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Contents 1 a Brief History of Nuclear Physics PHY401 - Nuclear and Particle Physics Monsoon Semester 2020 Dr. Anosh Joseph, IISER Mohali LECTURE 07 Monday, September 7, 2020 (Note: This is an online lecture due to COVID-19 interruption.) Contents 1 A Brief History of Nuclear Physics 1 1 A Brief History of Nuclear Physics 1896: Discovery of radioactivity by Henri Becquerel. This marked the beginning of the field of nuclear physics. Becquerel studied the radiation emitted by phosphorescent materials. He was intrigued by Roentgen’s recent discovery of X-rays and looked for X-rays in Uranium salts. But the unexpected happened. Becquerel discovered that these salts emit a new form of radiation, different from both phos- phorescent light and X-rays. He called them Uranic rays. 1898: Separation of Radium by Marie and Pierre Curie; Discovery of α, β and γ rays While Becquerel went on to do research in atomic physics, Marie Curie was interested in his discovery of the Uranic rays and began to investigate them systematically. Soon afterwards, her husband, Pierre Curie, joined her in this research. Their studies led them to propose that the radiation was emitted from single atoms. These ideas, based on the not yet fully confirmed theory of atomic structure of the elements, led them to the discovery of new elements Polonium and Radium. They showed that other elements besides Uranium emitted such rays. They introduced the term radioactivity. 1911: Rutherford scattering - Theoretical picture of an atom with nucleus as a central part. PHY401 - Nuclear and Particle Physics Monsoon Semester 2020 Rutherford observed alpha particles under go large back angle scatterings when they hit a gold foil. The experiment suggested that most of the mass inside an atom was concentrated in a tiny region called the nucleus. Rutherford’s atom was made up of a nucleus of Z positive charges and also A − Z pairs of positive and negative charges surrounded by a sphere of Z uniformly distributed electrons. This discovery of the atomic nucleus would have far-reaching impact not only in physics, but also in war and politics. Rutherford’s nuclear model pointed the way to the new world of modern physics. But it was Niels Bohr who opened its door. In 1913, Bohr constructed a dynamical model of the hydrogen atom with an electron circulating a hydrogen nucleus, (which later acquired the name proton) in stable orbits called stationary states. By allowing the electron to emit light only when it jumps between these stationary states, Bohr was able to explain the known energies of light emitted by excited hydrogen atoms. Bohr’s model was soon developed by others into a mathematical formulation called Quantum Mechanics. This theory and Einstein’s theory of relativity provide the conceptual basis for the theoretical description of all physical phenomena known to us today. 1913: Frederick Soddy (English radiochemist) and Theodore Richards (American chemist) elu- cidate the concept of nuclear mass: isotopes are born 1915: William Harkins (American chemist) notes that the mass of a helium atom is, in fact, not exactly four times that of a proton. It is slightly less. He states that the excess mass has been converted to energy via Einstein’s E = mc2 relation and that this is the source of nuclear energy. 1919: Rutherford carries out first transmutation (He + N ! p + O). By 1920, there were only six elements missing from the periodic table, which at that time ended with Uranium(Z = 92). All six missing elements would be discovered in the next 30 years. 1923: George de Hevesy (Hungarian chemist) uses radioactive tracers to study chemical pro- cesses such as in the metabolism of animals. 1928: Theory of alpha decay by George Gamow. One of the early successes of quantum mechanics was its explanation of alpha decay. The idea is that the α-particle, held inside the nucleus by a potential barrier caused by the positive nuclear charges, cannot escape from it, according to classical physics. However, quantum mechanics does allow the α-particle to escape by “tunneling” through the barrier, with an energy-dependent half-life consistent with experiment. 1929: First cyclotron (Ernest Lawrence). A cyclotron is a type of particle accelerator. 2 / 7 PHY401 - Nuclear and Particle Physics Monsoon Semester 2020 V(r) B Coulomb barrier wave function representing α particle in the nucleus ikr −ikr Ψ0(r < R) = Ae + Be Ψ0(r < R) γ e− Emitted α particle E(α) R b r −U Figure 1: Gamow’s theory of alpha decay. A representation of α particle as a wave function. The amplitude of the wave function decreases after tunneling through it. It accelerates charged particles outwards from the center of a flat cylindrical vacuum chamber along a spiral path. The particles are held to a spiral trajectory by a static magnetic field and accelerated by a rapidly varying (radio frequency) electric field. Magnetic field bends path of B each charged particle Square wave electric field accelerates charge at each gap crosing e+ Figure 2: Cyclotron. Lawrence was awarded the 1939 Nobel Prize in Physics for this invention. 