Table of contents

Several factors limit the precision and accuracy which can be achieved in the isotopic analysis of uranium hexafluoride by a mass spectrometer. These include inadequate resolution, memory errors, statistical fluctuations in ion currents, amplifier noise, current noise, inefficient collection of ions, voltage coefficients in resistors and chemical contamination of samples. A description is given of a 90° sector field mass spectrometer of 30 cm radius, which has been built and used to investigate the accuracy which can be achieved in practice. The precision (1 σ) of this instrument when measuring the fractional difference in isotope ratio between two samples is 5 × 10^-5 at the 50% level of enrichment and 1.4 × 10^-4 with natural uranium, for comparisons lasting 12 minutes. By suitable design of ion source and inlet system, the memory error in the measurement of the concentration difference between two samples has been reduced to ±0.3% of the difference.

A modified version of the Beevers-Macewan machine using dekatron counters is described. The machine calculates a Fourier synthesis on exactly the same principle as the Beevers and Lipson strips. A switch selects Cos or Sin; one set of push buttons selects the wave number (0-99) and a second set selects the amplitude (1-100). On pushing the + or - button the machine adds in the numbers on the corresponding strip to 16 counters simultaneously. When all the terms have been put in the result for 0-15/60 of the period, in steps of 1/60, is registered on the counters, negative numbers being given as complements.

The main difficulty in previous designs, which had not been surmounted, was to obtain reliable counters which would work backwards as well as forwards and could be quickly zeroed. This has now been accomplished by utilizing dekatron counters and running the Post Office selector switch at 20 steps per second, the maximum for reliable single stepping operation. A term of amplitude 100 thus requires 5 seconds to put in, but a term of amplitude 10 takes under 1 second because the function-generating uniselector finishes the rotation from position 11 onwards at its normal speed of 200 positions per second.

Condensation may occur in a vacuum gauge which is used for measuring the pressure in a vacuum system in which a liquid is being evaporated at elevated temperature and the pressure of the liberated vapour exceeds the saturation pressure of this vapour at the temperature of the vacuum gauge, which is usually at ambient temperature. In this paper, a pressure-measuring method is described which prevents condensation or liquefaction occurring in the manometer or in the line connecting it to a vacuum system. The essence of this method is the reduction of the vapour pressure in the vacuum gauge, or in the pipeline leading to it, by a small stream of air, to a value at which condensation or liquefaction cannot occur. A short note is also added on the use of mercury manometers on `wet' vacuum systems.

The effect of varying the diameter of a polystyrene disk on the value of its dielectric constant when measured in a resonator of fixed diameter is investigated. Agreement is obtained with results predicted by perturbation theory.

The paper describes a hydrogen discharge pulse generator, and gives an application of it to nuclear scintillation counter techniques.

A bakeable vacuum window seal suitable for use as an optical window or as a radio-frequency window for high power microwave applications is described. Fluid pressure, applied externally, compresses the window frame on to the edge of a dielectric disk. This pressure prevents the frame from expanding away during heating cycles irrespective of the differential thermal expansion between metal and dielectric. Silica and ceramic disks have been successfully sealed to stainless steel or copper frames and baked at 450° C.

Details are given of low inductance capacitors, and capacitor banks with energy storages in the range 1 to 10 kJ capable of giving maximum currents of 1.2 MA and rates of rise of 6 × 10¹² A/S. The working voltage in the work described is 10 kV, but the techniques have been used at higher voltages, up to 40 kV. An outline of the measurement techniques, used in the study of linear pinched discharges in low-pressure deuterium with these banks, is given.

The calorimeter described is suitable for the continuous study of the heat production of microbial systems. The instrument is self-balancing, heat production in a reaction vessel being simulated by electrical heating of an identical balance vessel. A continuous record of the electrical heat supplied to the system is presented on a strip chart. Provision is made for stirring, sampling and maintenance of a controlled atmosphere. The vessels may be sterilized and are inert towards the systems to be studied.

An instrument of the type comprising a float in a vertical tapered tube is often suitable for measuring low rates of fluid flow but difficulties arise in obtaining remote indication, recording or control owing to the small physical size of the float and the consequent failure of conventional methods of detecting its position. This paper describes a method in which a radioactivated float and low-voltage, high-current Geiger tubes have been used to overcome these difficulties.

Analogue methods of solving the Poisson equation in two dimensions are discussed briefly. The electrolytic tank equivalent must have a uniform current source density; an approximation to this is achieved by introducing equal currents through conductors spaced in a square lattice. As examples, the flow velocity distribution in a tube of rhombic cross section is shown, and the equivalent radii for rhombic tubes of various angles are found.

A liquefier cryostat is used to produce the liquid hydrogen and the crystal is cooled in situ to liquid-hydrogen temperature. This method introduces problems involving crystal cooling, crystal mounting, the rotation of the crystal and the provision of X-ray transparent windows in the cryostat walls. These are discussed, together with some preliminary results obtained with the apparatus.

A simple volumetric apparatus for the determination of the surface area of solid materials is described. Isotopically labelled krypton (⁸⁵Kr) is used as the adsorbate and the pressure measured using a combined counting cell and Geiger-Müller tube. The apparatus can be used to measure areas of 0.01 m² to 1 m² with a precision to 5% for 0.025 m², 1% for 0.1 m² and 1.5% for 1 m².

A simple and easily constructed plasma oscillator is described, giving power of the order of microwatts, and tunable over rather wide ranges, e.g. 40-55 cm, by varying the anode voltage.