Patrick Maynard Stuart Blackett Biography

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World of Scientific Discovery on Patrick Maynard Stuart Blackett

Blackett was born in London on November 18, 1897. He attended the naval colleges at Dartmouth and Osborne and, when World War I began, joined the Royal Navy. At the conclusion of the war, Blackett enrolled in Magdalene College at Cambridge University, where he studied physics under Ernest Rutherford. He received his bachelor's degree in 1921 and his master's in 1923.

Blackett's earliest research involved the use of C. T. R. Wilson 's cloud chamber, invented in 1911. The cloud chamber contained air saturated with a vapor. When the chamber was suddenly expanded, the vapor cooled and condensed on any charged particles present in the chamber. Since radiation produces such particles in its "wake," the cloud chamber created a vapor trail surrounding the path of the radiation which could be photographed.

The problem with Wilson's original design was that if the chamber was not expanded at the same time radiation happened to pass through it, no trail was created, so an enormous number of photographs had to be taken on a "hit-or-miss" basis to record one radiation trail.

In 1925, Blackett used the cloud chamber to restudy a famous experiment conducted by Rutherford in 1919. In that experiment, Rutherford bombarded nitrogen gas with alpha particles. Rutherford was convinced that nitrogen atoms struck by alpha particles were converted to oxygen atoms and protons. Unfortunately, he had only the crudest of detection devices--a scintillation counter--with which to analyze this reaction.

Blackett modified Wilson's cloud chamber design so that expansion took place automatically every fifteen seconds. The chance of capturing an event in the chamber increased dramatically. One result of this modification was that Blackett was able to produce photographs that clearly supported Rutherford's interpretation of the alpha-nitrogen experiment. He took more than 20,000 photographs showing more than 400,000 alpha particle tracks. Among these were eight that showed conclusively that nitrogen had been transmuted into oxygen.

In the early 1930s, Blackett turned his attention to cosmic rays. Working with G. P. S. Occhialini, he used the cloud chamber to analyze reactions produced by these rays. Again, the problem was that, of all the photographs taken with the cloud chamber, only a small fraction (less than 5 percent) actually recorded cosmic ray events.

To solve this problem, Blackett and Occhialini placed the cloud chamber between two Geiger counters wired to each other. The circuit was set up so that any cosmic ray that passed through both Geiger counters also had to pass through the cloud chamber. When the two Geiger counters both recorded a cosmic ray, they immediately activated the cloud chamber. The system operated so quickly that tracks produced in the cloud chamber cloud could still be photographed after the cosmic ray had activated the Geiger counters. Using this device, Blackett and Occhialini were able to record cosmic ray events on 80% of the cloud chamber photographs they took.

By late 1932, Blackett and Occhialini had obtained hundreds of cosmic ray photographs. One of the most common events they observed was a large number of tracks all radiating from a single point in the chamber. They named this phenomenon a cosmic ray shower. About half of the particles in a shower were electrons. The other half were particles with the same mass as the electron, but with a positive charge. Blackett and Occhialini concluded that they were the positrons originally hypothesized by Paul Adrien Maurice Dirac in 1930 and discovered by Carl David Anderson only a few months before Blackett's observation. They were convinced that electron-positron pairs were produced when high energy gamma rays pass through matter. The calculations they made for this conversion corresponded to the predictions of mass-energy equivalence (E = mc²) made by Albert Einstein in 1905.

During World War II, Blackett made important contributions to the development of radar and the atomic bomb. After the war, he became active in efforts to limit the spread of nuclear weapons technology. He received the Nobel Prize in 1948 for his improvements of the cloud chamber and the discoveries resulting from its use. He died in London on July 13, 1974.