Cecil Powell's discovery of the pion 50 years ago led to a Nobel prize and opened the doorto modern particle physics. But Powell was more thanjust a physicist - he supported peace, had strong left-wing views and was a keen advocate of the public understanding of science Cecil Powell: pions, peace and politics Owen Lock Ii\ 1947 a relatively unknown physi- covery marked Powell's first step on cist at Bristol University in the UK, a path that led him to international Cecil Powell, discovered a new par- fame, which extended far beyond ticle - the pion - in the cosmic radi- the confines of sub-nuclear physics. ation. At the time, the pion was For Powell was far from being an thought to be the particle that had "ivory tower" physicist. His political been predicted in 1935 by Hideki views were well to the left, and the Yukawa as the carrier of the strong award of the Nobel prize allowed nuclear force that binds the protons him a wider audience for his ideas on and neutrons in the nucleus. Al- the abolition of nuclear weapons though we now know that this is not and the need for international col- the case - the pion is just one of a laboration in science. In 1954 he family of strongly interacting mesons became president of the Association - Powell's discovery earned him the of Scientific Workers - a UK trade Nobel Prize for Physics in 1950. union - and in 1956 he was made Powell discovered the pion using president of the World Federation specially manufactured photogra- of Scientific Workers. He was also phic emulsions as detectors. The first one of the 11 signatories of the major discovery of the post-war Russell-Einstein manifesto of July era, it showed the potential of cos- 1955, which called upon scientists mic-ray studies using the emulsion to "assemble in conference to technique and opened the door to appraise the perils that have arisen as the discovery of many other funda- a result of the development of mental particles. The international weapons of mass destruction". The collaborations that grew up between manifesto led to the foundation of die Pugwash movement, with Powell the various emulsion groups were PoweTl circa !L§i§ holding a montage o'f six photomicrographs, also the forerunner of what was to each showing the decay of pions into muons of constant energy. succeeding Bertrand Russell as happen later around the big high- chairman in 1967. energy particle accelerators, which gradually replaced cosmic- On the European scene, Powell played a major role in the ray experiments in the study of elementary particles. establishment of CERN, the European laboratory' for particle Charles Frank and Donald Perkins, both close colleagues of physics in Geneva, and in developing its research programme. A Powell at Bristol, wrote in their Royal Society biographical mem- leading exponent of the need for pure science - and a great oir of him: "The impact of the discovery of the pion on the sci- believer in the public understanding of science - he became a entific world in general and on physics in particular was highly respected figure in the international science community profound. It stimulated the building of a new generation of before his untimely death in 1969, at the age of 65. accelerators with which to probe the newly discovered domain of sub-nuclear physics and to uncover the richness of completely Early years new and unexpected phenomena. In a very real way Powell can Cecil Powell was a quiedy spoken man of slight build and be said to have been the father of particle physics". medium height. He was born in Tonbridge, Kent, on 5 Decem- As well as being a milestone in physics history, this major dis- ber 1903. His father was a gunsmidi and his mother was the PHYSICS Worno NOVEMBER 1997 35 eventually chose Powell, who arrived in Bristol in the spring of 1928 and stayed there for the rest of his life. Tyndall and Powell carried out extensive measurements on the mobility of positive ions in gases. Tyndall was greatly im- pressed by Powell's experimental ingenuity and skills, which included carpentry, an interest he had acquired as a boy. Powell was appointed lecturer in 1931 and the following year he mar- ried Isobel Artner, the German-born daughter of an Austrian industrialist and a Scottish mother. The first of Powell's two daughters was born in 1933 and an immediate consequence was that Powell found that his university income was inad- equate. He secured a job at twice the salary with the electrical- engineering firm British Thomson-Houston at Rugby, but his wife was sure that he would not be happy in industry. Fortunately, Tyndall arranged an increase in Powell's salary at Bristol, and a crisis was averted. Tyndall, who was always looking to the future, began to" wonder if any