection.

Lecher wires can be used to guide microwaves through a region whose dielectric properties are to be measured. It is shown that 3 cm microwaves can be spatially confined to a diameter of approximately 0.2 cm by using wires of 0.2 cm spacing. Power transmission decreases and reflections increase as the wire spacing is reduced but by a suitable and simple coupling of the Lecher wires to the waveguide the transmitted signal is very adequate for interferometric purposes.

A new type of DC linear motor has been developed. The stationary field system is excited by the ferrite permanent magnets, and the properties and reasons for choosing a ferrite material are explained. This new configuration improves the reliability of the motor and can be precisely controlled.

A velocity monochromator is proposed which is based on the use of the inhomogeneous, oscillatory electric field derived from the potential distribution V(x,y,z,t)=(A+B cos omega t)( alpha _xx²+ alpha _yy²+ alpha _zz²).

A furnace controller employing a Pt resistance temperature sensor has been very simply modified to control the temperature of a cooling gas flow in the range between room temperature and 77K. At 91K the mean temperature drift is 1.5K h^-1 with a control ripple of +or-1.5K at a frequency of 3 min^-1. At 243K these figures become 1K h^-1, and +or-0.3K at a lower frequency.

A simple but accurate circuit is described to integrate photomultiplier anode pulses produced by viewing a nanosecond lamp. The integration time can be varied from seconds to minutes. The circuit is used in measuring nanosecond time resolved spectra in which each point of a spectrum, as viewed on a multichannel analyser, is the result of equal light intensity falling on the fluorescent sample.

Modifications are described that permit a previously described root-law circuit (see abstr. B2556 of 1975) to perform the computation (x²+y²)¹2/, regardless of the x and y signal polarities. Inputs may be independently represented as either voltages or currents.

A fringe mode laser-Doppler velocimeter is described that instantaneously measures two orthogonal velocity components that have fluctuating signs. The optical system utilizes an equiangular configuration of three intersecting beams which allows measurements of the normal velocity component to be taken in close proximity to a wall. Acousto-optic frequency shifting of two beams is used to eliminate cross channel interference and to provide detection of the velocity polarity. The instrument is applicable to flows having velocities in the range of +or-1 mm s^-1 to +or-10 m s^-1.

An experimental arrangement is described which controls both the calibration and stepping of a double monochromator. Calibration is achieved using 'channel spectra' from a Fabry-Perot etalon. The counts from the signal and from the calibration fringes are stored consecutively in digital form at the same transmission frequency of the monochromator. The collection times of the signal and calibration data are independently variable over a wide range. The system allows the transmission frequency of the monochromator to be measured to better than 0.05 cm^-1 at any point in the scan.

A method is described for measuring linear compressibilities of solids under very small hydrostatic pressure. The authors use a three-terminal capacity method for measuring the change in length of a sample, produced by increasing the pressure applied to it. The method is checked by measuring the known isothermal compressibilities of three solids (Si, Mo and NaCl).

A new method of measuring the critical tension which a liquid contained in a steel Berthelot tube can sustain is described which enables one to monitor the growth of this tension prior to its critical value at which the enclosed liquid breaks. The pressure and tension changes in the liquid inside the cylindrical Berthelot tube are measured by attaching a pressure transducer to the tube in such a way that the diaphragm of the transducer forms one of the end walls of the tube. Earlier methods used for the calculation of the critical tension are discussed and compared with the new method.

The application of Ge thin films as temperature probes is investigated. The shift of the transmission spectrum and of the reflection spectrum between wavelengths of 0.5-1.5 mu m was measured at temperatures up to 430 degrees C. The temperature sensitivity was found to be 0.3% intensity change per 1K temperature variation in the case of transmitted He-Ne laser light ( lambda =0.633 mu m) and 0.1% in the case of reflection ( lambda =1.15 mu m).

The construction and operation of a fast acting gas valve is described. The valve, which has a measured sampling time of 150 mu s, has been used to extract samples from a transient rotating neon plasma for the purpose of studying isotope separation effects in the plasma. The motion of the valve was investigated optically. The valve sampling time was determined by a novel method, which utilized the discharge itself.

A minimal computerized laser and plasma diagnostic system, suitable for small and medium laser installations, is presented. This system is now being operated successfully; typical diagnostic sheets are given for illustration.

A simple method has been devised of coupling the light emitted by many LED devices into matching optical fibres, which is especially applicable to systems where the emitting and receiving linear and numerical apertures are identical or similar.

Describes a new method of generating low frequency high alternating voltages by modulating two high voltage DC supplies of opposite polarity. The modulation is accomplished by the use of triode valves situated at high voltage whose grids are supplied by control signals, transmitted from the control circuit through fibre optic light guides. The first generator built is capable of driving a 1 nF load at 10 kV peak over the frequency range DC-50 Hz, or a 0.1 mu F capacitor at up to 0.1 Hz. In order to increase the voltage and current ratings of the generator, a series-parallel arrangement of valves may be used, with the appropriate control signals fed to the grid of each valve. Because the generator is relatively free from any electrical noise, it can be used to energize insulation samples in connection with partial discharge detection investigations.

