Obituary: Professor Kenneth Allen

Kenneth Allen will be best remembered for his outstanding contributions to nuclear structure physics and for his advocacy of the use of electrostatic nuclear accelerators in other areas of science. Accelerators - otherwise known as "atom smashers" - are machines used for studying nuclear reactions by creating beams of high-energy particles.

Under Allen's direction, as Professor of Nuclear Structure from 1963 to 1991, Oxford University developed a very strong team of electrostatic accelerator designers and builders. As well as their use for nuclear research, Allen saw that these machines could have applications in other areas, and he pioneered the development of accelerator mass spectrometry.

In this technique atomic nuclei are extracted from a specimen and accelerated to high energy. The different elements of the specimen are then identified by the amount of their deflection in a magnetic field. Using in-house accelerators initial work was done on Carbon 14 dating by this technique, and the success of these experiments led directly to the establishment in Oxford of the Research Laboratory for Archaeology and the History of Art, which later played a prominent part in the dating of the Turin Shroud.

Such carbon dating is now also established in Peking using one of the original accelerators Allen installed in Oxford. Allen himself later used similar techniques to search for evidence of nucleon decay by measuring the amount of Xenon 129 gas particles in old rocks. The Xenon 129 would be the stable nucleus eventually resulting from the decay of either a proton or neutron in the rock material.

Allen was born in London in 1923, and educated at Ilford County High School, from where he obtained a Draper's Scholarship to Queen Mary College, London. After graduating with a first class BSc in Physics in 1943 he undertook wartime service with the Ministry of Supply. As the Second World War drew to a close he was able to take up a research studentship at St Catherine's College, Cambridge, studying nuclear physics.

In 1947 he obtained his PhD and moved to Canada, where he spent four years studying uranium fission and the fusion of light nuclei in the Physics Division of Atomic Energy of Canada, Chalk River, at that time headed by Sir John Cockroft. This was the beginning of a long, happy and fruitful collaboration with Canadian physicists and Chalk River in particular.

On returning to England in 1951 he became Leverhulme Research Fellow and Lecturer at Liverpool University, where his research centred on the continuing study of nuclear reactions induced by Helium 3 and Hydrogen 3 beams, which were of higher energy than those he had been using in Canada. This area of interest continued to be a theme throughout his career and has recently re-emerged as an important part of nuclear structure research with the advent of even more advanced and energetic radioactive particles.

From 1954 to 1963 Allen served with the United Kingdom Atomic Energy Authority based at AWRE in Aldermaston. He joined the small group of scientists who worked out, in a very short time, the principles of the hydrogen bomb and who went on to design such a device, which was tested successfully at Christmas Island in 1958. Their work also involved evaluating the state and maturity of the Soviet Union's hydrogen bomb programme via studies of samples collected after tests and this may well have led to Allen's long-term interest in mass spectroscopy, which was the method used to identify the trace atoms in the samples.

Allen's efforts at Aldermaston were not solely directed towards the weapons programme. With the support of the Chief Superintendent, Sir William Penny, he built the Van de Graaff type of accelerator for the study of nuclear physics and produced some of the first negative ion beams. From such beams higher-energy neutral atoms could be obtained, which are important in fusion research and led to Allen's participation in the initial programme at Culham, for three years from 1960.

In 1963 he was appointed to the newly created chair of Nuclear Structure Physics in Oxford University and became a Fellow of Balliol College. At that time the Department of Nuclear Physics, which also carried out research in particle physics, was in full expansion under the leadership of Professor Denys Wilkinson. Considerable resources were put at Allen's disposal and he was able to build up one of the strongest Nuclear Physics research teams in the country. The new building in Keble Road was to house two of the then state-of- the-art Van de Graaff accelerators and provided a wide variety of secondary beams for use by physicists from Oxford and also from several other universities, both in the UK and abroad.

It was during this period that Allen carried out the work for which he is well known. His particular interest was in the measurement of the lifetimes of nuclear states undergoing gamma-decay. This is essential input to theories of nuclear structure, but, because of the extremely short lifetimes, less than one million millionth of a second (a picosecond), severe experimental difficulties were posed. The experimental solutions used by Allen and his collaborators are of remarkable elegance. They depended on the observation of the Doppler shifts of gamma rays emitted from nuclei decaying in flight. The relative amount of the Doppler shift depends upon the relation between the lifetime of the state and the flight time before it is brought to rest, either in the target itself, or in an external absorber. Another technique developed in Oxford was the gas target, using very low-temperature pumps, with which he was able to study states in fluorine produced by alpha particles captured in nitrogen.

The beam design work carried out in Allen's group also led to the development of the Proton Microprobe Technique. Beams of a few micrometers across are absorbed in the sample and the resulting characteristic X-rays enable the composition of the material to be determined. Allen encouraged the initial work on this technique, which led to the establishment in the Nuclear Physics building of a dedicated unit which now provides researchers in physics, metallurgy, biology, medicine, archaeology and the arts with very finely detailed chemical analyses of their material.

When Wilkinson left Oxford in 1976, Allen became Head of the Department of Nuclear Physics, a position he occupied from 1976 to 1979 and again from 1982 to 1985. Although his administrative duties left less time for his research, he continued to take a detailed interest in the progress of the Nuclear Structure Group, of which he was a strong supporter. Allen retired as Professor in 1991, but continued to live locally and was a frequent visitor to the laboratory.

Besides his university duties, Ken Allen was also very active in Balliol College. Unusually for a Professorial Fellow, he took on the duty of Estates Bursar from 1980 to 1983 and again in 1991. He brought to this task his characteristic enthusiasm and disdain for bureaucracy and his periods of office were most successful.

During his career Allen was a prominent figure in Nuclear Structure Physics, both in the UK and internationally. He took a full role in the development of Nuclear Structure research policy in Britain and served from 1970 to 1973 on the Nuclear Physics Board of the Science and Engineering Research Council.

He will be especially remembered by his generations of graduate students. His enthusiasm for the field was contagious and he was a careful and considerate supervisor. His students, many of them now prominent figures in the world of physics, remember their time in Oxford with pleasure and gratitude.

Although Allen clearly found the greatest satisfaction in nuclear physics in all its diversity, he was also a lover of music and a keen chess player.

Roger Cashmore and Gerald Myatt

Kenneth William Allen, nuclear physicist: born London 17 November 1923; member, Physics Division, Atomic Energy of Canada, Chalk River 1947-51; Leverhulme Research Fellow and Lecturer, Liverpool University 1951-54; Deputy Chief Scientist, UKAEA 1954-63; Professor of Nuclear Structure, Oxford University 1963-91 (Emeritus), Head of Department of Nuclear Physics 1976-79, 1982-85; Fellow, Balliol College, Oxford 1963-92 (Emeritus), Estates Bursar 1980-83, 1991-93; married 1947 Josephine Boreham (two sons); died Oxford 2 May 1997.