Obituary - Bill Cochran (1922 - 2003)

Bill Cochran (1922 - 2003)

ACA RefleXions, Winter 2003

cochran

Bill Cochran was born on a remote sheep farm, some 15 miles south of Glasgow. Forebears of the Cochran family had farmed in the area continuously since the middle of the seventeenth century although Bill’s father took the family to a new farm, some eight miles west of Edinburgh in 1928.

As Bill himself said, “It is surprising particularly as I have no brothers, that I did not become a farmer.” Had he done so it would have been a sad loss to science.

Bill was educated at Boroughmuir High School where his first interest was in languages, but the fortunate gift of a Mecanno set turned his interest first to engineering and later to physics. He entered Edinburgh University in 1939 to read physics, graduating in 1943. He made repeated efforts to become involved in the war effort but he was always directed towards academic activity and from his graduation until 1946 he was an Assistant in the Edinburgh Physics Department, mainly involved in teaching electronics. For the first part of that period he worked under the direction of Professor C G Barkla who had won the Nobel Prize for Physics in 1917 for his work on the characteristic emissions of x-rays. When Professor Barkla died in 1944, Bill moved to the Chemistry Department to do research in x-ray crystallography under the guidance of Arnold Beevers. After an exhaustive, but unsuccessful attempt to solve the native sucrose structure, Bill succeeded in solving the isomorphous NaCl and NaBr adducts of sucrose, which enabled him to obtain his PhD.

In 1946 Bill moved to Cambridge as research assistant to Sir Lawrence Bragg. The research group was under the direction of W H Taylor and also contained June Broomhead and C J B Clews. Professor Alexander Todd of the Chemistry Department had suggested to Bragg the problems of solving the structures of pyrimidines, purines, nucleosides and nucleotides as a way of throwing light on the structure of DNA. Cochran and Broom- head found that they could determine the electronic structures of pyrimidines and purines sufficiently accurately to find the positions of hydrogen atoms and Bill found that this was greatly improved by the use of an (Fo – Fc) Fourier synthesis that removed the effect of termination errors. Later he built a Geiger-counter diffractometer with which he measured the electron density in salicylic acid to reveal the presence of hydrogen atoms and electron density associated with covalent bonding.

In 1949 Bill met David Sayre, an Oxford-based American postdoctoral worker, and this began his interest in direct methods of solving the phase problem. As early as 1948 papers on inequality relationships by Harker & Kasper, Gillis and others had appeared in Acta Crystallographica but such relationships were restricted to very small and simple structures. In 1952, in one issue of Acta Crystallographica, there appeared three papers separately written by Sayre, Cochran and Zachariasen that could be said to be the starting point of modern direct methods.

In 1951, after an extended visit to the USA, Bill was promoted to a full Lecturer in Cambridge and so obtained the tenure and stability he needed to anchor his subsequent research career. Travelling home by ship from Stockholm after the second International Congress on Crystallography in 1951, Bill met his future wife, Ingegerd Wall, and they married in Sweden in 1953. Also, at about this time, he collaborated with Henry Lipson in the production of The Determination of Crystal Structures, a very influential text for many years.

In the 1950s Bill acted as a consultant to the protein crystallography group in the Cavendish Laboratory and in 1952 he, Francis Crick and Vladimir Vand published a notable paper on the diffraction pattern of atoms on a helix. It was this paper that enabled Crick and Watson to interpret Rosalind Franklin’s diffraction photographs of the A-form of DNA.

Bill saw quite early the potential of computers as a crystallographic tool and in 1955, with Sandy Douglas, he applied EDSAC, a primitive early computer, to a direct method for solving centrosymmetric structures. This pointed the way to the subsequent development of computer applications in this field, which had an important impact on structural crystallography.

In the mid-1950’s Professor Dingle had created a bit of a stir by expressing doubts about the validity of the ‘twin paradox’ in relativity and for some time Cochran and Dingle carried out a lively public debate which ended in Cochran’s favor. During this period Cochran was becoming somewhat disenchanted with direct methods and had concluded, incorrectly as he later con- ceded, that they could not solve more than very simple structures. Protein crystallography was clearly the future but, having decided early on that it too was unlikely to succeed, he was reluctant to “climb on the bandwagon” now that it was rolling successfully. By 1958 Cochran was ready to embark on a new field.

In 1957, at the fourth International Congress on Crystallography in Brookhaven, Bill had attended a lecture by Bert Brockhouse on neutron diffraction applied to lattice dynamics. He arranged to spend a year at Chalk River, funded by Atomic Energy of Canada, where Brockhouse, later to win a Nobel Prize, was just using his newly-designed triple-axis spectrometer to measure phonon dispersion curves for sodium iodide. Bill spotted a theoretical paper by Dick and Overhauser on the dielectric constant of alkali halides and he realized that their shell model was just what was needed to explain the lattice dynamics of sodium iodide. Later he extended the theory to germanium and two papers he wrote at this time became classics in the field. Later development, while he was still in North America, led to the idea of the soft-mode concept for the onset of ferroelectricity.

On returning to Cambridge in 1959 he briefly dallied with the phase problem once more but then turned his full attention to lattice dynamics. He was fortunate in having two gifted research students, Stuart Pawley and Roger Cowley, later to become colleagues at Edinburgh. In the next few years, work was done on the lattice dynamics of molecular crystals, the theory of the lattice dynamics of sodium, and the shell model was extended to GaAs. At this stage Bill was at the forefront of the lattice dynamics field and it was almost inevitable that, at the comparatively early age of 40, he was elected a Fellow of the Royal Society in 1962.

In 1964 he was appointed Professor of Physics at Edinburgh and quickly established a research group on Condensed Matter Physics. Further work was done on phase transitions leading to the onset of ferroelectricity, mainly using the Chalk River facility for neutron spectroscopy. For the next ten years Bill and his group established a leading position in the general field of lattice dynamics. He also developed a new model for the structure of amorphous materials.

In 1975 Cochran succeeded Norman Feather as the Professor of Natural Philosophy and Head of Department in Edinburgh. Although his research activity continued for a while it quickly tailed off as he bore an increasing administrative burden.

Cochran’s work was recognised by several awards and appointments to scholarly bodies. He was a Fellow of the Royal Societies both of Edinburgh and London and an Honorary Fellow of Trinity Hall, Cambridge. He was awarded the Hughes Medal of the Royal Society, the Guthrie Medal of the Institute of Physics and the Potts Medal of the Franklin Institute. Amongst his many interests were the writing of poetry and tracing the genealogy of his family. He leaves behind his wife, Ingegerd, and children Margaret, Robert and Jennifer, to whom he was a devoted husband and father.

- Michael Woolfson
Reprinted from the Winter edition of the British Crystallographic Association Newsletter