Kent Scientists

Cecil Powell (1903-69)

Born and educated in Tonbridge.

Nobel Prize-winner, described as 'the father of particle physics'.

In physics circles, 1947 has been called ‘the year of the pion’. In February of that year Kent-born Cecil Powell and a colleague at Bristol University announced the detection of a hitherto unknown subatomic particle. They named it the pi-meson but it is known today as the pion. Its importance in the great scheme of things is that it plays a part in binding together the other particles within the nucleus of almost all atoms.

Cecil Powell

Cecil Powell in 1950 by Elliott & Fry, bromide print, NPG x181834

© National Portrait Gallery, London

Cecil Powell was born in 1903 in Tonbridge, where the family ran a gunsmith’s shop for many years. He grew up in the town and attended the Judd School.  An early introduction to science came from a chemistry primer given to him by his grandfather. It included instructions for how to make hydrogen from sulphuric acid and zinc, and young Cecil decided to try this for himself, using ingredients purchased from a local chemist’s shop. He assembled his homemade apparatus in a ‘lab’ in a garden shed and began the experiment. But when no hydrogen appeared he began to look for possible leaks, using a naked flame. What happened next is described in a memoir he wrote in later life:

‘There was an explosion which in that confined space, and with that corrugated iron roof, seemed absolutely tremendous. The candle was blown out, and I was left in the dark, dazed and deeply impressed, but otherwise unhurt. Of all the apparatus, glass-ware, acid, granulated zinc, I never discovered the slightest trace …’. When he emerged from the shed his ‘mother, who had been frozen in her armchair where she had been reading the newspaper, … breathed again’.  

But it was physics rather than chemistry that was to become his life’s work. He joined the staff at Bristol University in 1928 and remained there for the next 40 years, working in the field that eventually became known as particle physics – the study of the atomic nucleus and its components. The technique today is to use huge machines, such as the Hadron Collider at CERN, to observe what happens when particles collide at high energy, and the technique – the study of collisions –  was exactly the same in the 1930s and 40s but on a much smaller scale. For a source of high-energy particles, Powell used what nature provides, cosmic rays. And to observe the results he used photographic emulsion. This is the gelatinous light-sensitive coating that was used on photographic film prior to the digital age. Protected from light, emulsion also detects fast-moving charged particles, whose tracks become visible under a powerful microscope as tiny black specks within the emulsion, resembling beads on an invisible string. Over the years Powell and his colleagues showed great determination in improving and refining this technique into an extremely valuable tool for studying sub-atomic events.  

The Pic du Midi observatory in the French Pyrenees in 1948. The pi-meson was first identified in photographic emulsion exposed to cosmic rays here. Photo © Yan, Fond Essalet

Cosmic rays are high-energy particles – mainly protons – which originate beyond the solar system and bombard the Earth’s upper atmosphere. Because they rapidly lose their energy colliding with air molecules in the atmosphere, they are best studied at high altitudes. Powell and his colleagues initially sent stacks of their special photographic plates aloft attached to plastic hydrogen balloons (whose appearance in the dawn sky sometimes prompted reports of flying saucers). The scientists chased these balloons by car in order to retrieve the plates for developing and analysis. Later they took to installing their emulsion stacks at mountain top sites – in the Pyrenees and Bolivian Andes – where they could remain for months at a time. This increased the chance that cosmic rays would collide with atomic nuclei in the emulsion, causing them to disintegrate. It was among the tiny tracks left in the emulsion by these disintegrations that the hitherto unknown pi-meson was discovered. For this discovery, and for his work in developing the technique that made it possible, Powell was awarded the Nobel Prize for Physics in 1950.  

Cecil Powell was an outstanding man. In addition to his scientific expertise, he was an exceptional university teacher, a wise administrator, modest and approachable. Profoundly concerned about the relationship between science and world affairs, he was the founding chairman of the international ‘Pugwash Conference’ movement, which continues in a quiet way to bring people together across divides, and give technical advice to governments.  

‘In a very real way Powell can be said to have been the father of particle physics’, according to his Royal Society obituary published after his death in 1969, just a few months after he retired from his post at Bristol. From his discovery of the pion has grown the vast edifice of modern particle physics, in which a whole ‘zoo’ of exotic elementary particles has since been discovered.   Powell is commemorated by a blue plaque in Bristol, a lecture theatre at Bristol University, and at the Judd School in Tonbridge where he was educated.