A technical description is given of a system that has been developed and used for a number of years in work reported on the assessment of silver halide and other light sensitive photographic materials. The instrument uses coherent light both for generating test patterns and for measuring the properties of the exposed samples. The parameters that may be evaluated in this instrument are transmittance as a function of exposure, diffraction efficiency of gratings, modulation transfer function (MTF), and scattered flux spectrum. The heart of the system is an arrangement that generators sinusoidal irradiance distribution from laser two-beam interference over a continuous spatial frequency range from about 3 to 3000 cycles/mm. Accessory units adapt the system to various evaluation tasks. One configuration is the multiple-sine-slit microdensitometer (MSSM), which measures transmittance modulation with constant and high accuracy up to 1500 cycles/mm.

A new method has been developed for continuous recording of changes in ultrasonic velocity. The system is consistent with classical equipment for automatic attenuation measurements (working by the pulse echo technique). This is based on phase comparison measurement between two ultrasonic pulses, one travelling through the sample tested and one through a reference sample. The phase comparison is made with high speed logic circuits after adequate shaping of the RF pulses. The resolution thus obtained is about 1 ns for frequencies up to 10 MHz. A description of the major functions of the system is given. Some typical results are reported for dislocation studies in aluminium.

New shapes for high field superconducting lenses, are computed from Poisson's equation taking into account the strong saturation of the magnetic circuit. The optical properties of these lenses are better than those of lenses of classical design working in the same conditions.

Describes a superconducting lens with holmium polepieces and presents the measured optical properties of this lens for various excitations and several gap values. An objective focal length of 7.5 mm and a projector focal length of 9.5 mm can be achieved at 3 MV. The aberration coefficients are also smaller than those of classical lenses operating with the same accelerating voltage. Finally, a limit of resolution of 1 AA can be expected.

An UHV apparatus is described which allows energy analysis of low energy rare gas ions backscattered from surfaces in the energy range from 100 to 2000 eV and in a large range of scattering angles. Surface composition is measured by ion scattering and Auger electron spectroscopy. Surface structure of monocrystalline samples is evaluated by LEED. Examples of surface composition analysis and ion impact desorption are given.

A moire method for recording the surface topography of living animals is presented. Moire fringes representing contours of equal distance from a reference plane may be produced when two gratings, one projected onto the surface of the animal and the other an equispaced line grating are superimposed. The levels at which contours are observed may be adjusted on a single photographic record by lateral movement of the equispaced grating. In this way any point on the surface may be made to coincide with a contour line and its coordinates determined. The technique is suitable for measuring surface gradients and a method of automating the reading of surface coordinates is suggested.

The performance of water filled acousto-optic cells is quantified with particular reference to their use for laser-Doppler anemometry. With the cell operating at a frequency of 9.2 MHz, the intensities of the +1 and -1 order beams are shown to account for 76% of the transmitted light intensity for an electrical power input of 0.56W. The influence of the incident angle and the performance of a cell at frequencies close to but different from the fundamental frequency are quantified. Operation of the cells at a frequency of 30 MHz, i.e. close to a third harmonic, is shown to require significantly greater electrical power input but can without introducing thermal connection effects, allow the transmission of a first and zero order beams with equal intensities and a total loss of 8% of the incident intensity. To show that an acousto-optic cell can be used as a beam splitter and frequency shifter, measurements of three components of velocity are reported in the region of recirculation immediately downstream of the centre disc of an annular jet.

A current pulse modulator is described for use in frequency-temperature characterization of IMPATT diodes. It has an amplitude substantially independent of PRF up to 100% duty cycle for pulse widths down to 200 ns with a risetime of 20 ns.

A new method for scanning an absorption spectrum under digital control, particularly suited for very low flux levels, is demonstrated. The scan is divided into small integral steps digitally controlled with feedback such that each step represents a constant number of input photons. High resolution spectra are thus obtained with the asynchronous scan of a single beam at constant signal to noise ratio without any ratio devices.

The authors have constructed a high speed digital autocorrelation spectrometer suitable for use as a sensitive receiver in millimetre spectral line astronomy. The instrument has 32 channels and employs the one bit digitization technique to analyze spectra contained in bandwidths available between 0.94 and 30 MHz. Schottky clamped transistor-transistor logic is used in the digital processor to permit a maximum sampling rate of 60 MHz, which is three times faster than any similar instrument of which we are aware. Stability tests indicate that for 1000 s integration times the receiver sensitivity is within 15% of its theoretical value at all available bandwidths. The receiver is presently being used to study emission at 22.2 GHz from interstellar water molecules.

A voltage contrast detector is described for measuring the potential of a sample in a scanning electron microscope by monitoring the energy shift in the differential secondary electron spectrum. The longterm stability of the measurement is +or-0.5 V and is independent of changes in topography of typical microelectronic devices. Use of this detector does not interfere with either the logical operation of the device or with the stroboscopic operation of the SEM.

A hydraulic pressure technique has been used to measure the strength of glass discs. The apparatus is simple to construct and to use, and its behaviour agrees well with theory. Results are presented which demonstrate the use of the pressure tester in examining the reduction in strength of glass specimens, caused by high velocity water-jet impact; this situation has relevance to the practical problem of the rain erosion of brittle materials. For this study the pressure apparatus has proved to have important advantages over other experimental methods.