Table of contents

The principles of operation of atomic frequency standards are described. Details are given of their physical construction. Their characteristics as well as their availability, principal uses and impact are specified. The basic concepts involved in atomic frequency standards and the terminology commonly used in this field are summarized. The role of quartz crystal oscillators which are slaved to active and passive atomic frequency standards is emphasized. The traditional atomic frequency standards are reviewed, i.e. the hydrogen maser, the caesium beam tube and the passive rubidium cell frequency standards.

Electronic gating of the video signal, which synchronizes with the sinusoidal drive magnetic field, is used to obtain stroboscopic images. Preliminary observations of the dynamic response of the magnetic domain in Fe-Si samples are also described. This simple technique is easily applicable to the conventional scanning electron microscope without any reconstruction of the electron optical system.

Mica windows with a thickness down to about 0.3 mu m can be prepared by a novel method and sealed helium-tight to metal tubes. A window with a diameter of 6 mm and 0.6 mu m thick held atmospheric pressure. Such a window has useful optical transmission upwards from 160 nm (30% at 160 nm, 65% at 170 nm and about 80% at 200 nm).

An application of a commercially available ratiometric voltage to frequency convertor to the problem of light source fluctuations in optical absorption spectroscopy is described. The performance of the device is evaluated using a tungsten-halogen lamp and a xenon arc source.

A modification of the Oxford Instruments CF100 continuous flow cryostat is described which enables variable temperature high resolution electronic absorption spectra to be measured on milligram quantities of powdered samples.

An account is given of a resistance bridge network for four-electrode resistivity observations and its use with AC energization in the off-balance condition for continuous observations. The bridge is of particular use for geophysical measurements.

By adopting a metrological approach to avoid lost motion and conflicting constraint, the feasibility of displacing a large component to occupy either of two positions to an accuracy of better than one second of arc angle with a simple automatic controlling mechanism has been realised and is described.

A mechanical technique for producing radial tension in flat diaphragms to eliminate buckling and the need for differential temperature assembly is described. Results obtained from a manometer thus constructed are given.

Reports the successful operation of a simple low noise charge sensitive linear preamplifier at temperatures down to 4K. It is unaffected by magnetic fields up to 16 T at 4K.

A large copper sample was neutron irradiated at liquid helium temperature and transferred into a neutron spectrometer cryostat without warming above liquid helium temperature. This paper describes the procedure and includes a method of constructing a small aluminium Dewar required in the process.

An improved thermal ionization source for solid sample analysis by a mass spectrometer is described. By proper biasing of the side filaments with respect to the centre filament, the correct choice of their relative orientation and simultaneous evaporation of the sample from both the side filaments, an overall improvement in sensitivity of an order of magnitude is obtained over the conventional source.

A simple ion source producing H₃⁺ and different molecular rare gas ions is described. To stabilize the extracted ion current the discharge current is stabilized by regulating the gas flow. With hydrogen the extracted ion current consists of more than 95% of H₃⁺ ions. Thus a primary mass selection is not needed. In rare gases the content of molecular ions rises to about 1% to 4%.

Measurements of relative permittivity at high pressure require test leads of relatively high impedance. This can introduce appreciable errors at frequencies of 1 MHz and above even when transformer coupled ratio arm bridges are used. Experimental and theoretical analyses are presented which permit these errors to be corrected.

The controller was designed to obtain small-angle X-ray scattering curves by step scanning. Several angular ranges, having different step intervals, can be chosen. The controller enables the scattering curve to be scanned by a multiple cycle technique to eliminate the effects of long period drift of the incident beam. The technique is applied to obtain the corrected scattering curves of glassy carbon with an X-ray beam of varying intensity.

The technique of using a PIN diode to calibrate broadline NMR spectrometers is discussed. A circuit described permits one to reproduce consistently a calibration signal to within about 0.1%.

Measurements are presented of noise spectra and conversion gain of type 1N26 diodes at 24.0 GHz. Optimum operating conditions for these devices are determined, and the characteristics are compared to those of diodes designed for 8-12 GHz operation.

Experiments were performed to record Fourier transform holograms with carbon K_alpha X-rays at 4.48 nm. Attempts were made to reconstruct the object from the holograms with He-Ne laser light. The results are compared with those obtained from holograms that were recorded with a visible analogue source of similar coherence conditions.

An apparatus is described for making friction tracks on thin foils under the electron microscope beam. The process is displayed on the screen whilst the friction is measured. Sensitivity is about 0.1 mu N for the normal and tangential channels and the first results showed that, on annealed aluminium, the friction force is nearly zero until plastic deformation begins, i.e. dislocations appear on the screen.

