Nuclear Physics

1. Early nuclear physics

1.1 Alpha rays

New Zealand physicist Ernest Rutherford and British chemist Frederick Soddy were the first to show that one element can naturally radiate enough matter to turn into another. They did this in 1902, after experimenting with thorium[1][2][3].

By 1907, Rutherford had shown that the thorium was emitting alpha rays, which he identified as helium ions - helium atoms that are devoid of electrons, and are therefore positively charged[4][5][6].

Rutherford showed that you can also turn one element into another by adding alpha rays to atoms, rather than removing them, in 1919. He did this by firing alpha rays at nitrogen atoms. This produced oxygen and a positively charged particle, which Rutherford identified as a proton[7][8].

We now know that alpha rays - helium nuclei - contain two protons and two neutrons. When Rutherford fired alpha rays at nitrogen atoms, a proton was knocked out of the nitrogen, and the helium nuclei merged with what was left to make oxygen.

1.2 Neutrons

Rutherford discovered the nucleus of the atom in 1911[9], and predicted the existence of the neutron in 1920[10], shortly after he discovered the proton. The neutron was finally discovered by British physicist James Chadwick in 1932[11].

Earlier that year, French physicists Irène and Frédéric Joliot-Curie (Marie and Pierre Curie's daughter, and her husband), had been investigating a new type of neutral radiation, which had been discovered by German physicists Walther Bothe and Herbert Becker, in 1930[12].

Although Bothe and Becker had assumed this radiation was composed of gamma rays, the Joliot-Curie's found that it was composed of something that may be even more energetic[13].

Rutherford and his student, Chadwick, were convinced that this radiation was composed of neutrons, and Chadwick soon proved this by measuring the mass of the neutral particles that formed the radiation.

The discovery of neutrons explained how isotopes form. Isotopes are elements that occupy the same place in the periodic table but have slightly different masses. Soddy had first suggested the existence of isotopes in 1913[14], and it was now clear that heavier isotopes contain more neutrons.

1.3 Fission and fusion

The atom was split for the first time weeks after Chadwick's discovery of the neutron, in what would later be called nuclear fission. This was achieved by Rutherford's colleagues, Irish physicist Ernest Walton and British physicist John Cockcroft. Walton and Cockcroft fired protons at lithium atoms in order to split them into two helium nuclei[15].

Australian physicist Mark Oliphant became the first to fuse hydrogen isotopes together in 1934[16]. Oliphant fused 'heavy' hydrogen nuclei (hydrogen nuclei containing one proton and one neutron, rather than just one proton), and this created helium nuclei.

2. Nuclear energy

2.1 Nuclear binding energy

British physicist Robert d'Escourt Atkinson and Dutch-Austrian-German physicist Fritz Houtermans had first suggested that a large amount of energy could be released by fusing small nuclei together in 1929, three years before the discovery of the neutron[17].

It was later shown that both fusion and fission release energy[18][19]. Energy is produced as long as the new nuclei are more tightly bound, and hence more stable, than the old. Whether fusion or fission makes the nucleus more stable depends on the binding energy per proton and neutron.

Binding energy is the energy required to keep two protons or neutrons bound together, and the same amount of energy is needed to tear them apart. This was later explained by quantum chromodynamics, a quantum field theory of the strong nuclear force.

The existence of binding energy means that the total mass of a proton and neutron combined is less than the mass of an individual proton plus an individual neutron.

The binding energy adds to the mass of the nucleus because energy is related to mass via German-Swiss-American physicist Albert Einstein's theory of special relativity[20]. Special relativity states that the rest mass (m0) of an object equals e/c2, since e = m0c2, where e is energy, and c is the speed of light.

The binding energy per proton and neutron varies, depending on how many protons and neutrons make up the nucleus. The most stable nucleus is iron, elements heavier than this undergo fission in order to become more stable, and lighter elements undergo fusion.

When an atom becomes more stable, it moves to a lower energy state, and so energy is released.

Plot of atomic weight against binding energy per neutron. Energy is release from fusion for atoms less massive than iron, and from fission in atoms that are more massive.

The binding energy of different elements. Image credit: modified by Helen Klus, original image by Persino/CC-A.

2.2 Nuclear chain reactions

In 1934, American physicist Leo Szilard realised that neutrons could be used to mediate a self-sustaining nuclear chain reaction, which would be able to generate vast amounts of energy[21]. This could happen if a nuclear reaction released high velocity neutrons that went on to collide with other atoms, splitting them and releasing yet more neutrons.

Italian physicist Enrico Fermi claimed to have created the new heavy elements ausonium (now known as neptunium) and hesperium (now known as plutonium) by firing neutrons at lighter elements in 1934[22].

Influenced by Fermi's results, German chemists Otto Hahn and Fritz Strassmann began performing similar experiments in Berlin. By 1938, they had created a new heavy element, barium, after bombarding uranium with neutrons[23].

Austrian physicist Lise Meitner showed that this provided evidence of nuclear fission[24], and Austrian-British physicist Otto Robert Frisch confirmed this experimentally in January of 1939[25].

That same year, German-American physicist Hans Bethe[26] and Indian-American physicist Subrahmanyan Chandrasekhar[27] showed how stars are fuelled by nuclear fusion chain reactions.

