Obituary: Professor Cormac O Ceallaigh

Cormac O Ceallaigh, Emeritus Professor of Physics at the Dublin Institute for Advanced Studies, was one of the most distinguished physicists in Ireland. His research field was that of cosmic rays and elementary particle physics, to which subjects he made many seminal contributions.

The son of a prominent Dublin obstetrician, O Ceallaigh's career as a scientist started at University College, Dublin. His postgraduate research was carried out at the Cavendish Laboratory, Cambridge from 1935 to 1938, after a year in Paris with the great French cosmic ray physicist, Pierre Auger. At Cambridge, O Ceallaigh worked in the field of nuclear physics, coming directly under the eye and the influence of Lord Rutherford. A brilliant scholar, he gained an 1851 (Commonwealth) Fellowship, the award being announced to him by Rutherford in a chance encounter with the admonition "I see that you've wangled it again, O'Kelly!"

After some years as a lecturer at the University of Cork, O Ceallaigh joined C.F. Powell's group at Bristol, from 1949 to 1952. This was something of a golden era for physics in Britain, when revolutionary new discoveries in cosmic rays occurred almost daily. Of several important experiments undertaken by O Ceallaigh, one regarding the nature of the neutral particle (neutrino) emitted in the decay of an elementary particle called the pion, stands out and has ramifications to this day. The exact nature of this neutrino remains a puzzle: it impacts on the observed deficit of neutrinos from the sun, and on how very massive stars, in their final death throes, explode as supernovae.

In 1953, O Ceallaigh took up the post of Senior Professor in Cosmic Ray Physics at the Dublin Institute for Advanced Studies. There he continued his association with Bristol University, concentrating on the study of the superheavy nuclei in the cosmic rays (those heavier than iron and nickel), using special plastic detectors carried in the stratosphere by unmanned balloons in very long flights (sometimes even crossing the Atlantic).

These experiments continued through the 1960s and early 1970s, to be followed by even more ambitious ventures. Huge detector arrays, 10 square metres or more in area, were prepared by O Ceallaigh and his colleagues in Dublin, to be carried and launched into earth orbit on flights of the US Space Shuttle. Intended for a one-year exposure, due to various failures the equipment was to stay in orbit for six years before being recovered. This had the fortunate consequence that it provided by far the most prolific data available on the fluxes of the very heaviest nuclei.

In particular, these included nuclei heavier than bismuth (those in the actinide series, such as uranium, plutonium and californium). We believe that these are produced as a result of very rapid neutron capture processes which occur, and only occur, in the course of supernova explosions.

In a sense therefore, O Ceallaigh's work came full circle: 99 per cent of the prodigious energy released in a supernova outburst is in the form of invisible neutrinos, and although not yet fully understood, our computer models of the explosive mechanism - whether it blossoms or stalls - seem to depend critically on the nature of those neutrinos and their interactions: indeed, precisely those questions that O Ceallaigh had been addressing in a quite different context 40 years previously.

In 1951, O Ceallaigh was elected Fellow of the Royal Irish Academy. He was also a Council Member of the European Physical Society, and a Boyle Medal winner.

Many of us will remember him as an accomplished and brilliant speaker at international conferences, with a mordant wit and great sense of humour. Aside from physics, Cormac O Ceallaigh's diverse tastes included linguistics, cabinet-making and gardening; and his passion for sailing was such that he and his late wife Millie arranged to race in different classes of boat so as not to compete with each other.

D.H. Perkins

Cormac O Ceallaigh, physicist: born Dublin 29 July 1912; married 1939 Millie Carr (died 1987; three daughters); died Dublin 10 October 1996.