Table of contents

An X-ray micro-beam technique is described which depends on the use of X-ray beams down to approximately 1 μ diameter, obtained by means of lead glass capillaries The micro-beam precision camera incorporates collimator holders for aligning the capillaries, and enables simultaneous transmission and back-reflection photographs to be taken The effect of the divergence of the X-ray beam on the photographs is considered, and a formula is derived for the mean value of the divergence at the specimen. The optimum collimating conditions are discussed in relation to maximum resolution and minimum exposure The use of totally reflecting glass collimators is considered. An example is given of the application of the micro-beam technique to the study of cold-worked metals. It is found that, using a high intensity generator, single reflections can be obtained from crystals of diameter of the order of 2 μ in about 10 hours.

A high intensity X-ray generator has been constructed which combines the use of small foci and a rotating anode. Improvements in target loading per unit area of about fifty times compared with ordinary stationary target tubes* have been obtained Any width of focus between about 1 mm and 50 μ can be produced by means of an adjustable, biased electron gun. A detailed experimental investigation of the focusing properties of the high voltage gun has been made, the conditions for obtaining small foci and maximum current are derived. The magnitude of the negative bias is found to be the most important factor controlling the focusing action of the electron gun The effect of bias on focal structure is discussed for several types of filament, and is explained on the basis of suppression of emission from different parts of the filaments.

The paper contains an account of an X-ray study of the phenomena observed when polycrystalline pure aluminium is subjected to heavy compressional stresses. The rate of recovery to the metastable state under various conditions has been examined in some detail.

During recovery recrystallization occurs; the crystals grow rapidly at first, but afterwards the rate of growth decreases until finally, after the metastable state has been reached, there are no further changes in crystal sizes The recovery phenomenon and recrystallization are two different processes but they are not independent of each other. Crystals grow when the value of the stress varies from point to point in the material; when the stresses become evenly distributed, as is found to be the case in the metastable state, the crystals stop growing

The change from the state immediately following plastic deformation to the equilibrium crystalline state takes place in two stages, one brought about by strain energy and the other by thermal energy. The results are considered in the light of the dislocation theory.

Oxide layers on the cathode produced marked irregularities in the Paschen curves for air and hydrogen, especially in the neighbourhood of the minimum V_m. V_m in hydrogen was measured at different stages during the removal of oxide layers from the cathode, and also during the deposition of one metal on another as base. Removal of the oxide layer produced smooth Paschen curves, while a thin film of one metal on a different metal as base gave values of V_m which were not characteristic of either metal in bulk. The deposition of thin metallic films had considerable effect on cathode emission owing to the alteration of the work function

The distribution of space charge and potential in thermionic tubes with two hot electrodes (`double diodes') is derived by the method of statistical mechanics for plane parallel electrodes, first for tubes in true thermodynamic equilibrium, and then, approximately, for tubes with applied external voltage or with electrodes at slightly different temperatures. The dependence of current on voltage in the first case (characteristic), and on temperature difference in the second case is calculated Finally a general expression for the current fluctuations (`noise') in double diodes is given and discussed. The application of this formula to other types of conductors is also indicated.

A technique based on well-known interferometric procedures is described for determining the corrections for phase-change dispersion at reflection in Fabry-Perot interferometers. Results obtained during precise determinations of mercury-198 waveengths are quoted to illustrate its application. A marked difference is found between the dispersion corrections for the silvered and aluminized mirrors actually used, and it is established that the latter have negligible corrections in the region 4047 A. to 6438 A.

The method of measuring the thickness of thin films by multiple-beam Fizeau fringes has been studied in order to assess the accuracy attainable. In this connection the small-scale irregularities on the surfaces of optical flats and other optically worked glass have been studied by multiple-beam methods. The effect of these irregularities on the accuracy of the thickness measurements is shown and the conditions for optimum accuracy are discussed.

Thickness measurements have been made on films of silver and of lithium fluoride with several different metals as reflecting layers. The measurements cover the range 120 A. to 1500 A. The results for the different reflecting layers show agreement to within the observational error provided that measurements are made within a short time of the preparation of the films. With silver as the reflecting layer an accuracy of ±10 A. can be expected if suitable precautions are taken.