Details of the construction of a high resolution UV photoelectron spectrometer are presented. As analyser an electrostatic spherical condenser with a mean radius of r₀=109.56 mm and a deflection angle of 161.7 degrees was chosen. The base pressure of the vacuum system is 10 nPa. Control of the analyser and data acquisition is performed by means of a computer based CAMAC system. A simple bakeable gas discharge source for rare gas resonance radiation is described. The spectrometer can be used for investigations of clean surfaces and of gases. Electron energies of less than 1 eV can be distinguished directly. During investigations of solids the photoelectrons are emitted into a field free region immediately after excitation. Thus the spectrometer is especially suitable for studies of single crystal faces and directional photoemission. The energy resolution obtained when measuring solids is demonstrated by photoemission spectra of an evaporated Au film and of an Ag single crystal. The width of the Fermi edge of Au is appreciably smaller than that previously obtained. Using Ne-resonance radiation (h(cross) omega =16.85 eV) the FWHM of the Ar⁺²P_3/2 line was found to be 13 meV.

For purposes concerning rheological research in general and drag reduction phenomena in particular, a cone-and-plate viscometer has been developed. In this viscometer the torque transmitting part rests completely on air bearings. The results of calibration with Newtonian liquids show this viscometer to be highly sensitive and accurate, especially in the range of low viscosities. In addition, some measurements on dilute aqueous polymer solutions are included.

An application of the molecular beam time-of-flight technique is presented providing a physically simpler method of overcoming the principal uncertainties encountered in measurement of the velocity dependence of total collision cross sections. Explicit expressions are given for the velocity resolution and transmission of the time-of-flight (TOF) system. The considerations and procedure for design of a molecular beam TOF apparatus are detailed. This method gives greater velocity range and freedom of control of velocity resolution and transmission than traditional methods. The apparatus is described with emphasis on the considerations for optimizing the signal to noise ratio. An example of the method of extraction of the total cross section from the data is given, together with the observed velocity dependence and resulting intermolecular potential information.

Gallium arsenide surface photovoltage variations with temperature are studied near the band gap, by wavelength modulation spectroscopy. It is thus possible to measure the band gap with the same accuracy as in electroreflectance but with a better signal-to-noise ratio and without making any contact with the surface. The results are compared with similar ones obtained by classical methods.

A multichannel system has been built to sample and store a single or repetitive waveform. In the repetitive mode the effects of noise on the signal can be reduced by averaging. The instrument dissects a 400 ns long signal of nominally 1 V peak amplitude through 21 sampling channels. In each channel the sample value is stored as an 8 bit digital word. The sampling can be repeated and added into store for signal averaging purposes up to 256 times. Readout is via either a CRO display or an X-Y recorder.

A versatile and flexible digital pulse programmer for two-pulse, three-pulse, saturation burst and Carr-Purcell sequences is described. Independently variable controls for pulse widths (0.2 mu s to 100 mu s), delay between pulses (0.2 mu s to 100 s) and for number of pulses (1 to 99) for the saturation burst and for the Carr-Purcell sequence, are brought to the front panel. The programmer can be used for one-shot experiments as well as for repetitive experiments.

An analogue technique is described which recovers a de-noise, amplitude-averaged waveform from a noisy, synchronous signal of short duration. The system is advantageous where the changing or non-stationary nature of the event prevents signal averaging by slow scanning and 'real-time' analysis equipment is not available. If part of the noise is outside the signal bandwidth, the signal to noise ratio may be immediately improved by filtering. The remaining noise within the signal bandwidth must be removed by averaging. This is accomplished by recording several repetitions of the noisy signal on a loop of magnetic tape and then playing it back many times into a boxcar integrator which averages out the noise. Data are presented to show the capability of the system in recovering a signal which is non-stationary in amplitude and buried in noise. The technique has been used with an optical homodyne spectrometer to de-noise the Rayleigh spectrum of laser light scattered by biological samples which permit only brief experiments.

A stopped-flow spectrograph is described which enables the UV-visible spectra (220-650 nm) of transient intermediates to be recorded in solution on the millisecond timescale. The method is illustrated by results obtained for the formation and decay of the unstable peroxynitrous acid in acid solution, and a new, more intense absorption band is recorded for this species at ca 233 nm.

The recent paper by Bragg and Seehra (1976), although welcome for its rigorous approach to the problem of the theoretical EMF obtainable from pickup coils used in vibrating sample magnetometers, is incomplete since it does not pin-point the crucial factors governing coil pickup efficiency. In addition these authors make some statements which would be misleading to the uninitiated and thus require further comment. An attempt is made to clear up these shortcomings and demonstrate an extremely simple approach to coil design which follows directly from a little-quoted paper by Mallinson (1966).