Table of contents

Today's high-performance electron microscopes have achieved an optical resolution below 0.2 nm. The radiation sensitivity of biological specimens inhibits structure analysis with a corresponding resolution, thus several attempts are being made to reduce the electron dose impinging on the specimen to an amount which is physically unavoidable. Development trends are toward higher voltages, correction of aberrations in the lower energy range, application of image reconstruction methods, and low specimen temperature. The technical state of the various components of conventional transmission electron microscopes is assessed, i.e. beam sources and condensers, objective and imaging systems, specimen stages, image-recording and image-processing devices, and energy-filtering systems. Properties of commercial instruments are described, and non-commercial high-resolution projects now under way are mentioned.

A piezoelectric drive which produces rapid axial motion of a microelectrode has been developed. The device is easy to assemble and to align accurately. The amplitude of the movement can be precisely controlled over a range of 0.25-100 mu m. The motion can be completed in a few milliseconds. The device has been used to penetrate successfully the cell membranes of the large free-swimming protozoa Stentor. The device should prove useful for the penetration of other cells regardless of size.

A low-temperature infrared cell is described which is capable of operation at a controlled temperature or under controlled rates of heating and cooling down to liquid nitrogen temperatures. Careful consideration is given to thermocouple placing and avoidance of paths with high thermal resistance so that the lag between measured and actual temperatures during heating and cooling cycles is reduced to a minimum. Although the cell is primarily designed for investigations of hydrated protein films a variety of other sample preparations can be used.

The design and advantages of a digital system for reading the ohmic drop compensation level in electrochemical cells are described.

A universally adjustable goniometer polishing device for precise single-crystal orientation and sample preparation is described. This device allows orientation and polishing of single surfaces as well as multifaceted crystals with defined surface orientation requiring only one orientation procedure. The simplicity and compact construction of this device further facilitate its use with other commercially available equipment for crystal orientation and polishing.

A simple discharge model is used to evaluate the field uniformity requirements for TEA CO₂ lasers and to predict the electron density distribution which results with Chang profile electrodes.

The beam divergence of a highly multimode, pulsed, electron beam CO₂ laser has been measured and the results have been compared with simple predictions. Good agreement has been obtained between experiment and theory.

An expendable microwave refractometer suitable for use in a meteorological or a tethered balloon is described. The design is based on the Crain system, but with the TE₀₁₁ mode sampling cavity remote from the reference cavity which is the local oscillator of a ground-based receiver. Oscillator stability of both the disposable refractometer and the receiver is achieved by the use of a modified Pound stabilising system. The microwave circuit of the disposable part of the instrument is constructed in microstrip and uses a Gunn oscillator operating at 10.588 GHz with a power output of 10 mW. It weighs 1¹/₂ kg and will operate over a temperature range of 258-303K. The refractometer has a dynamic range of 80 N units with a resolution of +or-0.1 N units (N=(n-1)10⁶, where n is the refractive index).

The method of determining dielectric constants, based on the transmission properties of a partially filled rectangular waveguide, has been extended to solid or liquid multilayers. The procedure is suitable for frequencies between 1 and 100 GHz. Data obtained between 7 and 24 GHz from various solid and liquid materials are presented demonstrating the high accuracy and the simplicity of this method.

To evaluate the energy involved in the setting up and switching of electrical polarisation in polar materials, a method, based on the comparison of the energy graphically plotted on an hysteresis cycle with that measured simultaneously in a differential calorimeter, was tested with ceramic samples of PLZT type. These measurements confirmed the existence of an antiferroelectric-ferroelectric first-order transition, induced by the application of an electric field at room temperature on a PLZT-type compound (5% La, 85% Zr, 15% Ti) and on a PZT type (94% Zr, 6% Ti, doped with 1% ThO₂), both materials exhibiting inverted phase transitions with temperature.

A simple and inexpensive resistance bridge is described which can monitor small changes in resistance with very high resolution. It has been used to measure the temperature dependence of (four-terminal) resistances with values in the range 10^-8-10^-5 Omega . The sensitivity is limited only by Johnson noise generated in the sample, comparison and contact resistances; with a sample current of 100 mA the resolution is 1 in 10⁶ to 1 in 10⁷. The highest temperature at which the bridge can be used is set by the superconducting transition temperature of the leads to the sample, 9K in this case. The circuit uses an RF SQUID as a null detector, and the only critical components are the resistors for the arms of the bridge; kept constant by maintaining them at liquid helium temperature. A current supply of the requisite moderate stability (about 1 in 10⁵) is also described.

A compact and simple instrument is described for the measurement of mercury vapour concentration in the atmosphere. The detector element is a vacuum-deposited gold film about 10 nm thick. Absorption of mercury vapour by this film produces a change in electrical resistance, the time derivative of which is proportional to the concentration of mercury vapour in the vicinity of the detector. The instrument compensates for the fall in detector sensitivity with continued exposure to the vapour. Detector elements may be regenerated by electrical self-heating, to drive off absorbed mercury. Vapour concentrations of less than 10 mu g Hg m^-3 may be measured.

An ion implanter with high mass resolution has been connected to a conventional electron microscope. Preliminary results on the evolution of defect clusters created by ions of 10-50 keV in Ni are given. The resolution achieved is better than 1.5 nm for irradiation temperatures above 90K, using conventional holders. In the temperature range 20-100K irradiation experiments are performed using a liquid-helium-cooled holder built in the laboratory. The resolution is then about 3 nm.

The principles of design and practical use of a digital circuit for the correction of spherical aberration effects in selected-area electron-channelling experiments are described. The apparatus employs spiral scanning and the dynamic focusing of the final lens, with a range of mathematical functions for the correction signal. A method is described for the calibration of the microscope and an example of its use is presented.

A detailed calculation is presented of the resolution properties of a three-axis neutron spectrometer equipped with a neutron spin-echo device. The calculation shows that the Fourier transform of the spectral density of an elementary excitation may be measured with a resolution which is substantially better than that available with a conventional three-axis spectrometer. In contrast to conventional spectrometers the spin-echo technique allows focusing of both longitudinal and transverse phonon modes.

A method for an accurate determination of the efficiency curve of planar solid state detectors at very low photon energies is described. It is based on the measurement of the intensity ratio of doublets of characteristic X-ray lines emitted by suitable chemical compounds irradiated by ⁵⁵Fe and ²⁴¹Am radioactive sources.

A gamma ray Compton spectrometer, which uses a 1.85*10¹¹ Bq (5 Ci) ²⁴¹Am annular source and an intrinsic germanium detector, has been designed and built. Compared with previous instruments using weaker ²⁴¹Am sources an order of magnitude improvement in intensity and reproducibility has been realised. The potential of the technique, particularly for the study of electron momentum distributions in light materials is discussed. The effect of multiple Compton air scattering upon the symmetry of the final profile is stressed.