Table of contents

Speckle is the peculiar appearance acquired by a diffusion object when illuminated by a laser beam. A laser-illuminated surface gives rise to a randomly distributed amplitude in any plane parallel to its plane. The real and imaginary parts of the amplitude at any point of the plane under consideration have zero mean value and the contrast of the speckle pattern is maximum. The autocorrelation function of the intensity distribution is proportional to the Fourier transform of the aperture limiting the surface, and the average size of a speckle in the considered plane is given by the radius of its Fraunhofer diffraction pattern. The results are applied to the optical processing of information, to speckle photography and to speckle interferometry.

A digital-to-analogue converter linear to about 1 ppm and requiring little adjustment is described. Its accuracy is obtained by a simple, symmetrical application of the mark:space ratio principle.

A circuit is described which will automatically cut off the EHT supply to a photomultiplier tube if accidental overload of the tube should occur. The EHT remains cut off, and resetting must be done manually.

An all-metal flow cryostat is described which will maintain temperatures between 3.5 and 300K. The experimental chamber has an inner diameter of 1.4 cm and the overall outer diameter of the cylindrical cryostat is 2.4 cm allowing its use in a standard 1 in magnet pole gap. The cryostat features simple top loading at low temperatures and a particularly low refrigerant consumption rate.

The problem of detecting extremely low count rates of VUV photons or metastable atoms, at the maximum possible target gas density and opening angle, can conveniently be solved by using a double channel plate detector. In an electron-impact experiment, the first of two channel plates has been operated as a separative wall between the target gas region (0.1 Pa) and a high vacuum in order to achieve an opening angle of 0.8 sr.

A description is given of a device which allows the precise orientation of a single crystal in any given direction with respect to the static magnetic field. It particularly facilitates the ESR analysis as it gives the angular dependence of the ESR signals in the principal planes.

Studies on a counter system comprising two groups of photomultipliers have shown that the effects of photomultiplier afterpulses are not eliminated when the two groups are run in coincidence.

A metallic bellows dilatometer has been carefully calibrated and the effects of hysteresis, nonlinearity, temperature changes and pressure changes have been analysed experimentally.

Details are given of the performance of an array of flashlamps powered from a homopolar generator via a transfer indicator. It is found that if the lamps are suitably derated reliable performance is assured.

A simple method is described for stabilising the optical thickness of the air gap in a piezoelectric Fabry-Perot etalon. The spacing between the plates is controlled by an error signal proportional to the difference in intensity of two orthogonally polarised modes of a frequency stabilised He-Ne laser.

A simple design is described for a constant-current generator that covers seven orders of magnitude of current (5*10^-10-5*10^-3 A) and may be operated with high insulation from earth.

A temperature-controlled oven is described which is inexpensive and easy to construct. The temperature-dependent resistance of the filament is used as the temperature sensor. The circuit is based on a low-drift operational amplifier. Long-term stability (more than 20 h) of +or-0.6 mK, a regulation factor of 3.5*10^-5 and a reproducibility of Delta T/T=1*10^-3 are reported. The temperature range is essentially limited by the oven material.

A time-of-flight (TOF) atom-probe field-ion microscope (FIM) specifically designed for the study of defects in metals is described. This atom probe features (i) a variable-magnification internal image-intensification system based on a channel electron multiplier array for viewing the FIM image; (ii) a liquid-helium-cooled goniometer stage which allows the specimen to be maintained at a temperature anywhere in the range 13-450K; (iii) a low-energy (<or=3 keV) gas ion gun for in situ irradiations; (iv) an ultra-high-vacuum (about 4*10^-8 Pa) chamber to minimize specimen contamination; (v) a high-vacuum (about 1.33*10^-4 Pa) specimen-exchange device; (vi) a Chevron ion detector; and (vii) an eight-channel digital timer with +or-10 ns resolution for measuring the TOFS of the pulsed-field-evaporated ions. The entire process of applying the evaporation pulse to the specimen, measuring the DC and pulse voltages and analysing the TOF data is controlled by a Nova 1220 computer.

Thirty of the more intense Cu II vacuum ultraviolet lines from a liquid-nitrogen-cooled hollow-cathode lamp have been studied to determine which are sufficiently narrow and symmetrical for use as wavelength standards. The wavelength shift and width variations caused by changes in the helium gas pressure and the discharge current have been measured for four of the lines. It is concluded that many of the lines could be used as wavelength standards with an accuracy of +or-0.01 pm (0.1 mAA) if the current is kept between 200 and 300 mA and the helium pressure between 400 and 800 Pa (3-6 Torr).

