Nuclear

Part 9 - Positrons

The year 1932 was very productive for nuclear physics in the Cavendish Laboratory, with discoveries of the neutron, artificial nuclear disintegration, by the Cockcroft–Walton particle accelerator, and the positron.

In 1928, in Britain, Paul Dirac introduced the Dirac equation, a unification of quantum mechanics, Einstein's special relativity and a new concept of electron spin, which allowed electrons to have either positive or negative energy as solutions. Electrons could have both a positive and negative charge!

Hermann Klaus Hugo Weyl, was one of the most influential mathematicians of the twentieth century. He was one of the first to combine general relativity with the laws of electromagnetism and his research significantly influenced theoretical physics on space, time and matter. In 1929, he proposed an equation to replace the Dirac equation which described massless fermions; quasi-particles that behave in a form of crystals known as Weyl semi-metals. They were discovered in 2015.

Hermann Weyl's mathematical implications of the negative energy solution puzzled Dirac. The positive-energy solution to the Dirac equation explained experimental results, but the negative-energy solution allowed by the mathematical model was equally valid. Classical mechanics permitted the negative energy solution to be simply ignored, but this was not allowed in quantum mechanics; the dual solution made it possible for an electron to spontaneously jump between positive and negative energy states but no such transition had ever been experimentally observed.

In December 1929, Dirac attempted to explain the negative-energy solution for the relativistic electron but the American physicist, Robert Oppenheimer argued strongly against this and, in 1931, Dirac was persuaded to predicted the existence of an "anti-electron". It would have the opposite charge to the electron but the same mass, and it would mutually annihilate an electron on contact!

Cloud Chambers

The Scottish physicist, Charles Thomson Rees Wilson, began developing expansion chambers to study optical phenomena in moist air in 1894. He discovered that ions could act as centres for water droplet formation and perfected the first cloud chamber in 1911. The air inside a sealed chamber was saturated with water vapour, and he used a diaphragm to expand the air inside the chamber thereby cooling the air which caused the water vapour to condense. When an ionizing particle passed through the chamber, water vapour condensed on the resulting ions and the trail of the particle was visible in the vapour cloud. Wilson, along with Arthur Compton, received the Nobel Prize in Physics in 1927 for his work on the cloud chamber.

Patrick Blackett improved the design with a stiff spring to expand and compress the chamber very rapidly, making the chamber sensitive to particles several times a second. A cine film was used to record the images.

Wilson Cloud chambers showed the track of ionizing radiation and were important particle detectors from the 1920's to the 1950's, when they were superseded by bubble chambers. They were used in the discoveries of the positron in 1932, the muon in 1936, both by Carl Anderson (awarded a Nobel Prize in Physics in 1936), and the kaon by George Rochester and Clifford Charles Butler in 1947. In each case, cosmic rays were the source of ionizing radiation.

In St. Louis, USA, in 1923, Arthur Holly Compton recorded the scattering of a photon by an interaction with a charged particle; typically an electron. If it causes a decrease in energy (a decreased frequency) of the photon (typically an X-ray or gamma ray photon), where part of the energy of the photon is transferred to the recoiling electron, it is known as the Compton effect. Inverse Compton scattering occurs when a charged particle transfers part of its energy to a photon. The effect is significant because it demonstrates that light cannot be explained purely as a wave phenomenon. Compton earned the 1927 Nobel Prize in Physics for the discovery.

Starting in 1923, Dmitri Vladimirovich Skobeltsyn, a Soviet physicist, pioneered the use of the cloud chamber to study the Compton effect. Skobeltsyn adding a magnetic field to his cloud chamber and discovered high energy, charged particle cosmic rays which led to the discovery of the positron.

Carl David Anderson discovered the positively charged electron (positron) on August 2, 1932, for which he won the Nobel Prize for Physics in 1936. This was the first evidence of anti-matter and was discovered when Anderson allowed cosmic rays to pass through a cloud chamber and a lead plate. A magnet surrounded his apparatus, causing particles to bend in different directions based on their electric charge. The ion trail left by each positron appeared on the photographic plate with a curvature matching the mass-to-charge ratio of an electron, but in a direction that showed its charge was positive.

Anderson credited Skobeltsyn and Chung-Yao Chao for their contribution and acknowledged that Frédéric and Irène Joliot-Curie in Paris had evidence of positrons in old photographs, when Anderson's results came out, but they had dismissed them as protons.

Patrick Blackett and Giuseppe Occhialini at the Cavendish Laboratory in Britain had also recorded the positron in 1932 but they had delayed publication, to obtain more solid evidence, so Anderson was credited with the official first discovery.

The positron or anti-electron is the anti-particle or the anti-matter counterpart of the electron. The positron has an electric charge of +1 e, a spin of 1/2 (the same as the electron), and has the same mass as an electron. When a positron collides with an electron, both are annihilated and if the collision occurs at low energies, it results in the production of two or more photons. (It is now the basis of Positron Emission Tomography (PET) used for cancer treatment and research).

Positrons can be created by positron emission radioactive decay (through weak interactions), or by pair production from a sufficiently energetic photon interacting with an atom in a material.