Table of contents

Reviews the experiences and the progress in understanding made during the last half-decade in the domain of both the discontinuous and the continuous types of electron multiplier. The introduction compiles the essential notions and recalls some theoretical statements which are necessary for a pertinent judgment of the detection limits. The discontinuous type has been explored mainly with respect to the detection efficiency. Knowledge of the operational behaviour is relatively well established and is not discussed in detail. The situation is different for the continuous type; and their operation, performance stability and detection efficiency are discussed.

A film holder/exit slit movement for a fixed-grating, vacuum ultraviolet spectrograph is described. Film clamped on a holder lying along the Rowland circle is used conventionally in the spectrograph mode. For use as a monochromator, the exit slit moves along the film holder and a phosphor/flexible lightguide combination couples the output radiation to a photomultiplier outside the vacuum system. The spectrum is covered from 25 nm to 200 nm using two gratings. The combination instrument is suitable for time resolved spectroscopy of a transient plasma with a complicated spectrum when reliable identification of spectral lines and accurate wavelength settings are necessary.

The use of a modified paramagnetic oxygen analyser as a detector for permanent gases in gas chromatography is described. The detector functions by using oxygen as the carrier gas and measures the deficiency of oxygen caused by the separate elutriation of gases from an injected sample. The sensitivity of the apparatus has been shown to be the same for hydrogen, methane and nitrogen and the response linear with sample size. The sensitivity to carbon monoxide was 30% lower than to the other gases mentioned above and the reason for this was not discovered. Samples of 0.3 ml for each gas gave peak areas amenable to measurement.

A technique for rapid calculation Mossbauer spectra in the presence of electronic relaxation is described. It involves approximate diagonalization of the supermatrix that represents the Fourier transform of the correlation function of the resonant system.

A circuit is described which gives an output voltage which sweeps linearly or exponentially up or down with time, or whose reciprocal varies linearly with time. Sweep rates or modes can be changed during a sweep without causing step changes in the output. It has found particular application in magnetic cooling and magneto-resistive oscillation experiments.

The authors describe an experimental device which delivers two powerful (> MW) and synchronous ruby laser light pulses (jitter <or approximately=10 ns); the frequency difference can be smoothly and accurately tuned from 0 to 30 cm^-1. By mixing these two laser beams in a LiNbO₃ crystal tunable coherent infrared radiation is generated and detected.

Rotation between the normal image seen in the scanning electron microscope (SEM) and electron channelling patterns (ECP) has previously been found for the deflection focus system for generating selected area ECP (SAECP). The rotation is examined for the after-lens deflection system also used for generating SAECP and difference between various modes of SEM use are characterised.

A simple and versatile automatic frequency control loop suitable for low power klystrons is described. The use of such a loop has proved valuable in the accurate measurements of the magnetic tensor susceptibility of ferrites.

To correct the variation of the mean video level due to the sample tilt, an important effect for electron channelling patterns, the black level is controlled by a linear signal derived from the frame scan sawtooth.

The response of a hot wire anemometer in bubbly two-phase air-water flow was studied. For the conventional hot wire design and operation at low overheat, it was found that the sensor does not enter the bubble completely, thus making measurements inside the bubble virtually impossible. This is due to a liquid film which attaches to the wire. For small bubbles it results in splitting of the bubble into two. Three types of interactions are identified-direct, glancing and partial hits. Direct and glancing hits result in identical signal level changes and hence discrimination between bubble and no-bubble signals, whereas partial hits give considerably reduced signal level changes. Application of such measurements to determination of local void fraction by use of signal probability density plots is also considered.

Describes a practical method for analysing the calibration of constant temperature hot wire anemometers whose output voltage is not linearized. This applies to a two-dimensional incompressible flow whose velocity ranges between 2 and 16 m s^-1. For a single wire, normal to the flow direction, King's law written in the form U²= alpha + beta Vⁿ is verified; alpha , beta and n are constants and are obtained by computation. Corrections for the ambient temperature drift effect are presented. When the flow direction is at an angle phi with the normal to the sensor (0<or= phi <75 degrees ) the preceding law is generalized by taking beta as a function of phi . Furthermore alpha cannot be taken as a constant for phi greater than 55 degrees . A new law: U²= alpha ( phi )+ beta ( phi )Vⁿ is proposed and is applied to the case of cross-wire probes.

A general hot wire apparatus, earlier employed for a variety of heat transfer investigations such as thermal conduction through gases, thermal accommodation at the metal wire-gas interface, radiation from heated metal wires and resistivity of metal wires is described in its developed form. New results are given on the radiative heat transfer from platinum in the temperature range 400-1500K. These data are successfully analysed to determine the hemispherical total emittance of platinum as a function of temperature. The use of such an instrument to determine the electrical resistivity of metal wires at high temperatures is discussed with results on platinum.

An empirical method has been developed for determining the tip shapes from field ion micrographs utilizing digitization and computer graphics techniques. Application of this method to tungsten and iridium specimens indicates that the tip profiles deviate significantly from a hemisphere, and on a fine scale the shape varies from region to region over the specimen surface. However, for practical purposes the entire end form can be represented by a polynomial of degree 4 or 5.

A microwave T-jump apparatus was built for the time range extending from a few microseconds to about 20-50 ms. This mode of heating has the major advantage of enabling one to work with non-conducting solutions, the requirement being just that the solvent be polar. The high repetition rate of the microwave pulses made it possible to electronically average over a large number of experiments with corresponding gain in signal to noise ratio.

The design, construction and performance of a precision strain gauge monitoring system are described. Low drift DC amplifiers are used, suitable for converting the typically small output of a strain gauge bridge into an easily monitored voltage. The system operates from a single battery supply, using an inverter to derive the various supply voltages required. Emphasis is placed on careful construction and component selection, and cost. Some applications of the system in practice are described.

The planar motion of a charged particle in a logarithmic potential is studied in the context of an electrostatic cylindrical spectrometer. The trajectory is found to be bounded and to have a period, 2P, lying in the interval pi <2P<or=2^1/2 pi . General spectrometer design considerations are discussed, and a method for calculating resolution is described. A specific example of spectrometer design is included.

It is well known that a metal tip blunts if it is heated in vacuum. Sharpening may occur in the case of an evaporation of the tip material. To test this, molybdenum tips were heated in vacuum. A typical result: at 2400K, the tip radius decreases from 8 mu m to 0.3 mu m thus confirming the hypothesis. Measured and calculated final radii agree roughly. The pure material evaporation can be replaced by a surface reaction followed by the evaporation of the reaction products. This is shown on tungsten tips heated (1750K) in the presence of oxygen (0.5 mTorr). The evaporation of tungsten oxides results in a sharpening to a radius to 0.05 mu m. Under special conditions, in connection with the formation of solid drops, radii down to 0.01 mu m are obtained.

A device is described which enables the transfer in vacuo of electron microscope specimens from an ultrahigh vacuum (UHV) environment to the JEOL 100B electron microscope. The method allows not only the full tilt of the goniometer cartridge (+or-30 degrees ) but also a resolution of 0.34 nm to be realized.

A method is described for determination of the electric permittivity of fairly conducting solutions (specific conductivity of the order of magnitude of 1 Omega ^-1 m^-1) in the megahertz region. Use has been made of a commercial measuring bridge (the RX meter type 250-A of Boonton Radio Company) in conjunction with a cell constructed in the laboratory. The balance equations of the bridge have however been revised for this purpose. It is shown empirically that for proper evaluation of the unknown capacitance a correction term has to be introduced which is dependent on the measured conductance and is justified by a theoretical evaluation assuming a non-ideal representation of the bridge's circuit. The coaxial construction of the variable cell permits a description in terms of transmission line theory in so far as the part is considered which contains solution only. The measuring method is applicable at frequencies between 1 MHz and 100 MHz for solutions with specific conductivities smaller than 0.1 Omega ^-1 m^-1 and is limited to frequencies between 5 MHz and 20 MHz for solutions with specific conductivities of approximately 1 Omega ^-1 m^-1.

A description is given of a new high pressure viscometer which is capable of measuring viscosities up to 10⁶ P s (10⁷ Pa). The instrument consists essentially of a high pressure linear variable differential transformer (LVDT) which is used to detect the free fall of an unguided cylinder with a coaxial hole. The motion of the cylinder, the dimensions of which are optimized for maximum fall velocity, is recorded automatically by triggering the output of the LVDT, which is sensitive to changes in distance of 2 mu m. The sinker is returned to its starting position by inversion of the pressure vessel.

Describes the design and performance of a sealed-off, single-frequency, single-mode CO₂ laser. Similar lasers were used in the latest measurement of the speed of light, by the c=f lambda method, at the NPL and now provide an output power of 8-12 W over many of the lasing transitions. Using a stabilization technique, employing carbon dioxide as a saturable absorber, a frequency stability of better than +or-1 part in 10¹⁰ over a 1 s measurement time and frequency reproducibility of better than +or-2 parts in 10¹⁰ have been achieved.

A new target holder is described for use under high intensity ion bombardment. The target temperature may be varied from 77K to 1500K by applying liquid nitrogen cooling and/or electron gun heating, respectively, directly to the target. The characteristics of the assembly such as time constants and stability of the temperature control are reported.

A low light level camera tube has been fitted on a Cameca ion analyser IMS 300. Set at the position of the optical microscope, it aims at the fluorescent screen of the apparatus. This device improves the sensitivity as well as the ease of visualization of the image. Moreover, a quantification of the image is made possible thanks to the fact that addresses and levels of brightness are stored.

A new LHe cryostat allowing measurements of IR matrix spectra with high M/A ratios by means of an ATR technique is described. By use of a multiple-reflection double-pass ATR element, matrix spectra showing approximately the theoretically expected relation to transmission spectra are obtained. However, considerably thinner matrices and shorter deposition times are required for spectra of comparable intensity. Special attention has been given to unwanted deposition of water layers.

In May 1975, Hellier et al. (see ibid., vol.8, p.352) reported on their integrated circuit pulse echo overlap (PEO) facility for measurement of sound velocity. They proposed their integrated circuit as part of a do-it-yourself alternative to purchasing a commercial PEO instrument, mentioning, as a footnote, only one of the manufacturers from whom such PEO equipment is available. To maintain correct historical perspective, it is pointed out that PEO equipment employing similar integrated circuitry, has been available from another manufacturer in standard commercial PEO instruments since 1972.

An assembly of instruments was realized for the following purposes: (i) particle-gamma correlation measurements, (ii) to minimize contamination of carbon deposit on the target, (iii) easy vertical and horizontal adaptability to the accelerators of a nuclear research centre, (iv) facilities of fixing Ge(Li) detectors of different geometries and (v) rapidity of beam alignment.