Ernest Thomas Sinton Walton, 1903-95
Ernest Thomas Sinton Walton, Nobel Laureate and Honorary Fellow of the Institute of Physics, who died on 25 June 1995, was one of the legendary band of nuclear physicists who made 1932 an annus mirabilis. With John Cockcroft he caught the world's attention when he 'split the atom' (more properly its nucleus) by artificial means. This was no large-scale, extravagant experiment, and still less did Walton crave the great fame which he achieved. A modest man, his great skills lay in his keen mind, love of physics, and exceptional dexterity, qualities which remained with him throughout his long life.
His achievement realised the alchemist's age-old dream of transmutation in a dramatically novel way. In 1919, Rutherford had already induced the nuclear reaction 147N + alpha = 178O + p using alpha particles from a radioactive source. Driven by the great man's call for 'a million volts in a soapbox', Walton and Cockcroft built an accelerator which provided a far greater and more controllable particle flux than had previously been available from radio-isotopes. The particles being accelerated were singly charged protons rather than doubly charged nuclei of helium atoms. This increased the chance of nuclear reactions occurring, as the electrostatic repulsion between projectile and target nucleus was smaller.
The accelerator was built in a disused room in Rutherford's Cavendish Laboratory at Cambridge, and was supplied with several hundred kilovolts from a voltage multiplier circuit designed and built by Cockroft and Walton. Odd bits and pieces of equipment and even recycled pieces of wood and nails were used, as was standard practice in the Cavendish. Part of the apparatus is now in the Science Museum in London.
Sitting in a small box, Walton turned the proton beam on to a lithium target. The date was 14th April 1932. He was able to observe scintillations characteristic of alpha particles on a zinc sulphide screen. These were produced by the reaction 73Li + p = alpha + alpha. Walton loved to recount how Rutherford hurried over, as he always did when something important was happening, and how the two young physicists had to manoeuvre his imposing bulk into their little box. According to Walton, when Rutherford saw the scintillations, he exclaimed: 'These look mighty like alpha particles. I should know - I was at their birth'. 'Indeed', Walton would say, ' he should have added that he was at their christening also, for it was he who named them'.
New Physics
While inside the box on that day, Walton was also present at another birth - that of the era of accelerator-based experimental nuclear physics. The subject grew rapidly from these string-and-sealing wax beginnings. Nevertheless to this day the Cockcroft-Walton circuit is often still used to supply a high voltage at the injector stage of large particle accelerators. In the apparatus used by Cockcroft and Walton, the voltage of a transformer was rectified and multiplied fourfold by an arrangement of capacitors and thermionic rectifiers.
The rectifier system consisted of a 4 m high tower containing four continuously evacuated glass cylinders of the type used in petrol pumps of the period. The cylinders were placed end to end, with electrodes and hot filaments mounted inside. The arrangement gave a voltage of up to 700 kV, steady to within a few per cent. This voltage was applied to an accelerator tower built from two glass cylinders containing focussing electrodes. At the top a hydrogen discharge tube supplied the protons. The small experimental observation chamber at the bottom was shielded by lead and was electrostatically screened. Proton currents of up to 10 microamps were produced.
During the period April-June 1932 the alpha particles from the splitting lithium nuclei were observed not only on a scintillation screen but also with an expansion chamber, which made the alpha tracks visible, and with an ionisation chamber connected to an amplifier and oscilloscope. From these measurements the kinetic energies of the alpha particles could be deduced. In addition, counts were made in coincidence from two scintillation screens placed on either side of a thin lithium target. This showed that the alpha particles were being emitted in pairs. It also demonstrated that the two alphas from each disintegration emerged in approximately opposite directions, as expected from momentum conservation.
The letter published by Cockcroft and Walton on 30th April in Nature under the title 'Disintegration of lithium by swift protons' is a model of elegance - economical in words but not in content. They showed that a substantial net energy release could be obtained from a nuclear reaction. 'The evolution of energy ... is about sixteen million electron volts per disintegration, agreeing approximately with that to be expected from the decrease of atomic mass involved'. This was a major experimental verification of Einstein's mass-energy equivalence E = mc2.
The experiment was also a striking demonstration of one of the predictions of the new theory of wave mechanics. Because an accelerating voltage as low as 125 kV was all that was required for the reaction to be observable, it was seen that the protons must be tunnelling through the potential barrier surrounding the nucleus. There was no need for the much higher voltages necessary if the protons were to surmount the high barrier. This possibility had been first appreciated by Cockcroft in conversations with the Russian theoretical physicist George Gamow in 1928. It was a key which Lawrence and others in the USA apparently failed to grasp in their strivings to obtain MeV energies before looking for nuclear reactions.
