Obituary: Sir Clifford Butler

CLIFFORD BUTLER was the co- discoverer with George Rochester of "V-particles", a discovery which in hindsight can be seen as the first step towards our current understanding of the nature of matter.

He was born and educated in Reading. From Reading School he went on to spend the war years at Reading University, gaining both his BSc and PhD there. His first research contributions were in the area of electron diffraction, but by 1946 Rochester and Butler were young members of the group established by Patrick Blackett at Manchester University. There they started, under Blackett's guidance, to use a cloud chamber operating in a magnetic field to study the products of the interactions of high-energy cosmic rays in a block of lead placed above the chamber.

On 15 October 1946 they found an unusual event recorded in the photographs of tracks in the chamber - a forked track looking like an inverted "V". This was subsequently interpreted as the spontaneous decay of a neutral particle, but, strangely, the particle had to be enormously long-lived on the time-scale of the nuclear processes produced by the cosmic rays in the lead.

In May 1947 they found a second long-lived particle, this time electrically charged. They had thus seen both neutral and charged examples of what later became called K-mesons.

To increase the observed flux of cosmic rays the Manchester magnet and its chamber were transported to an observatory on the Pic du Midi in the French Pyrenees where they were able to observe a significant number of these "V-particles", and it became clear that they fell into two separate classes of long-lived particles, now known as hyperons and K-mesons.

The strange behaviour of their long life was only explained in 1952 by Abraham Pais, who suggested that the particles were created in pairs. They could be created in quick nuclear reactions, but once separated from their partner could no longer interact rapidly. Murray Gell-Mann and K. Nishijima clarified this picture by identifying a new quantum number, still called "strangeness" in recognition of the apparently strange behaviour.

The creation of the new particles in pairs resulted from the need to balance positive and negative values of the new quantum number if processes are to take place in the fast nuclear interactions; subsequent decay could violate this quantum number, but only on a long time scale.

The observation of these "strange" particles was therefore the first step towards an understanding of the quark structure of matter. Considering the range and importance of the physics that then flowed from the original Rochester and Butler papers, it is hard to understand why their discoveries were never recognised by the Nobel Committee.

In 1953, Blackett and many of his staff, including Butler, moved to Imperial College, London, where Butler became a full professor in 1957, heading the High Energy Nuclear Physics Group there. He quickly recognised the importance of the invention of the bubble chamber by Donald A. Glaser in 1952 and the development of such chambers filled with liquid hydrogen by Roger Hildebrand at Chicago in 1953 and Luis Alvarez at Berkeley in 1954. (Unlike the low- density gas in a cloud chamber, the liquid in a bubble chamber can at once serve as a target for a nuclear interaction and a detector of its products.)

The first operating hydrogen bubble chamber in Europe was created by his group at Imperial College. This led to the 1.5-metre National Hydrogen Bubble Chamber which was used both at the Rutherford Laboratory and at Cern. An important development in such research programmes at Cern was the establishment of international collaborative teams in order to analyse the large number of photographs using the bubble chambers (an approach to research which became the norm for particle physics), under the planning of the Cern Track Chamber Committee which Butler chaired between 1962 and 1965.

By this stage, Butler had become deeply involved in science administration. He became Head of the Physics Department at Imperial College, by now one of the largest multi-group research departments in Europe, whilst still leading the High Energy Nuclear Physics Group. Between 1966 and 1969 he simultaneously also served as Dean of the Royal College of Science. Clifford Butler was a good and kind man who cared for the welfare of his staff and led a department that was proud of the standards and achievements in both its teaching and research.

In 1970 he left Imperial College to become Director of the Nuffield Foundation, a position he held until 1975; then between 1975 and 1985 he served as Vice-Chancellor of Loughborough University of Technology.

Yet he never lost his interest in physics, and played a vital role in IUPAP, the International Union of Pure and Applied Physics, being its Secretary-General, 1963-72, Vice-President, 1972-75 and President, 1975- 78. IUPAP generally, and Butler in particular, were of great importance in maintaining relations between physicists on both sides of the Iron Curtain during the worst periods of the Cold War.

Ian Butterworth

Clifford Charles Butler, physicist: born Reading, Berkshire 20 May 1922; Demonstrator in Physics, Reading University 1942-45; Assistant Lecturer in Physics, Manchester University 1945-47, Lecturer 1947-53; Reader in Physics, Imperial College, London 1953-57, Professor of Physics 1957-70, Head of Physics 1963-70; FRS 1961; Dean, Royal College of Science 1966- 69; Director, Nuffield Foundation 1970-75; Vice-Chancellor, Loughborough University of Technology 1975-85; Kt 1983; married 1947 Kathleen Collins (two daughters); died Leicester 30 June 1999.