Table of contents

The authors describe an inexpensive intelligent controller for laboratory torque motors, or any other analogue positioning device. The instrument is based on a single-board microcomputer, and is programmed in the high-level language FORTH. It may be interfaced to a general purpose computer via one of two serial ports, and may also function as a stand-alone controller. The compact hardware, and the flexible software, also allow the controller to function as a general purpose remote data logger. Operation of the system to produce perturbing torques in studies of human finger movement is described.

The response of AT-cut quartz crystal microbalances is analysed theoretically to show how mass sensitivity varies over the electroded and bare quartz areas. Analytical solutions are presented for the radial and polar dependence of mass sensitivity as a function of the electrode areal density for both planar and plano-convex crystals. In many surface science applications it is desirable to minimise the off-electrode sensitivity of the QCM, so as to make the fractional on-electrode response as close to unity as possible. This fractional response is shown to increase with the electrode mass, but to be in the range 0.90 to 0.94 for the planar crystals in common use. On the other hand, for plano-convex crystals this response readily exceeds 0.99. To check the accuracy of the calculations a simple method has been devised to deposit small dots of well defined mass at points on the crystal surface. These measurements show that the derived expressions for mass sensitivity are correct to within the measurement accuracy. The dots are deposited using a permanent marker pen giving a mass per dot of 0.21 mu g. After measurement, the dots are readily removed by solvent washing leaving the crystal calibrated and ready for other studies.

An elastic cantilever is used for the measurement of the mass of a small quantity, by measuring the shifted resonant frequency when the unknown mass is attached to the free end of the cantilever. The measurements of resonant frequencies of the cantilever with and without an attached mass should allow the calculation of mass. An optical glass fibre is used as the cantilever and is excited by an acoustic wave. The resonant frequency of the optical glass-fibre cantilever is determined by measuring the maximum displacement by optical means while the exciting frequency is swept. Since the optical glass-fibre is chemically stable, this system can be used to measure a chemically active viscous fluid as well as small soft materials because it is easy to attach these materials to the end of the glass fibre without any glue joint. The experimental results obtained by this method for small masses (10-300 mu g) are in good agreement with those measured by the commercially available microbalance.

Mass flow measurement in pneumatic conveyors with very low solids loading is difficult, even when using capacitance techniques, due to the baseline drift and limiting sensitivity of the transducer. The authors describe a measurement system specifically designed for low solids loadings. The relationship between number of particles in the sensing volume of the capacitance electrode and the solids loading in the conveyor is derived and shown to be linear and independent of the solids velocity. When the number of particles in the sensing volume is low, a novel capacitance transducer measuring the average value of capacitance fluctuations can be used to determine solids loading. This measurement can be combined with a solids velocity measurement to give mass flow. Good agreement is obtained between calculated and measured results using 4 mm PVC cubes for solid concentrations from 0.05 to 0.30 V/V and velocities from 5.2 to 18.9 m s^-1.

The design and testing of a new type of silicon sensor to measure the magnitude and direction of a fluid flow is described. The measurement principle is based on the detection of flow-induced temperature differences on a hot sensing surface. By measuring temperature differences in two directions perpendicular to each other, a full 360 degrees sensitivity can be obtained. The sensor is made of a silicon IC, on which the flow-induced temperature differences are measured with integrated thermopiles. The use of an IC sensor allows the realisation of a compact multicomponent sensor, which can easily be combined with electronic circuitry. Calibration measurements are presented for air flow.

A method is described which allows one to measure the detection efficiency of a MCP simply by tracing the output current pulse in the saturated mode. The dependence of the detection efficiency on the gain of the input stages of a detector is calculated and the result is applied to a MCP. An approach is presented to enhance the detection efficiency of a MCP for 100 keV electrons up to more than 90% by means of the field-enhanced secondary electron emission.

The authors present developments in the design of a new type of voltage contrast detector. Voltage contrast measurements are made by recording transit times of secondary electrons as they are emitted from a specimen and collected above the final lens of an SEM. The study shows that such a voltage contrast detector can be designed to combine a high spatial resolution with a large field of view: voltage measurements using sub- mu m probe sizes can be maintained over a 6 mm by 6 mm square region. The detector is also predicted to have data-acquisition times that are at least a factor of 30 times shorter than those of conventional retarding field-type detectors. The detector is expected to sample waveforms in its normal multi-stroboscopic mode of operation at frequencies up to 20 MHz.

A microwave heterodyne spectrometer consists of a radiometer front-end and a real-time spectrum analyser back-end. Common spectrometer types are filterbanks, autocorrelators and acousto-optic spectrometers (AOS). A high-resolution chirp transform spectrometer using reflective array compressor (RAC) filters with a time bandwidth product of 6400 is presented. The spectrometer has 1600 channels within a 40 MHz input bandwidth and a linear dynamic range of 63 dB for a CW input. It seems to be very promising for future space applications.

Electron impact dissociative excitation of molecules is used to measure the lifetime of metastable species which cannot easily be produced in the laboratory by direct excitation. Two illustrative examples are given. The lifetime of the N₂(a¹ Pi _g) state is measured to be 120+or-2 mu s and the lifetime of the O(⁵S) state to be 185+or-30 mu s, both in good agreement with earlier measurements. The method described may be extended to many other metastable species of current interest.

