On scanning electron diffraction. II†

Abstract

The design, advantages and applications of automatic electron diffractometers are discussed with reference to an improved system. The power to measure electron scattering profiles from which loss electrons are virtually eliminated is the fundamental difference between the new instruments and traditional photography of electron diffraction patterns. The basic limitations of the diffractometers are discussed. A new treatment of the electron gun is given, based on angular distributions of current which prove to be Gaussian. The treatment is necessary when maximum signal intensity is required. A number of improvements in electronic circuitry are described which include servostabilization of the gun emission; automatic two-dimensional scanning, and control of CRT display brightness or of xy writing rate in accordance with the intensity of the peaks. The fields of application where the now diffractometers have advantages are when accurate, that is filtered, intensities must be determined, and when growing or changing specimens are studied. The applications are illustrated by the performance of the improved system and the first measurements of two-dimensionally scanned growing films are presented. It is shown that the speed of the new instrument, both in producing growths and in recording them, is important for thin film studies. The speed is itself an aid to consistency, to effective improvement in vacuum conditions, and extensive tests for consistency become practicable while parameters of the specimen growth may be systematically varied.