The flow near the tip of a choked-nozzle aspirating probe for gas density measurement is examined. Such probes are currently being used for studying how two dissimilar gases mix in unsteady, free-shear layers. The response of the probe to a step change in gas species is computed using asymptotic limit analysis. Such a change occurs when an interfacing vortex sheet separating the gas species passes over the probe. The limits used in the analysis correspond to small suction and large suction.

A description is given of a fully electronic precision temperature controller. Its construction, properties and operation are described. The temperature setting of the controller is directly calibrated in the temperature range -200 to +500 degrees C using a platinum resistance thermometer. The temperature resolution of the controller amplifier is better than 2*10^-5 degrees C and temperature stability of the controller components is better than 10^-4 degrees C per 1 degrees C change in external temperature. The typical precision realized with the controller is about 10^-4 degrees C. The design of the controller resistance bridge makes the controller a versatile device. It can be used as either a constant-temperature controller, a differential-temperature controller, a temperature programmer with a linearized sensitivity of 1 Omega degrees C^-1 over -200 to +500 degrees C or a simplified transfer function analyser of thermal systems.

The construction of a highly adiabatic reaction calorimeter for the determination of the kinetic constants of liquid reactions at high concentrations is described. The performance of the calorimeter is demonstrated with the acid-catalysed hydrolysis of 1,2-epoxy-butane.

The minimum measurable voltage in the SEM is estimated for a voltage contrast linearization scheme with restricted- and unrestricted-aperture analysers, using approximations for the shapes of the energy distribution curves and assuming that the only significant source of noise is on the collected electrons. A similar estimate is also made for Auger electron voltage measurement schemes. A numerical example shows that the hemispherical retarding-potential analyser system (unrestricted aperture) provides the lowest measurable voltage.

A compact electrostatic energy analyser is presented. Its main characteristics are an electrically adjustable resolving power with Delta E/E ranging from 1.5 to 0.1%, a high sensitivity provided by an incident solid angle of 12.5% of 2 pi , a large source area (up to 4 mm in diameter) and a large working distance (25 mm). The high sensitivity allows measurements to be made at very low primary beam intensities (down to 1 pA) and makes this analyser very suitable for scanning Auger microscopy. The defocusing of the low-intensity primary beam drastically reduces the electron bombardment of the sample and permits the study of electron-induced phenomena on the surface. The good resolving power makes this analyser suitable for X-ray photoelectron spectroscopy.

The stepwise heating method for measuring thermal diffusivity of molten salts is based on the electrical heating of a thin metal plate as a plane heat source in the molten salt. In this method, it is important to estimate the errors due to the following: (i) the finite plate thickness and (ii) the determination of the effective distance between the plate and the point at which the temperature is being measured. Both have great influence on the accuracy of the measurement. An improved apparatus is described and adequate measuring conditions are suggested on the basis of error estimations. The measurements for distilled water and glycerine were made first and the results agreed well with standard values. The thermal diffusivities of molten NaNO₃ at 320-80 degrees C and of molten Li₂BeF₄ at 470-700 degrees C were measured.

The equilibrium and dynamic aspects of the gelatinisation of starch granules in a variety of aqueous dispersive media have been investigated by recording the concomitant loss of birefringence of the starch using a small-angle light-scattering photometer. An inexpensive photometer was developed which employed a solid state photodetector and a He-Ne gas laser light source. A directed airflow temperature-control system was also developed to enable both the equilibrium and dynamic aspects of the gelatinisation process to be studied. Equilibrium measurements require long-term temperature stability, whereas dynamic measurements require a rapid increase in the sample temperature without any overshoot. The temperature and time dependences of the gelatinisation process are presented for typical starch dispersions investigated using the integrated, anisotropic light-scattering system.

A simple temperature regulator for the 1-20K range that can be used with either a resistive or capacitive sensor is described. The former is useful for applications involving no applied magnetic fields, whereas the latter is essential when a strong field is present. When used with a resistive sensor, short-term and long-term stabilities of 0.1 and 1 mK, respectively, are easily achieved. When used with a capacitive sensor, the stability is determined by the sensor drift and is limited to 1 mK short-term and 3-5 mK long-term.

A spectrometer for the study of surface barrier detectors using either a collimated alpha particle source or a particle beam from an accelerator is described. A Peltier cooling module and associated temperature control system maintain the temperature of the detector constant to within +or-0.2 degrees C over the range of room temperature down to -40 degrees C. A technique for measuring the window thickness is described and sample results are presented.

An energy analyser for neutral hydrogen and noble gas atoms is described. The neutral particles are ionized by electron stripping in a gas cell and the resulting ions are energy-analysed in an electrostatic spectrometer. The instrument is absolutely calibrated with nitrogen in the gas cell. The lowest energies which can be detected are 200 eV for hydrogen, 800 eV for helium and 1 keV for neon atoms.