Table of contents

In this review article, two aspects of the role of superconductivity in electron microscopy are examined: (i) the development of superconducting devices (mainly lenses) and their incorporation in electron microscopes; (ii) the development of electron microscope techniques for studying fundamental and technological problems associated with superconductivity.

A simple electronic circuit has been developed to eliminate the secondary pulses obtained when mercury-wetted reed relays are used for producing high-voltage pulses in the nanosecond region at high repetition rates.

A test facility is described which can be used for the measurement of very small resistances (a few micro-ohms). While the facility was designed primarily to measure the changes in resistance of bimetallic solid-phase welds during intermetallic layer growth at the joint interfaces, the techniques developed have applications in other areas of metallurgical research.

A simple non-magnetic technique is described for carrying out mechanical functions, such as shifting or turning diaphragms and samples, or actuating adjustment screws in small, near-inaccessible volumes. The device utilizes the sizeable forces accompanying the thermal expansion and contraction of a straight piece of relatively thin wire. Controlled heating of the wire is achieved electrically with small currents. A system of levers and springs operated by the wire permits storage, release and transmission of the potential energy needed for the mechanical functions. The device was applied in high-pressure hydrogen atmospheres up to 2 GPa.

A low-cost water bath temperature control device, and an apparatus designed to monitor temperature variation from a null point both inside a precision calorimeter and in the surrounding bath, are described. Bath control to better than +or-0.002K in the 280-350K region can be achieved. Temperature variations in ranges between +or-2K and +or-0.02K can be measured to better than 5*10^-2K and 5*10^-4K respectively.

A versatile system is described for measuring electrically active impurity distributions in semiconductors. The system uses an improved double AC Hall system for sensitive measurement of the Hall effect in conjunction with chemical etching for removal of successive layers. All processing is done with samples mounted on standardized discs, increasing the convenience and speed of the technique for repetitive measurements. A measured profile of Be-implanted GaAs obtained with this system is presented as an example.

Bridges employing inductive voltage dividers are known to be more accurate and more stable than comparable bridges employing resistances as their adjustable elements. Inductive voltage dividers have other desirable properties that have received less attention. Among these are high input impedance, and low output impedance. These secondary advantages are discussed and a typical circuit is solved, both in the balanced and the unbalanced states. It is shown quantitatively that unbalance currents can flow without the necessity of magnetizing the dividers and that this can lead to output impedances that are almost entirely resistive, with negligible reactive components. This desirable state of affairs can exist because magnetic field is a vector quantity and the fields of two currents in an inductor can cancel. Similar possibilities seem to be lacking when resistors or capacitors are used.

The construction of a simple high-frequency (100 kHz) modulation technique for conduction-electron spin resonance cavities is described. In K-band cavities, modulation field amplitudes of more than 7 mT can easily be obtained without introducing any noticeable modulation-induced microphonics.

To further the understanding of the biological effects of radiation a knowledge of the spatial distribution of ionizations in small volumes is required. A cloud chamber capable of resolving the droplets formed on individual ions in the tracks of low-energy electrons has been constructed. It is made to high-vacuum specifications and contains a mixture of permanent gases and vapours, unsaturated before expansion, at a total pressure of 10 kPa. Condensation efficiencies close to 100% are obtained without significant background from condensation on uncharged particles and molecular aggregates. This paper describes the chamber, associated equipment and method of operation and discusses the performance of the system.

A computer-controlled microdensitometer has been designed and built mainly for use in X-ray crystallography. The instrument is a hybrid of two earlier microdensitometers and combines accurate positioning with fast measurement. It incorporates a stage mechanically driven by stepping motors. The source of light is a high-resolution cathode ray tube which is imaged on to the film plane, the optical densities being measured by photomultipliers. The instrument is used routinely to scan oscillation photographs of biological macromolecules.

A pulse modulator is described for frequency-temperature monitoring of transferred electron devices. It has a fully variable pulse height with a stability of 150 mV for a current demand of up to 1 A and is largely unaffected by variations of pulse repetition frequency and pulse width.

