Table of contents

In the first part of this review the theory and operation of a conventional dilution refrigerator are explained in simple terms. The discussion of heat exchangers is, however, somewhat more detailed. In the second part several of the latest machines are discussed, including a refrigerator which reaches 2.0 mK and another which operates by an unconventional gas circulation cycle.

A new approach is presented for designing a thin film thickness monitor using the frequency demodulation technique. The monitor permits film thicknesses to be measured to an accuracy of +or-1%. Provision is made for automatic termination of deposition and for measurement of rate control.

Constructional details are given of an inexpensive and easily made electromechanical shutter. The device may be placed at any convenient point in an optical system , without modification to that system, and causes no scanning of the final image.

A double-action pump using the liquid piston principle has been constructed to ensure efficient circulation of gas at moderate and low pressures. The oscillation of mercury is caused by a reciprocating piston, actuated by means of an external solenoid. Quantitative gas transfer of the order of 20 cm³ s^-1 can be easily obtained.

A muzzle flange configuration and flange alignment device are described for obtaining precision impact planarity in gas gun experiments. The alignment procedure provides a simple and convenient means for adjusting the target flange for minimum impact tilt.

A versatile digital mass read-out system for magnetic sector mass spectrometers is described. The system operates when the mass spectrometer is in the positive- or negative-ion mode and facilitates display of mass spectra on linear mass scales. It is suitable for use with instruments having large or small pole gaps and can be set up and calibrated quickly. With the mass spectrometer for which the system was initially designed reliable mass read-outs were obtained having a maximum error of +or-0.2 u up to a mass of 550 u.

A simple and inexpensive electronic circuit is described to modify the width and amplitude of signals derived from p-i-n photodiodes exposed to mode-locked laser pulses. The device provides output suitable for triggering many kinds of detection electronics and the authors describe its application in a laser-excited time-resolved fluorescence spectrometer.

With the advent of methods for the pulsed annealing of materials using laser radiation, it has become increasingly important to achieve good beam uniformity. The authors describe a device based upon a diffusing screen coupled to a light guide which accepts a multimode beam directly from a laser and, irrespective of input beam inhomogeneities, yields an output beam with an exceptionally flat and uniform intensity distribution. The optical efficiency of the device is high and the output radiation power density can be controlled by adjusting diffuser design parameters. Deleterious speckle patterns in the output beam can be virtually eliminated.

A high-speed laser diffractometer is described which employs a novel scanned array or optical light guides to record the angular intensity variation of light scattered radially by a particle throughout an arc of 180 degrees C in approximately 2.8 ms. The instruments has proved successful in the analysis of micrometre-sized fibres and individual microspheres in liquid flow. Furthermore, potential exists in the analysis of precipitation reaction kinetics and aerosol dynamics involving rapid particle growth and coagulation.

The development is described of a fast-response hydrocarbon gas detector, suitable for monitoring fluctuating gas concentrations in dispersing vapour plumes produced by spillages of liquefied flammable gases. The improved response time has been achieved by extensively modifying an existing hydrocarbon gas detector and adding an electronic signal-conditioning circuit. The new instrument measures gas concentrations over the range 1-100% gas with a response time of less than 0.5 s.

A new sensitive heat flow calorimeter combined with a hydrogen reactor was developed. Metal-hydrogen reactions can be investigated in the temperature and pressure ranges 243<or=T<or=353K and 1<or=p_H2<or=20 MPa, respectively. The calorimeter permits detection of heats of 10 mJ and is suitable for precise measurements of 50 mJ and more in periods of a few seconds up to some 10³ s. In its first application the apparatus has been used to determine differential heats of solution of hydrogen in FeTi at 298K.

Presents a detailed comparison of different existing cryostats for a range going from liquid nitrogen to room temperature. None of these was really adapted for continuous operation between these two temperatures in a large volume with a good stability. The new apparatus is a double cryostat adapted for this range. The output cryostat is cooled by the evaporation of a servo-controlled flow of liquid nitrogen and the inner cryostat is maintained by a slightly higher temperature by a servo-controlled electric heater. This device gives a stability of 0.001K with a temperature difference of 0.01K in a volume of a few litres.

A direct heating pulse calorimeter was designed and constructed for measuring the heat capacity of semiconducting materials at high temperatures. Heat leak from the sample to the surroundings due to the increasing radiation at high temperatures was reduced by providing an adiabatic shield to enclose the sample. The error of the method has been analysed, and the condition for measurement of the heat capacity with a limited inaccuracy is discussed. It is shown that a sample with electrical conductivity in the range from 10^-1 to 10⁶ Omega ^-1 m^-1 can be measured with an imprecision of less than 0.5% at 1000K.

Describes the testing of an ultraviolet light source giving a line spectrum in the 20-100 nm range, emitted by discharge in a gas. Identification of the Ar, Kr and Ne lines was obtained by intensity variations according to voltage and pressure in the discharge tube. The optimum experimental values are given as well as the absolute light intensity available.

