Intermediate Physics for Medicine and Biology: Patrick Blackett and Pair Production

Friday, June 21, 2024

Patrick Blackett and Pair Production

In Chapter 15 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I describe pair production.

A photon… can produce a particle-antiparticle pair: a negative electron and a positron… Since the rest energy (m_ec²) of an electron or positron is 0.51 MeV, pair production is energetically impossible for photons below 2m_ec² = 1.02 MeV…

Pair production always takes place in the Coulomb field of another particle (usually a nucleus) that recoils to conserve momentum.

I often wonder how surprising or unexpected phenomena are discovered. Pair production was first observed by English physicist Patrick Blackett. Here is part of the entry about Blackett in Asimov’s Biographical Encyclopedia of Science & Technology.

Asimov’s Biographical
Encyclopedia of
Science & Technology.
BLACKETT, Patrick Maynard

English physicist

Born: London, November 18, 1897

Died: London, July 13, 1974

Blackett entered a naval school in 1910, at thirteen, to train as a naval officer. The outbreak of World War I came just in time to make use of him and he was at sea throughout the war, taking part in the Battle of Jutland.

With the war over, however, he resigned from the navy and went to Cambridge, where he studied under Ernest Rutherford and obtained his master’s degree in 1923. In 1933 he became professor of physics at the University of London, moving on to Manchester in 1937.

It was Blackett who first turned to the wholesale use of the Wilson cloud chamber [a box containing moist air which produces a visible track when an ion passes through it, condensing the moisture into tiny droplets]…

In 1935 Blackett showed that gamma rays, on passing through lead, sometimes disappear, giving rise to a positron and an electron. This was the first clear-cut case of the conversion of energy into matter. This confirmed the famous E = mc² equation of Einstein as precisely as did the more numerous examples, earlier observed, of the conversion of matter to energy (and even more dramatically).

During World War II, Blackett worked on the development of radar and the atomic bomb… After the war, however, he was one of those most vociferously concerned with the dangers of nuclear warfare. In 1948 he was awarded the Nobel Prize in physics for his work with and upon the Wilson cloud chamber.

More detail about the discovery of pair production specifically can be found at the Linda Hall Library website.

In 1929, Paul Dirac had predicted the possibility of antimatter, specifically anti-electrons, or positrons, as they would eventually be called. His prediction was purely a result of his relativistic quantum mechanics, and had no experimental basis, so Blackett (with the help of an Italian vsitor, Giuseppe Occhialini), went looking, again with the help of a modified cloud chamber. Blackett suspected that the newly discovered cosmic rays were particles, and not gamma rays (as Robert Millikan at Caltech maintained). Blackett thought that a cosmic particle traveling very fast might have the energy to strike a nucleus and create an electron-positron pair, as Dirac predicted. They installed a magnet around the cloud chamber, to make the particles curve, and rigged the cloud chamber to a Geiger counter, so that the camera was triggered only when the Geiger counter detected an interaction. As a result, their photographs showed interactions nearly every time. They took thousands, and by 1932, 8 of those showed what appeared to be a new particle with the mass of an electron but curving in the direction of a positively charged particle. They had discovered the positron. But while Blackett, a very careful experimenter, checked and double-checked the results, a young American working for Millikan, Carl Anderson, detected positive electrons in his cloud chamber at Caltech in August of 1932, and he published his results first, in 1933. Anderson's discovery was purely fortuitous – he did not even know of Dirac's prediction. But in 1936, Anderson received the Noble Prize in Physics, and Blackett and Occhialini did not, which irritated the British physics community no end, although Blackett never complained or showed any concern. His own Nobel Prize would come in 1948, when he was finally recognized for his break-through work in particle physics.

If you watched last summer’s hit movie Oppenheimer, you might recall a scene where a young Oppenheimer tried to poison his supervisor with a apple injected with cyanide. That supervisor was Patrick Blackett.

The Nobel Prize committee always summarizes a recipient’s contribution in a brief sentence. I’ll end with Blackett’s summary: