The British physicist Patrick M. S. Blackett (1897-1974) used a modified Wilson cloud chamber to obtain the first photographs of the tracks left by the particles involved in a nuclear disintegration as well as those produced by showers of cosmic rays.

Patrick Maynard Stuart Blackett was born on November 18, 1897, in London, England. At age 13 he entered naval training school, graduating four years later at the outbreak of World War I. He served in the Royal Navy as an officer throughout the war, participating in the Battles of Jutland and the Falkland Islands. In 1919 Blackett resigned his naval commission to study physics at Cambridge University. There he met, and eventually worked under, the physicist Ernest Rutherford.

The opening of Blackett's scientific career coincided with two important developments in physics. The first was the work of C. T. R. Wilson, who had built a small cloud chamber in which the paths taken by atomic and subatomic particles could be momentarily viewed with the naked eye or photographed. As particles moved through the supersaturated air of the chamber water droplets were formed, marking their trails. The second was Rutherford's investigation of nuclear disintegration. Rutherford directed alpha particles (nuclei of helium atoms) emitted by radioactive materials into various gasses with the intention of disintegrating the nuclei of the gas molecules. When alpha particles sped through nitrogen they occasionally collided with nitrogen nuclei, knocking out protons and transmutating the nitrogen into oxygen. Rutherford was able to prove that alpha particles and nuclei had collided, but he was unable to determine precisely what occurred during the process of collision. He assigned Blackett the task of obtaining visible cloud chamber evidence of what happened before, after, and at the moment of impact of alpha particles and nuclei.

Blackett altered the cloud chamber so that it was capable of automatically taking 270 photographs per hour of the condensation trails left by particles moving through it. With this apparatus he obtained 23,000 photographs, which contained 415,000 alpha particle tracks. Of the 415,000 tracks, only eight revealed that an alpha particle had collided with a nitrogen molecule. By a careful study of these tracks Blackett was able to show that in each collision two particles emerged. The first particle was a proton; the second a heavy isotope of oxygen. These experiments, which took place in 1924, provided the first photographic evidence of the process of nuclear transformation.

Early in the 1930s Blackett turned from the analysis of alpha particle tracks in cloud chambers to the study of cosmic rays (charged particles coming from outer space). This change was precipitated by the arrival in England of the Italian physicist Giuseppe Occhialini, who brought with him new techniques for counting the incidence of cosmic rays. Blackett and Occhialini, working as a team, devised an apparatus that recorded the random appearance of cosmic rays. This new piece of equipment used Geiger counters to detect the rays and activate the cloud chamber so that the condensation tracks formed would be photographed automatically. Blackett's cosmic ray photographs revealed that this natural phenomenon manifested itself in showers of roughly equal numbers of positively and negatively charged particles. Further investigation of cosmic rays enabled Blackett to confirm Carl Anderson's earlier discovery of the positron. In 1948 Blackett was awarded the Nobel Prize in physics for his improvement of the Wilson cloud chamber and its uses in the fields of nuclear physics and cosmic ray study.

Blackett's inquiries into the nature of cosmic rays led him to become interested in astronomy and astrophysics and finally in terrestrial magnetism. For Blackett, the study of the earth's magnetic field provided a means of exploring the geological history of the planet. Using a sensitive magnetometer of his own design he was able to measure changes in the earth's magnetic field over a period of 500 million years as compared to the 400 year period previously studied. Blackett's work in paleomagnetism, as well as his cloud chamber experiments, resulted in his winning the Copley medal of the Royal Society of London in 1956.

Blackett combined a busy life in science with a full involvement in educational, military, and political affairs. He is especially remembered for his 1948 book, Fear, War, and the Bomb; Military and Political Consequences of Atomic Energy, in which he argued that the atomic bomb was not a decisive weapon and warned that a preventive war might erupt because America's large stockpile of atomic weapons posed a threat to the Soviet Union.

Further Reading

• For a scholarly study of Blackett's scientific work see: Sir Bernard Lovell, "Patrick Maynard Stuart Blackett," in Biographical Memoirs of Fellows of the Royal Society, Vol. 21 (1975). On a more popular level Blackett is discussed in Emilio Segré, From X-rays to Quarks: Modern Physicists and Their Discoveries (1980). Blackett's ideas on atomic weapons are to be found in his three books: Fear, War, and the Bomb: Military and Political Consequences of Atomic Energy (1949); Atomic Weapons and East-West Relations (1956); and Studies of War (1962).