P M S Blackett, British physicist, at Gottingen, Germany, c 1925.
© Science Museum / Science & Society Picture Library
Blackett (1897-1974) was the son of a stockbroker and studied physics at Cambridge. During his research he took the first cloud chamber photographs of the transmutation of h nitrogen into the isotope oxygen-17. During World War II he pioneered the use of operational research to produce economies in military resources. After the war, he publicly opposed the growing role of nuclear weapons. He was awarded the Nobel Prize in 1948 for his work with G Occhialini. They constructed a cloud chamber which took a photograph when two aligned Geiger counters were triggered by vapour expansion. Blackett used this to identify the positron.