The construction of an interference-free, water-cooled, oil-insulated, sealed, coaxial N₂ laser is described. By introducing an adjustable self-inductance into the pulse generator the time evolution of the high-tension laser electrode voltage has been synchronized with the gas breakdown delay period in such a way that the laser output and stability are maximized. The electrical functions of the system are explained by comparison of the measured time evolution of the high-voltage electrode potential with the numerical results of an exact equivalent circuit analysis. Increasing the repetition rate from 10 to 400 Hz, the peak value pulse-to-pulse reproducibility remains better than 3%, but due to heating of the laser medium a fivefold decrease in peak output power is observed.

Describes the operation and properties of a novel, simple, multi-atmosphere photoionization-stabilized carbon dioxide laser. The device is energized by a simple Blumlein-type pulser and requires no auxiliary timing and switching circuits.

A modification of a calorimetric technique for heat capacity measurements on small samples of low thermal conductivity is described. The sample is connected to a sink of large heat capacity by a finite thermal resistance. Step-wise electrical heating of the sample gives rise to an increase in temperature as a function of time. Accurate evaluation of specific heat capacity is possible from the recorded curve. Theoretical analysis has been made using an electrical analogy. The influence of parasitic capacities as well as of the sample geometry on the response of the sample is considered in detail. The method has been used for high-precision measurements of the specific heat anomaly in the vicinity of the nematic-smectic-A phase transition in liquid crystals.

Small droplets for laser targets are continuously produced on the tip of a needle that emerges slightly from an insulating liquid. The method requires only a constant high-voltage source of 2-10 kV to polarize the needle, thus dispensing with the need for a capillary tube or pressure-controlled tank, or a sonic source. An optical system designed to collect the diffracted light is used for the detection of the droplets and laser triggering.

Describes an extended investigation of the behaviour of Pt-Pt/13% Rh thermocouples under the operational conditions of a large creep testing laboratory. Analyses of comprehensive records over several years led to the formulation of certain assumptions which were confirmed by subsequent tests, and eventually resulted in a significant reduction in thermocouple drift.

Rapid measurement of the second derivative of the probe characteristic is realized by using a delay circuit and a single probe swept linearly. The probe current is time-shifted by a prescribed amount before entering an operational amplifier which takes the difference between the shifted currents. The resolution and sensitivity are determined by the delay time and sweep rate. When the attenuation caused by the delay circuit and the displacement current due to cable capacitance are compensated, the apparatus works sufficiently well for one to observe fine structure in the electron velocity distribution function caused by a plasma wave.

Apparatus is described for magneto-optical measurements in solids at liquid helium temperature using a modulated magnetic field. The AC magnetic field is generated by a small coil placed at the centre of a static superconducting magnet. In this arrangement mechanical vibrations are avoided completely. Joule losses in the coil are minimized in order to reduce liquid helium dissipation. Interesting and original results have been achieved on excitons and Landau levels in CdS and CdSe.

A new opto-electronic instrument for linear velocity measurements in plasmas has been developed. It incorporates a linear photodiode array and measures the intensity profiles of light emitted by a plasma section. Velocities are determined from the shift of intensity variations (typical error: 10%). Velocities of NaCl powder injected into a thermal induction plasma and of the plasma gas itself were measured and the scatter of the results is explained. The advantages of this method are immediate display of the results, versatility and the possibility of on-line data treatment.

A data processing technique is described which enables useful laser light scattering data to be collected from liquid solutions contaminated with dust. Details of the electronic circuits are presented and their operation is described. The technique may be applied to any light scattering measurement in which a photon counting detection system is employed.

A high-resolution energy analyser of the spherical condenser type with variable predeceleration is described. The optimization of the resolution over a broad energy band while maintaining total transmission is discussed. Furthermore, following the study of the properties of a twin-tube lens, a decelerating system is proposed which makes use of only three lenses polarized by means of voltage dividers. Measurements of the performances of such an ion spectrometer for surface analysis are presented.