laboratory could become internationally prominent unless it did some sort of experimental nuclear physics. Powell had The first (left) and second (right) observed decays of a pion into a muon and a become excited by key breakthroughs that had been made at the neutral particle, later shown to be a neutrino. For this discovery of the pion, Cavendish, including the discovery of the neutron by James Powell was awarded the 1950 Nobel Prize for Physics. Chadwick and of electron-positron pair production by Patrick Blackett and Giuseppe Occhialini. Tyndall later recalled that in daughter of a schoolteacher. A lawsuit arising from a shooting 1933 Powell had been "dead keen on starting on a Blackett accident led to his father being declared bankrupt when Powell cosmic-ray experiment". However, it was eventually decided that was only three years old. Cecil's mother, who was forced to take Powell should be given the responsibility of building a 700 keV in lodgers to make ends meet, was anxious that he and his Cockcroft-Walton generator to accelerate protons and younger sister should escape from similar drudgery. Her elder deuterons. The deuterons would be used to bombard light- brother had succeeded in getting in to Trinity College, Cam- element targets to produce neutrons, and Powell would then bridge, to become an electrical engineer, and she was deter- study neutron-proton scattering using a Wilson chamber filled mined that her son should secure a similar emancipation. with hydrogen. With the aid of a brilliant young physicist, When he was 12, Powell won a scholarship tojudd School in Geoffrey Fertel, the accelerator became operational in May 1938. Tonbridge. Although he was initially interested in chemistry, he came under the influence of an able physics teacher and in 1921 Photographic emulsions obtained a state scholarship and an open scholarship to read In 1938 the theorist Walter Heitler, who was then at Bristol, drew natural sciences at Sidney Sussex College, Cambridge. Powell Powell's attention to the work of two Viennese physicists - graduated in 1925 with first-class honours in both parts of the Marietta Blau and Herta Wambacher - who had exposed pho- tripos, coming second in his year for physics. tographic emulsions to cosmic rays in the Austrian Alps. They Unsure what to do next, he obtained a post at a well known had observed the tracks of low-energy protons as well as "stars" public school, but wondered whether teaching would suit him. - nuclear disintegrations that had presumably been produced by He therefore asked Ernest Rutherford - who had succeededJJ cosmic rays. Powell was impressed by the relative simplicity of Thomson as Cavendish professor of physics at Cambridge in the method, and in the summer of 1938 Heitler took some "half- 1919 - if he could be accepted as a research student. Rutherford tone" emulsions made by Ilford to the Jungfraujoch in the Swiss duly arranged for him to work with the inventor of the cloud Alps, where they were exposed for 230 days. Subsequent exam- chamber, Charles Wilson. Powell built an all-glass cloud cham- ination in Bristol confirmed the Austrian observations. Since the ber to study how the condensation of water droplets in dust-free Wilson chamber was not ready when the Bristol accelerator was air was affected by the degree of expansion of the chamber. completed, Powell bought a box of normal half-tone photo- From this work he concluded that there was nothing to be gained graphic plates — which were normally used for making lantern in operating such chambers at temperatures other than room slides - and exposed one tangentially to his proton beam. temperature, although the results did help him to show how Impressed by the results, he considered the possibility of using steam flows through nozzles, which contributed to the design of the plates to detect and measure the energy of neutrons. steam turbines. Powell realized that if a neutron collides head-on with a proton in the gelatin of the emulsion and scatters the proton by less than The move to Bristol 5°, the proton takes up more than 99% of the neutron's energy University College Bristol, founded in 1876, became a university The neutron's energy could then be determined simply by in 1909, helped by generous benefactions from various members measuring the range distribution of the recoiling protons. To of the Wills tobacco family. In the early 1920s Henry Herbert test this method, Powell decided to study a reaction that had Wills gave £200 000 for the construction of a physics laboratory already been investigated using a Wilson chamber, namely and in October 1927 the H H Wills Physics Laboratory was for- B" + H?-> Cg + nj,. He therefore placed some half-tone emul- mally opened by Rutherford.