**PART XIV – NEWER DEVELOPMENTS IN ATOMIC AND NUCLEAR THEORY**

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**Chapter 81: Mesons – Cecil Frank Powell (1905 – 1991)**

**Mesons. **In 1947 Powell and Occhialini discovered pion tracks on special photographic plates exposed to cosmic rays. Powell received the Nobel Prize in 1950 for developing special photographic techniques for the study of cosmic rays and applying the techniques to the analysis of mesons found in such rays. This discovery confirmed Yukawa’s theory about the nature of nuclear force.

**Chapter 82: The Antiproton – Emilio Segrè (1905 – 1989) and Owen Chamberlain (1920 – 2006)**

**Antiprotons. **In 1955 Segrè and Chamberlain discovered the antiproton for which they received the Nobel prize in 1959. The antiproton was predicted by Dirac’s theory, but to produce it required vastly more energy, over a six-billion-volt proton as a bombarding particle. Collisions at this energy produced some 40,000 other particles. The recognition of antiproton required precise alignment of detectors and counters along with the demonstration that these particles annihilate protons and neutrons. The existence of the antinucleon greatly strengthens the belief of physicists that antimatter exists as the normal state of things in a different part of our universe.

**Chapter 83: Nuclear Magnetic Moment – Isidor I. Rabi (1898 – 1988)**

**Quantization in a Gyrating Magnetic Field. **I. I. Rabi developed the most precise and elegant method for measuring the size of the magnetic moment of a nucleus that was needed to construct a nuclear model. His starting point was the Stern and Gerlach experiment to which he added a longer path and auxiliary fields that could rotate and oscillate at adjustable frequencies. This finally led to the molecular beam resonance method that could precisely determine the magnetic moments of nuclei. His experiments won him a Nobel prize in physics in 1944.

**Chapter 84: Hydrogen and the Elementary Particles – Willis E. Lamb, Jr. (1913 – 2008)**

**Fine Structure of the Hydrogen Atom. **In 1947, Lamb designed a very ingenious and beautiful experiment, based on microwave techniques, to analyze the fine structure of the hydrogen lines for n = 2. The experiment showed that there is a 1000 megacycle-per-second separation between the 2S½ and 2P½ levels, in disagreement with the prediction of Dirac’s theory. This remarkable experiment led to the mass renormalization theories of Bethe, Schwinger, Feynman and Tomonaga, and indicated how the Dirac theory must be corrected to conform to the observed results. Lamb won the Nobel Prize in Physics in 1955 “for his discoveries concerning the fine structure of the hydrogen spectrum.”** **

**Chapter 85: Magnetic Moment of the Electron – Polykarp Kusch (1911 – 1993)**

**Magnetic Moment of the Electron. **Another discrepancy from Dirac’s theory detected experimentally was the value of the magnetic moment of the electron. It became clear that the intrinsic magnetic moment of the electron must differ from 1 Bohr magneton by about 1%. This suggested the need of a very precise determination for g-factor associated with spin of the electron. This was undertaken by Kusch. The agreement was about 1 part in a billion. This result is extremely important since it demonstrates the high degree of accuracy of the improved quantum electrodynamics in analyzing the interaction of an electron and an electromagnetic field.

**Chapter 86: High Energy Physics – Hans Bethe (1906 – 2005), Julian Schwinger (1918 – 1994) and Richard Feynman (1918 – 1988)**

**The Electromagnetic Shift of Energy Levels. **An error in the Dirac theory arises because it regards the electron as a point without a surrounding radiation field. There is therefore no limit as to how energetic the photons may be with which the electron could interact. This is equivalent to saying that the interaction of the electron with the radiation field surrounding it leads to an infinite correction to its mass. Bethe was the first to obtain a fairly accurate value by an approximate non-relativistic method. Schwinger and Feynman then independently came up with a precise relativistic procedure for mass and charge renormalization.

**Chapter 87: The Nuclear Shell – Johannes D. Jensen (1907 – 1973)**

**The History of the theory of Structure of The Atomic Nucleus. **There is a nuclear shell structure similar to the electronic shell structure. For electronic shells the numbers of electrons that completely fill the shells are: 2, 8, 18, 32, etc. For nucleon shells such numbers for neutrons or protons are: 2, 8, 20, 28, 50, 82, 126, and so on. When these nucleon shells are completely filled, we get an extremely stable and abundant nucleus. It was for shell structure theory of the nucleus that Jensen shared the 1963 Nobel Prize.

**Chapter 88: Radiocarbon Dating – Willard F. Libby (1908 – 1980)**

**Radiocarbon Dating. **Libby discovered C^{14}, with a half-life of 5,568 years, as the radioactive substance that could be used to date substances in the organic world. The method depends on the fact that all samples of atmospheric carbon dioxide are radioactive and consequently all plants, animals and humans are radioactive in a balanced way. When death occurs the balance immediately ceases, and the radiocarbon atoms become fewer and fewer as time goes on. Libby was honored by the Nobel Prize in chemistry for 1960 for his development of the C^{14} dating techniques.

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