A description of a nitrogen laser, transversely excited by a double Blumlein line with automatic corona preionisation is given. The preionisation capacitor is formed by the dielectric material of the laser chamber. The discharge volume is 77.4 cm³. The achieved energy per pulse and average power were E_p=10 mJ and P_m=600 mW when preionisation was used. without using preionisation these values were E_p=7 mJ and P_m=270 mW respectively. The effect of corona preionisation upon the energy and frequency characteristics of the laser was studied.

A long-pulse ( approximately 30 mu s) xenon-ion laser with a maximum output energy of 20 mJ has been developed by using a double-discharge technique. Design details of the laser tube and the discharge circuits are described. Laser performance for different discharge circuits and working parameters (gas pressure and excitation voltage) is also presented.

A combination of a Fabry-Perot interferometer, a grating monochromator and an optical multichannel analyser is used to measure argon ion line profiles with a spectral resolution lambda / Delta lambda of 2*10⁶ and a temporal resolution of 500 ns. A deconvolution technique using Voigt profiles allows one to determine the argon ion temperature in pseudospark discharges. The main advantage over conventional methods is the registration of the whole line profile in a single exposure, which eliminates line profile distortions caused by intensity fluctuations which occur during a scanning process. Using the monochromator as a filtering element allows attainment of high resolution even in wavelength regions of high spectral line density, whereas the optical multichannel analyser provides single shot capability.

A fully automated technique is described which uses a BBC microcomputer to obtain the I-V characteristics of a thin-film sandwich device. The potential difference across the sample can be accurately controlled in a series of 256 linear steps up to a maximum value of 25 V and the circulating current recorded and displayed.

Resistivity striations in a strongly inverted FZ grown n-type silicon wafer are non-destructively detected by photovoltage imaging, in which a 2 kHz AC surface photovoltage, induced by the irradiation of a chopped blue (peaking at 448 nm) photon beam with 30 mu W, is mapped as a 512*480 pixel black and white image. The image contrast is due to the interface trap distribution associated with inhomogeneous dopant concentration.

A method of imaging short T₂ materials is demonstrated that makes use of a large sinusoidally time-varying magnetic field gradient. The technique is experimentally verified by imaging both rubber and PTFE samples. A multiple pulse scheme is also described that is shown to provide a signal-to-noise improvement with no time penalty.

The authors describe how a low-cost teaching interferometer has been modified for use as a time-resolved instrument to study the infrared vibrational chemiluminescence of the products from atom-radical reactions and molecular photodissociations. This technique allows the entire temporal evolution of the emission spectrum to be obtained from a single interferometric scan, with spectral and temporal resolutions of 2 cm^-1 and 3 mu s respectively. Illustrative results are given for discharge-flow studies of laser initiated atom-radical reactions, where product emission from highly vibrationally excited CO₂ and HF is observed, and for emission from vibrationally excited products of the infrared multiple-photon dissociation of halocarbon molecules.

A single-shot autocorrelator for UV pulses is presented. It uses a tilted pulse-front geometry and three-photon fluorescence of XeF as a non-linear detector. The readout system consists of a CCD camera and an oscilloscope. The resolution of the device is a few femtoseconds.

A spectroscopic system used for the calibration of pressure using the ruby R-line method has been designed and interfaced to a BBC model B microcomputer. The methodology of the real-time software design is detailed including flow diagrams for the three main programs employed in the system. The software includes both ambient and high-pressure data acquisition and analysis programs. Suggestions are made for the application of the software system to other experimental tasks and also to other compatible microcomputers.

A data acquisition card is designed and constructed for the Amstrad PC 1512, a low-cost IBM compatible microcomputer. This card is installed in the expansion slot of the microcomputer and the software necessary for nuclear magnetic resonance data acquisition and analysis is developed. This system generates the pulse sequences necessary for a nuclear magnetic resonance pulse spectrometer and acquires data from the free-induction-decay signal for further analysis. The generation of the timing pulses and the capturing of the free-induction-decay signal are done with the help of an assembly language source program.

The improvement of the dynamics of electrostatic deflection systems is an important problem to consider. There are several circuits which determine the speed of the system. The author is concerned with optimisation of 8-pole deflectors and their electronic power-supply circuits, which can be applied to electron-beam lithography systems.

The technical set-up of a multislit bender (bent neutron guide) for the beam separation of a white cold neutron beam is presented. The transmission characteristics are discussed and it is shown that the transmission behaviour is dominated by reflectivity losses.

A simple vibrating chopper with variable frequency was constructed for modulated reactive molecular beam scattering in UHV glass apparatus. The chopper was tested using a laser beam and the obtained signal was found to be almost rectangular in the frequency range 0-100 Hz.

A simple demountable home-made spark gap switch is described. The device can be used in switching up to 50 kV and provides reliable operation.

A simplified vibration technique for the determination of Young's modulus for a wide variety of thin films and coatings is presented. Some representative data on chromium and copper films is reported.

The authors are preparing a very low-temperature electron beam in order to investigate the ultimate possibilities of the electron cooling method. They present the initial results relative to an electron source, utilising photoemission from negative electron affinity GaAs. Electron currents in the region of 1 mA were achieved with a CW 65 mW argon ion laser. A photocathode lifetime of 17 hours has been measured.