It was no accident that Walton was the first to observe the splitting. Cockcroft's biographers acknowledge that, despite his engineering and administrative abilities, he was not an experimental physicist of the first rank, whereas Walton 'had a fine brain, was capable with his hands and had perseverence'. Rutherford had already judged that Walton was 'original and able' and had 'energy and skill'. This can be well appreciated from the quality of Walton's early published work, which appeared under his name alone. The 1932 experiment was a true team effort of two complementary personalities. It was rewarded six years later by the joint award of the Royal Society Hughes Medal, and belatedly in 1951 by the Nobel prize in physics.
The life of a pacifist
Walton was born on 6 October 1903 in Dungarvan, Co Waterford, Ireland. His father was a Methodist minister, so the family frequently moved around Ireland from one manse (an ecclesiastical residence) to the next. Methodist College, Belfast, provided his secondary education. He took the BA and MSc degrees in Trinity College, Dublin, where his research was in hydrodynamics. John Joly, whose achievements also involved the alpha particle, recommended him to Rutherford, under whose supervision he received a PhD in 1931. Based in Cambridge's Trinity College, he was funded initially by an 1851 Exhibition Overseas Research Scholarship. After his PhD he stayed on until 1934 in the Cavendish Laboratory as Clerk Maxwell Scholar.
Before his momentous collaboration with Cockcroft, Walton worked on attempting to accelerate electrons in a circular electric field, starting from ideas of Rutherford and J J Thomson. This effort was a basis for the subsequent development by Kerst in the USA of the betatron. Following earlier work of Wideroe, he then tried to build a linear accelerator for positive ions. Walton also used the 1932 accelerator with Cockcroft to bombard a range of targets other than lithium, and to accelerate deuterons derived from a sample containing the newly discovered deuterium.
The enormous media publicity accorded to the 1932 breakthrough was in contrast to that received by James Chadwick when he discovered the neutron just two months earlier in the Cavendish. Nuclear energy, it was reported in the Reynolds News, implied 'nothing less than the complete abolition of irksome manual labour and a new era of prosperity for all'. By a striking coincidence the very issue of The Times which announced the work also reviewed a newly opened West End play, Wings over Europe, in which a scientist disintegrated and transmuted matter with nuclear energy, as a result of which the end of the world was threatened. Walton must have winced at this juxtaposition. A pacifist, he was always at pains to stress that he did not feel in some way responsible for the subsequent development of nuclear weapons. These relied on two things, neither of which, as he said, was attributable to him: the fission of very heavy nuclei such as uranium, and the chain reaction of the resulting neutrons.
Back in Ireland
After the annus mirabilis many of Rutherford's team in the Cavendish dispersed around the world. Walton could have gone to laboratories in the USA or elsewhere. Instead he returned to Dublin, and in 1934 was elected to fellowship of Trinity College. In 1947 he also became head of physics and the Erasmus Smith's Professor.
Walton's heavy teaching and administrative duties in an impoverished college were in complete contrast to his Cambridge career. He worked on designing and building another accelerator as best as he could, and was involved in various scientific projects connected with alleviating the hardship of the war years. He never wished to be singled out for special support. Nor was it in his nature to be attracted by opportunities elsewhere. He certainly had no wish to accept Chadwick's invitation to join a 'secret project' in the States, which turned out to be the Manhattan project to develop a nuclear bomb.
When better times arrived for the support of science in Ireland, he rejoiced in the relative abundance enjoyed by his successors, without showing a trace of regret. He gained special satisfaction from the excellence of his teaching over several decades. Even in his late seventies, he gave demonstration lectures which were models of simplicity, clarity and integrity. They were a revelation to those who were privileged to see their effect upon a youthful audience. He also continued to lecture about his life's work, returning for example in 1982 to Cambridge to give a memorable contribution to the Institute of Physics conference on the 50th anniversary of the discovery of the neutron.
Following his retirement in 1974, he retained his association with the Physics Department at Trinity up to his final illness. His was a familiar face in the tea-room. Visitors, on being introduced to the quiet, unassuming man in the corner, softly whistling a hymn or a Percy French melody, would be astonished to find that he was one of the great figures of the history of physics. Shortly before his death he marked his lifelong devotion to Trinity by presenting his Nobel medal and citation to the college.
Walton's life was governed by unshakeable principles, a conviction derived from his steadfast Methodist faith. He was President of the Irish Pugwash Group, concerned with reducing the dangers of nuclear war. In November 1994 the President of the Republic of Ireland, Mary Robinson, paid him a special visit in his Belfast nursing home to honour him as a man of science and a man of peace. Having attained the summit of his early ambition, he led a long and contented life, and inspired several generations of physicists. His four children, Alan, Marion, Philip, and Jean, are all scientists, the first three being physicists - though not, as he emphasised, through any pressure from him.
- Eric Finch and Denis Weaire
- Obituary published in Physics World, November 1995
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