A distributed computer system has been chosen to support the data acquisition and control requirements of experiments using the Daresbury synchrotron radiation facility. The reasons for adoption of a distributed computing philosophy are given and implementation of the system is described. Future developments in the support of a new synchrotron radiation source are briefly considered.

Formulae are derived for the expected shapes of delayed coincidence spectra when an exponential decay is measured using a simple circuit having a time-to-amplitude converter or equivalent digital timing device. In the limiting cases when the start pulse rate is low or when the fraction of start pulses which have a related stop is low, the formulae reduce to that of Coleman and Griffith (1973).

A driven torsion pendulum is described which is capable of directly and continuously measuring the energy loss of materials as a function of the amplitude and the number of repeated cycles. A high tensile stress can be imposed to enable the energy loss to be determined at various points on the tensile stress-strain characteristic of the material. Because the energy loss is obtained from the electrical energy required to maintain oscillation at a set amplitude, the damping can be obtained for specimens which exhibit amplitude-dependent behaviour.

The basic characteristics of an apparatus for experiments involving low-energy (0.1-5 keV) scattering of alkali ions from solid surfaces are presented. To produce alkali ions, a thermionic ion source was used. The ion beam obtained passes through the magnetic sector (R_m=10 cm, phi _m=90 degrees ), which operates in the stigmatic focusing regime. Due to the primary beam contamination effect, surface cleaning during the experiment was maintained by the deposition of a fresh film of the metal investigated. The backscattered ions were energy-analysed by the double cylindrical sector analyser (R_a=5 cm, phi ₁=90 degrees , E/ delta E approximately=400), which was continually swung around the target in the angular interval 0< theta <135 degrees . For monitoring surface cleanness during the experiment, an Auger spectrometer was used. Some results of the measured energy spectra of Na⁺ ions backscattered from an Ag surface are presented.

A series of modifications is described whereby a commercial video tape recorder was adapted for non-TV-format signals. The recorder used had a typical twin-head helical scanning system, with slow-motion and stop-playback capabilities. The signals to be recorded were developed during crack propagation of metal specimens under tension, i.e. stress-wave emissions. These occur at random intervals and are in the frequency range 20 kHz to 2 MHz.

A compact, high-intensity helium metastable source is described for use in surface spectroscopy experiments. This source, based on electron bombardment of a helium beam, produces a helium metastable flux of 4*10¹⁴ atoms s^-1 sr^-1, with a stability of 2%, and an effective ultraviolet photon content of less than 1%. Preliminary results, obtained from the study of the nickel (100) surface with this source, are presented and indicate that the metastable atoms are de-excited by a one-electron process.

For the study of anharmonic effects in lattice dynamics, valuable information is provided by the measurement of the change of phonon energy with strain by using inelastic neutron scattering. It is shown that the elements of the generalised Gruneisen parameter can be extracted by the application of uniaxial stress in various directions. A double-acting cylinder press is described that permits experiments with 350 kN compressive force or 150 kN tensile force acting on the sample. The results of compressive experiments along the crystallographic (100) direction of a single crystal of silicon are presented.

The multiple-hole SQUID is a useful instrument for high-resolution measurement of total magnetic field. A miniature five-hole SQUID with a preset point contact has been developed for use in a superconducting current comparator. A sensitivity enhancement of approximately 3.5 has been achieved for this SQUID over a two-hole unit using the same electronics. An electronic-noise-limited sensitivity of 5*10^-5 flux quantum for 1 s averaging time has been achieved at an operating frequency of 30 mHz.

A window shutter for X-ray crystallography apparatus is described that does not depend on gravity, springs or microswitches for its operation. The system can be adapted for older equipment such as the Unican S25 single-crystal camera and 9 cm powder camera and is also suitable for modern Weissenburg cameras and Philips Debye-Scherrer cameras. These shutters have been used for a number of years by undergraduate and postgraduate students and staff and have proved completely reliable.