Professor Andrew Lang: Pioneer of X-ray diffraction physics

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Distinguished for his pioneering studies in X-ray diffraction physics, especially for his development of the technique of X-ray topography, Andrew Lang will be greatly missed by many researchers for his detailed knowledge of crystal physics.

Crystals are regular three-dimensional arrays of atoms, packed together in evenly spaced unit cells. Departures from regularity, or crystal defects, significantly modify the useful physical properties of industrial materials, such as their strength, colour and electrical conductivity. Lang developed topographic techniques to image certain imperfections in crystals, such as dislocations, stacking faults, growth-sector boundaries and ferromagnetic domains. One such technique, the projection topograph, displays the internal imperfections throughout a crystal and this is often called the "Lang method". For the past 50 years, Lang's methods have been widely used in the non-destructive assessment of crystals for the electronics, diamond and other industries.

A diffraction pattern may be produced by visible light passing through a fine silk cloth or by X-rays passing through the regular lattice of atoms in a crystal. When a beam of X-rays enters a crystal at particular angles of incidence, it is possible for the planes of atoms to reflect the X-rays ( according to Bragg's law). The waves in the transmitted beam and in the reflected beam may interfere to produce another wave: in effect a moiré pattern.

Lang studied many types of X-ray diffraction phenomena, including variations from Bragg's law, X-ray moiré patterns and other types of fringes. His most important discovery in this category (with Norio Kato in 1959) was that of interference fringes in wedge-shaped perfect crystals, in which the fringe spacing gave a precise measure of the scattering power (or absolute structure amplitude) from a unit cell. Using synchrotron X-radiation, Lang and his research team were also able to measure and to map the small variations in the separation of carbon atoms across a 5mm synthetic diamond with an accuracy better than one part per million.

Lang also made significant discoveries using other techniques, especially electron microscopy and cathodoluminescence. For the latter, high-energy electrons are directed on to a specimen surface and the visible luminescence is recorded. Using these techniques separately and in combination with X-ray topography, he studied a wide range of crystalline materials, including metals, semiconductors, quartz and diamond. His first published paper (1947) was on the crystal structure of a potassium soap.

His work exhibited sustained innovativeness, craftsmanship in experimentation, and perceptiveness and thoroughness in the analysis of experiments. The topographic images which he produced were of the very highest quality and were often exceedingly beautiful.

Lang was born in 1924 at St Annes-on-Sea in Lancashire. He obtained first class honours in a London external BSc in Physics at the University College of the South West (now Exeter University) in 1944, a London external MSc in 1947 and a Cambridge PhD in 1953. He worked in industrial research in England (at Lever Brothers and Unilever Ltd) and in the United States (at the Philips Laboratories, Irvington-on-Hudson, New York).

From 1954 he was Assistant Professor of Physical Metallurgy at Harvard University before moving to a Lectureship in Physics at Bristol University in 1960. Lang spent the remainder of his career in Bristol, gaining promotion to Reader in 1966 and Professor of Physics in 1979. Lang was elected a Fellow of the Royal Society in 1975 and was awarded the society's Hughes Medal in 1997.

Even after retirement in 1987, Lang's dedication to research and scholarship was evidenced by his scientific productivity, his support of younger colleagues and his continual presence in the Physics Department at Bristol. He was a keen amateur geologist and a generous donor to charities.

A.M. Moore and R. Evans

Andrew Richard Lang, physicist: born St Annes-on Sea, Lancashire 9 September 1924; Research Assistant, Cavendish Laboratory, Cambridge University 1947-48; Research Assistant, North American Philips 1952-53; Instructor, Harvard University 1953-54, Assistant Professor of Physical Metallurgy 1954-59; Lecturer in Physics, Bristol University 1960-66, Reader 1966-79, Professor 1979-87 (Emeritus); FRS 1975; died Bristol 30 June 2008.