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OBITUARY: Isobel Powell

Isobel Powell was responsible for identifying one of the double mesons (subatomic particles) which led to the award of the 1950 Nobel Prize for Physics to her husband Cecil Powell. She was closely involved with her husband's work throughout his distinguished career at Bristol University and his work with the Pugwash organisation.

She was born Isobel Artner in Hamburg in 1907. Her mother was the daughter of a Scottish minister from Dornoch, on the north-east Scottish coast, and her father was an Austrian Jew. She was brought up chiefly in Vienna, but spent three years with friends in Denmark - she maintained an interest in languages all her life.

In her early twenties Isobel went to Paris, where she worked for a time as the composer Sergei Prokofiev's secretary - Peter and the Wolf was later a favourite record in her household. Through a mutual friend, Max Delbruck, she met Cecil Powell. Delbruck was a Research Fellow at Bristol University, sharing rooms with Powell, then a Research Assistant. In 1932 Isobel came to Bristol to be married to Cecil; shortly afterwards she started work in the Physics Department as his secretary and later, in addition, as a microscope observer or scanner. Remarkably, both Powell and Delbruck were, in due course, awarded Nobel Prizes for their work, Powell the prize for Physics in 1950, Delbruck that for Medicine and Physiology in 1969.

At Bristol, Cecil Powell with his chief collaborator G.P.S. Occhialini transformed the use of photographic emulsions for recording the tracks of subatomic particles into a powerful new technique. From the track of the particle one could, in favourable cases, determine both its energy and its mass - and see its behaviour. At that time Isobel Powell was a member of a small group of microscope observers employed to scan the processed emulsion plates that had been exposed either to the Cosmic Radiation at Mountain Observatories (for example, at Pic-du-Midi, and Jungfraujoch), or to laboratory sources of particles. These emulsions were mounted on glass plates and the task of the scanners was to search the whole plate area and thickness under a magnification of about 100 times, to locate and record tracks for further measurement by physicists.

The discovery of the p-meson in 1946, using the new technique, was a milestone for Bristol physics and the university. The parent p-meson comes to rest within the emulsion and gives rise to a second meson, always with the same energy and track length; careful measurement showed the parent to be a new particle and the daughter to be a m-meson, a particle whose existence was already known. The first two of these double mesons were decisive in their information, and Isobel found one of these in the course of her scanning. For his work Cecil Powell was awarded the Nobel Prize for Physics in 1950. Isobel Powell recorded in her diary of the award in Stockholm, "It was a very curious sensation when you suddenly see yourself on the screen. It was the first time that television had been used in Sweden for the Nobel Prize-giving ceremony."

After the p-meson discovery, there was a rapid growth in the size of the team at Bristol; the number of scanners rose to more than 20, many of them straight from school, and as chief scanner Isobel Powell was in charge of the team known affectionately as "Cecil's Beauty Chorus". There was a large throughput of postgraduates and scientists of many nationalities who came to join the exciting research. Morale was very high, and work continued at a great pace for many years.

Isobel also strongly supported the second strand of Cecil Powell's scientific career - his involvement with Joseph Rotblat in the politics of science. Powell was a scientific adviser to Bertrand Russell in his attempts, with Albert Einstein, to influence nuclear powers to consider the serious issues arising from modern weapon systems, and was one of the 11 signatories of the Russell- Einstein manifesto of 1955. These efforts became focused in 1957 in the Pugwash Organisation, with Powell as Deputy Chairman, and Rotblat as Secretary. Pugwash acted as a gadfly to prod governments into action in the Cold War period. After Russell's death in 1967, Cecil Powell was elected Chairman, a post he held briefly until his death in 1969. Just a few weeks ago it was announced that Joseph Rotblat and the Pugwash Organisation were to share the 1995 Nobel Peace Prize for their 40-year-long efforts. Cecil and Isobel Powell would have been very pleased.

After Cecil's death, Isobel Powell stayed in Bristol, and maintained her connections with the university. She continued her interest in languages, and took a diploma in Spanish. For as long as possible, she continued her weekly involvement with the university "Wives' International Group", which helped the families of overseas students and staff - she could speak to nearly all in their own home tongue. In 1987 there was a celebration of "Forty Years of Particle Physics" held in Bristol, attended by many from all over the world. Isobel much enjoyed meeting old friends.

Peter Fowler

Isobel Therese Artner, microscope scanner: born Hamburg 12 September 1907; married 1932 Cecil Powell (died 1969; two daughters); died 15 October 1995.