The design of a vacuum ultraviolet spectrofluorimeter for the measurement of luminescence from thin films is described. The instrument uses a pulsed continuum light source and a specially built detection system. The wavelength of excitation is selected by a double-grating monochromator of high spectral purity and large aperture. The excitation bandwidth employed may be adjusted without change of central wavelength or of sample illumination. Spectra are shown for thin films of anthracene and results are also cited for polyolefins and DNA films.

Describes a circuit which gives direct readings for the magnetic field measured by a nuclear magnetic resonance magnetometer. The display is expressed in 10^-4T units with automatic positioning of the decimal point. This circuit is easy to make and needs no special adjustment.

The mechanical construction, electronic logic control and data-logging systems are described of an automatic weather station capable of operating throughout the year in the severe icing and wind regime of the summit of Cairngorm in Scotland.

The improved psychrometer is constructed from high-thermal-conductivity materials to minimise temperature gradients within it. To minimise errors from condensation on the psychrometer, it is symmetrically constructed with respect to the porous windows where moisture exchange with the soil occurs. Zero offset of the new units is reduced by more than an order of magnitude compared with previous versions and measurement errors due to temperature gradients are less than one-third the value for the best designs previously available.

A description is presented of an improved design of Auger spectrometer of the cylindrical mirror analyser type, where the whole of the analyser geometry has been adjusted from the 'classical' type to allow both the input and output slits in the inner cylinder to remain free from field-smoothing grids, i.e. they are empty. The improved geometry enables a greater working distance to be used and in addition provides equivalent or improved performance especially at low energies. Typical spectra and mappings are given illustrating the instrumental performance.

A specimen stage for energy-dispersive analysis was built for the Stereoscan S-600 scanning electron microscope. It comprises 37 standards, embedded in a graphite disc, five positions for unknown samples, and 19 electron traps for absolute beam current measurement. The stage is described, as well as the principles of its use for quantitative analysis.

The probe was designed as a sensor of the measurement of static pressure acting on the surface of a building but the design also permits it to be used in free-stream flow. Details are given of the construction of the probe, the calibration procedure, the effects of Reynolds number and of the sensitivity of the probe to pitch.

Possible ways of using a transistor as an anemometer are examined to discover desirable properties, suitable for use in a battery-operated instrument. Two modes of operation are described: constant power and constant temperature rise above ambient. Bead transistors allow convenient operation at 30 mW with straightforward ambient temperature compensation. To verify what constitutes an adequate thermal model, a detailed analysis is given for a particular transistor and shown to be in agreement with experiment. The best of the arrangements tried gives an accuracy of the order of 5% of wind speed and a usable range of 0-5 m s^-1.

A method has been developed for determining the Reynolds stresses and the turbulence intensities from three measurements at the same position in space, by means of a single 45 degrees slant hot-wire probe, when the mean direction of flow is known, as in a duct. The Siddall and Davies (1971) hot-wire response equation was adopted in this study, since it has proved to give a better curve fit with the experimental data than the 'universal empirical law' derived by King (1914). Measurements of the turbulence characteristics quantities in a smooth pipe were shown to be in very good agreement with existing results.

Describes a frequency tracker for laser Doppler anemometry (LDA). The circuit makes use of the phase-locked loop principle. Special features include a data validation circuit and a twin counter for forming long-term averages of velocity data. A measured profile of average velocity in a turbulent open-flow channel is presented. By suitable programming, a commercial microprocessor is made to function as multichannel analyser for LDA velocity data. Accumulated data are processed for moments of the velocity distribution such as the mean, the mean square deviation and, using LDA optics at +or-45 degrees to the mean flow direction, the Reynolds shear stress. In a differencing mode the analyser gives the distribution function of acceleration which indicates the time scale of velocity fluctuations.

The shutter consists of cubic-phase lead lanthanum zirconate titanate (PLZT) ceramic operating in the quadratic mode between crossed polarisers and can be used from 0.4 to 8 mu due to the high optical transmission of the PLZT material. Typical minimum transmission pulse widths are 0.1 mu s in the visible and 1 mu s in the near infrared. They are not as fast as those of other liquid and crystalline electro-optic materials, but a definite advantage is the low driving voltage of some 100 V in spite of high pulse currents. The driving voltage increases with shorter switching times as well as with higher wavelengths.

Describes a Kerr cell which was built as a coaxial 50 Omega transmission line. A beam of light from a He-Ne laser operating in TEM₁₀ mode passes coaxially through the cell surrounding a central wire (0.1 mm in diameter). When an external voltage is applied to the cell a large radial inhomogeneous electric field forms through the cross-section of the laser beam. In comparison with common Kerr cells, the applied voltage required for on-off action is significantly reduced and shorter pulses may be transferred through it, allowing this cell to be used to transfer 1 ns single or low-repetition-rate pulses.