Shulou(Shulou.com)11/24 Report--

Zhao Zhongyao, a famous physicist in China, is regarded as one of the pioneers and founders of nuclear physics in China. While studying for his doctorate at the California Institute of Technology in the United States, he made an important physical discovery, that is, the abnormal absorption and special radiation of hard γ-rays in matter, which is the earliest experimental evidence for the generation and annihilation of positron pairs. it is of great significance to the development of quantum electrodynamics theory.

Hard gamma ray refers to the gamma ray whose energy is higher than that of 1MeV. It is a kind of high energy electromagnetic radiation that can penetrate matter. When hard gamma rays pass through matter, they interact with nuclei and electrons in matter, resulting in changes in the energy and direction of the rays. This change can be described by absorption coefficient and scattering cross section. The absorption coefficient represents the proportion of the energy loss of the ray in the matter per unit thickness, and the scattering cross section represents the probability of scattering between the ray and the atom of matter.

According to the Compton effect, when hard gamma rays are scattered by free electrons, part of the energy will be transferred to the electrons, so that the electrons can obtain kinetic energy, and the energy of γ photons will be reduced and the direction will be changed. Compton effect is an important result of classical electrodynamics theory, which can be used to calculate absorption coefficient and scattering cross section by Klein-Renke formula. However, in the late 1920s, quantum mechanics has begun to replace classical mechanics as the mainstream theory of physics, and quantum mechanics has a more profound explanation for the interaction between hard γ-rays and matter.

In 1928, British physicist Paul Dirac put forward an equation to describe the motion of electrons, that is, the Dirac equation. This equation accords not only with the principle of relativity but also with the principle of quantum mechanics. A surprising result of the Dirac equation is that it predicts the existence of a particle with the same mass but opposite charge as the electron, the positron. Positron is a kind of antimatter that annihilates each other when it meets electrons and emits two gamma photons with equal energy and opposite directions.

The Dirac equation also predicts the possibility that a high-energy gamma photon will split into a pair of positrons and positrons in a strong magnetic field or near a heavy nucleus. These two processes are actually reversible and both satisfy the laws of conservation of energy and momentum. However, in 1928, positrons had not been observed experimentally, so the Dirac equation was not widely recognized.

When Zhao Zhongyao went to Caltech for his doctorate, he didn't know about the Dirac equation and the existence of positrons. According to the arrangement of his mentor, Professor Milligan, he used the radioactive element thallium 208 as the radiation source to measure the absorption coefficient of hard gamma rays (energy is 2.6MeV) in different substances, and calculated with Klein-Renke formula. He found that when hard gamma rays pass through light elements, the experimental results are consistent with the theoretical formula, but when passing through heavy elements, there is an abnormal phenomenon, and the experimental results are about 40% larger than the theoretical formula. This shows that, in addition to the Compton effect, there are other mechanisms leading to the energy loss of hard gamma rays.

In order to explore the mechanism of this abnormal absorption, Zhao Zhongyao designed a new experiment to further study the scattered radiation of hard γ-rays and matter. He measured it with a high-pressure ionization chamber and a vacuum electrometer. He found that when hard gamma rays are scattered by lead, in addition to Compton scattering, there is a special kind of light radiation. The intensity of this particular radiation is roughly isotropic, and the energy of each photon is very close to the equivalent energy of an electron mass. Zhao Zhongyao wrote a paper on the experimental results and published them in the journal Physics Review in October 1930.

Zhao Zhongyao's two papers have attracted the attention of the international physics community. Many physicists begin to rethink the interaction between hard gamma rays and matter and look for evidence of the existence of positrons. In 1932, Zhao Zhongyao's classmate Anderson experimented with cosmic rays in a cloud chamber and found a track similar to the electron track but bent in the opposite direction in the magnetic field, thus discovering the positron. Anderson also observed that positrons and electrons disappear after meeting and emit two tracks in opposite directions, which is the annihilation process of positron pairs. Anderson won the Nobel Prize in Physics in 1936.

Although Zhao Zhongyao did not directly observe positrons, he was the first to find experimental signs of the annihilation process of positron pairs. The special radiation he observed were two gamma photons produced by the annihilation of positron pairs. He also observed that the abnormal absorption is the energy loss caused by the splitting of high-energy gamma photons into electron-positron pairs. His experimental results provide an important clue and basis for later discovery of positrons by Anderson and others. Zhao Zhongyao can be said to be a pioneer in the discovery of positron.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.