Table of contents

The ordinary expressions for the diffracted intensity and the centroid displacement resulting from transparency in counter diffractometry are applicable when either (i) the specimen is thin, or (ii) the specimen is highly absorbing. When the specimen is thick and not highly absorbing the intensity and the displacement become a function of the aperture of the receiving-slit system, or even of the dimensions of the specimen holder, as has been pointed out by Milberg and Vonk. In these circumstances the diffracted intensity may be very much less than would be expected from the usual formulae. In the present paper expressions are derived for the diffracted intensity, the centroid displacement and also the variance, applicable to cases in which the specimen is thick and weakly absorbing, or of a thickness comparable with the reciprocal of the absorption coefficient.

An apparatus is described which is suitable for the measurement of the spin-lattice relaxation times of paramagnetic ions by a pulse method. A superheterodyne paramagnetic resonance spectrometer forms the basis of the apparatus. The X-band spectrometer uses only one klystron and operates at an intermediate frequency of 10 Mc/s. Relaxation times in the range 5 μsec to 10 msec have been measured. The design limit of the apparatus is 1 μsec.

A description is given of the single-crystal equipment which has been in continuous operation at the DIDO reactor for the past two years. Two diffractometers are used on the same reactor hole and each employs a copper monochromator reflecting neutrons of wavelength 1.04 Å through 90°. The advantages of this high reflecting angle are discussed.

An apparatus providing pH stat or pH titration facilities is described. Increase or decrease in pH value is sensed automatically. The apparatus is mains operated and stabilized to within ±0.005 pH unit for a ±10% change in mains input. Calibration accuracy is ±0.01 pH unit over the range 1-10 pH and the stability at constant sample temperature is within ±0.01 pH unit over a 24-hour period. Liquid feed is through motorized microsyringes or solenoid operated valves. A pulsed system automatically ranges the liquid feed rate by 100: 1 as the balance point is reached.

An account is given of the conditions necessary to produce stable, highly reflecting films of lead monoxide with low absorption. At a substrate temperature of 200° C and in an atmosphere of oxygen at a pressure of 0.35 mtorr, a film of refractive index n_d = 2.74 was obtained with extremely low absorption. The oxygen atmosphere was produced in situ by heating manganese dioxide with the evaporant. In combination with magnesium fluoride a 13 layer stack was prepared with a reflectivity of 99.7%, falling to 99.1% after exposure in the laboratory for 2 months.

A frequency modulation technique for the detection of the small amplitudes of vibration involved in measurements of internal friction is described which may be realized using standard electronic equipment and is thus particularly suitable for the laboratory when only limited facilities are available. An absolute optical method is used to calibrate the system and confirm is linearity.

The design of a constant-current 30 A 4 kw d.c. power supply is described. A bridge rectifier incorporating silicon controlled rectifiers is followed by a low-pass filter and a series control element consisting of several power transistors in parallel. The current is controlled, and stabilization is provided, by the series transistors. By altering the firing angle of the rectifiers the bridge output voltage is controlled in such a way as to maintain a low collector voltage and a low transistor dissipation. The output current can be maintained constant to better than 0.1% at any value between 5 and 30 A. At maximum output the efficiency is 83% and the input power factor is 0.85 lagging.

Low level beta counting equipment is described which has been designed for portable use in tropical conditions. The beta counter is shielded by a hemispherical guard counter and when the whole assembly is mounted into a standard square castle (4.2 cm lead) a background count of 1.35 counts per minute is obtained. Transistors are used throughout the circuit which is operated directly from low voltage supplies.

The flexure of an engineer's length bar standard supported horizontally at two positions with a flat wrung to one end is analysed. Formulae are given for support positions which do not introduce errors in parallelism of the ends. The effect of the flexure on the length of the bar is shown to be negligible.

A centrifugal classifier is described for fractionating particles at sizes from about 4 μm up to about 30 μm at a throughput of up to 2 lb h^-1.

A new instrument for the measurement of particle size in the range 0.1 to 5 μm is described. It consists of a shallow bowl centrifuge with sampling tubes built into the base through which samples are taken while the centrifuge is rotating. Tests have given consistent and reproducible results on a wide variety of materials.

The `drum-pump' sampler described collects a single sample on a filter paper over a period of up to eight hours, the rate of aspiration being about one-third of a litre per hour. A size selection device rejects particles larger than `respirable size'. The instrument was designed for use in the working places of coal mines and is self-contained and portable. The novel design of water-sealed pump employed may find other applications.

In nuclear magnetic resonance experiments magnets are needed with the highest possible homogeneity of magnetic field throughout the volume of the probe. The problem of how to reduce the effects of inhomogeneities by spinning the sample and by using current shims is discussed.

The magnetic field inhomogeneities are expanded in spherical harmonics, and the terms cancelled by sample spinning determined. It is shown that current shims in the form of pairs of straight wires placed symmetrically on both pole pieces are able to compensate inhomogeneities not averaged out by spinning the sample.

Proton free-precession has been used to measure weak magnetic gradients in the earth's magnetic field by a direct comparison of the precession frequencies from two detectors spaced several metres apart. The comparison is made by timing the first half-cycle of the beat frequency. In the first instrument described the time is measured electronically, and field differences of 4 × 10^-5 oersted have been measured to ±10% in this way. The second instrument is semi-quantitative and is essentially an amplifier feeding an earphone. Nevertheless field differences of 5 × 10^-5 oersted can be detected easily, and the instrument is adequate for the magnetic location of archaeological remains.

A simple method is described for testing large numbers of block or cylindrically-shaped permanent magnets with flat parallel ends. During the test the magnet, which may be of any size, is sandwiched between a pair of identical soft iron plates. It is shown that an estimate of the unit permeance value of the working point of the magnet can be made from the dimensions of the magnet and the plates. Only the size and shape of the plates need be adjusted to vary this value over a wide range.

A quartz fibre torsion balance was built in order to measure the remanent magnetization of individual magnetic particles after the application of various magnetic fields. The balance suspension consisted of a quartz fibre approximately ½ μm in diameter, and the specimens were individual magnetic particles having linear dimensions of the order of 1 μm.

The construction and use of barium titanate ceramic transducers for fine-movement control is described. The movement obtainable for unit applied voltage is approximately 0.25 μm kv^-1, a maximum working voltage per unit thickness of the ceramic of 3.5 kv mm^-1 being possible. The maximum movement obtainable for a given overall voltage can be increased by lamination of the ceramics. Parallel movement of a plate is provided by the use of three transducers as supports; the expansions of such supports are identical to within 5 nm for a movement of 2.5 μm.

The machine consists of an accurately ground plate on which a carriage bearing a specimen can be positioned using lead screws. The deviations of the surface of the specimen are measured, with respect to a fixed pneumatic gauge head, with an accuracy to ± 1.5 × 10^-6 in.

A new high tempeature furnace of the resistance type employing graphite resistors is described in the paper. This furnace can be used for the performance of tension, compression or torsional load tests on materials for test zone temperatures up to 2100° C.

A rotating lead disk with radial collimating slits is placed between the active specimen and the x-ray film. The factor which determines whether the radiograph will be fogged by gamma rays then becomes the activity per unit length of the specimen rather than the total activity. The successful reduction of background fogging by the device is demonstrated. The extension of radiography to the study of very highly active specimens is indicated.