1930: Pauli predicts neutrino; Dirac predicts antimatter. Pauli proposed that the deficit between the maximum and the actual energies of the emitted electron is carried away by a new particle called neutrino. This postulate was readily accepted when Fermi succeeded in explaining the continuous β- spectrum with its help. 3 / 7 PHY401 - Nuclear and Particle Physics Monsoon Semester 2020 1932: Discovery of the neutron by James Chadwick; Discovery of positrons by Carl Anderson English physicist James Chadwick bombards beryllium with α-particles to knock out free neu- trons, and thus becomes the first physicist to detect neutrons directly. American physicist Carl Anderson is studying cosmic rays when he notices some tracks on his photographic plates that look exactly like electron tracks except that they are curving in the wrong direction. He realizes that that he has discovered a positively-charged electron, i.e., the anti-electron pre- dicted by Dirac. Anderson calls the new particle a positron. Figure 3: Cloud chamber photograph by Carl Anderson of the first positron ever identified. The chamber is separated by a 6 mm lead plate.The deflection and direction of the particle’s ion trail indicate that the particle is a positron. 1934: Fermi theory of beta decay; Walter Baade and Fritz Zwicky predict neutron stars. 1935: Yukawa predicts nuclear (strong) force through meson exchange. Japanese physicist Hideki Yukawa proposes that the neutrons and protons in atomic nuclei are held together by an intensely powerful force, which he calls the strong force. Working with the Dirac theory, he realizes that the fundamental forces must be carried by quanta, that is, they cannot exist as classical “lines” of force. The only way for such quanta to exist and still be compatible with classical physics is if they “steal” their energy by popping in and out of existence so fast that conservation of energy is not violated, since it is masked by the Heisenberg uncertainty principle. In other words, the uncertainty principle applies even to empty space – how do we know space is truly “empty”, when the principle will not let you measure its energy exactly? Yukawa predicts that the strong force is “carried” by what he calls an “exchange particle.” From the known sizes of atoms, and by assuming that the exchange particle usually moves near the speed of light, he calculated that it should have a mass about 200 times that of the electron. 1936: John Lawrence treats leukemia with Phosphorus-32. 4 / 7 PHY401 - Nuclear and Particle Physics Monsoon Semester 2020 1938: Stars are powered by nuclear fusion (George Gamow, Carl von Weizsaecker, Hans Bethe): p-p and CNO chain reactions. 1939: Nuclear fission (Otto Hahn, Fritz Strassman, Lise Meitner, Otto Frisch); Niels Bohr, John Wheeler explain fission. Austrian physicists Hahn and Meitner bombard Uranium with neutrons and discover nuclear fission. 1940: McMillan and Abelson produce a new element, Neptunium ( n + 238U ! 239U !239Np !238Pu). McMillan received the Nobel prize in 1951 for this. 1942: First self-sustaining fission reaction (Fermi); Chicago Pile-1 (CP-1): World’s first artificial nuclear reactor. Manhattan project (Oppenheimer). 1945: Atomic bomb. After World War II, many countries developed both nuclear weapons and nuclear power plants. In a nuclear power plant, a nuclear reactor creates a controlled nuclear fission chain reaction. This produces heat, which is used to heat water, creating steam. The steam then drives a turbine, which is connected to an electric generator. Nuclear energy from fission is controversial because the production of this sustainable energy uses dangerous materials, and accidents can have long term and tragic consequences. This was highlighted by the Chernobyl accident of 1986 and the Fukushima Daiichi accident of 2011. These problems do not occur with nuclear fusion, as none of the materials are radioactive. This can be achieved with the isotope Helium-3, but Helium-3 is too rare on Earth to be useful. 1947: π meson discovered (by studying cosmic ray tracks). 1948: Big Bang nucleosynthesis (Alpher, Bethe, Gamow). Electricity generated at the X − 10 Graphite Reactor in Oak Ridge. This was world’s second nuclear reactor. 1949: Nuclear shell model (Mayer, Jensen). The nuclear shell model is a model of the atomic nucleus which uses the Pauli exclusion principle to describe the structure of the nucleus in terms of energy levels. 1951: Nuclear collective model (Bohr, Mottelson, Rainwater). In addition to individual nucleons changing orbits to create excited states of the nucleus as described by the shell model, there are nuclear transitions that involve many (if not all) of the nucleons. Since these nucleons are acting together, their properties are called collective, and their transi- tions are described by a collective model of nuclear structure.
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