Diagram showing a chain reaction, where a fission reaction produces neutrons, which collide with other heavy elements, causing more fission, and more neutron collisions.

Fission chain reaction of uranium, which splits into krypton, barium, and three separate neutrons. Image credit: modified by Helen Klus, original image by Fastfission/Public domain.

Diagram showing how hydrogen fuses to make helium.

The nuclear fusion chain reaction that occurs in stars, known as the proton-proton chain. Image credit: Borb/CC-A.

In 1956, Japanese-American physicist Paul Kuroda suggested that nuclear fission might exist elsewhere in nature, since nuclear chain reactions only require natural materials[28]. This was proven in 1972, when evidence of natural self-sustaining nuclear chain reactions was found in uranium mines in Oklo, Gabon. The nuclear reaction is thought to have occurred about 1.5 billion years ago[29].

2.3 The Manhattan Project

Szilard and Fermi had moved to Manhattan, New York, in 1938. After hearing of Hahn and Strassmann's fission experiments, Szilard, Fermi, and American physicist Herbert Anderson successfully showed that uranium could mediate a nuclear chain reaction the following year[30].

This discovery prompted Szilard to write to other scientists, and ask them to refrain from publishing work on nuclear physics in case the Nazi government became aware of the possibilities. Not everyone agreed, and so Szilard drafted a letter, warning that Nazi Germany might be attempting to build an atomic bomb. This was signed by Einstein, and delivered to American President Franklin D. Roosevelt, on 2nd August 1939[31].

Szilard's letter resulted in the Manhattan Project, a top-secret research and development project that began in 1942. Szilard and Fermi created the first artificial nuclear chain reaction that year, with Chicago Pile-1, the first nuclear reactor.

The first atomic bomb was detonated in the Trinity Test, which was conducted in the New Mexico desert in July 1945. In August of that year, two more atomic bombs, one created from uranium fission and one from plutonium, were detonated over the Japanese cities of Hiroshima and Nagasaki.

After World War II, many countries developed both nuclear weapons and nuclear power plants.

2.4 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. Electricity was generated by a nuclear reactor - Experimental Breeder Reactor-I - for the first time in 1951.

Diagram showing how nuclear material produces steam, which drives a turbine.

Diagram of a nuclear power plant. Image credit: U.S.NRC./Public domain.

Nuclear energy is controversial because it is a sustainable energy source, however it 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[32].

3. References

  1. Rutherford, E. and Soddy, F., 1902, 'The radioactivity of thorium compounds. I. An investigation of the radioactive emanation', Journal of the Chemical Society, 81, pp.321-350.

  2. Rutherford, E. and Soddy, F., 1902, 'The radioactivity of thorium compounds. II. The cause and nature of radioactivity', Journal of the Chemical Society, 81, pp.837-860.

  3. Mlađenović, M., 1992, 'The History of Early Nuclear Physics (1896-1931)', World Scientific.

  4. Rutherford, E. and Soddy, F., 1903, 'A comparative study of the radioactivity of radium and thorium', The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 5, pp.445-457.

  5. Rutherford, E. and Soddy, F., 1903, 'Radioactive change', Philosophical Magazine Series 6, 5, pp.576-591.

  6. Rutherford, E., 1907, 'The velocity and energy of the alpha particles from radioactive substances', The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 13, pp.110-117.

  7. Rutherford, E., 1919,, 'Collision of alpha Particles with Light Atoms IV. An Anomalous Effect in Nitrogen', The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 37, pp.581-587.

  8. Soddy, F., 1920, 'Name for the Positive Nucleus', Nature, 106, pp.502-503.

  9. Rutherford, E., 1911, 'The scattering of alpha and beta particles by matter and the structure of the atom', The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 21, pp.669-688.

  10. Rutherford, E., 1920, 'Bakerian Lecture. Nuclear constitution of atoms', Proceedings of the Royal Society of London, Series A, 97, pp.374-400.

  11. Chadwick, J., 1932, 'The existence of a neutron', Proceedings of the Royal Society of London, Series A, 136, pp.692-708.

  12. Bothe, W. and Becker, H., 1930, 'Künstliche Erregung von Kern-γ-Strahlen' ('Artificial excitation of nuclear gamma rays'), Zeitschrift für Physik, 66, pp.289-306.

  13. Curie, I., 1932, 'Sur le rayonnement γ nucléaire excité dans le glucinium et dans le lithium par les rayons α du polonium' ('On the nuclear gamma radiation excited in beryllium and lithium by alpha rays of polonium'), CR Acad. Sci. Paris, 193, pp.1412-1214.

  14. Soddy, F., 1913, 'Intra-atomic charge', Nature, 92, pp.399-400.

  15. Cockcroft, J. D. and Walton, E. T. S., 1932, 'Experiments with high velocity positive ions. II. The disintegration of elements by high velocity protons', Proceedings of the Royal Society of London, Series A, 137, pp.229-242.

  16. Oliphant, M. L. E., Harteck, P., and Rutherford, L., 1934, 'Transmutation effects observed with heavy hydrogen', Proceedings of the Royal Society of London, Series A, 144, pp.692-703.

  17. D'E Atkinson, R. and Houtermans, F. G., 1929, 'Transmutation of the Lighter Elements in Stars', Nature, 123, pp.567-568.

  18. Weizsäcker, C. V., 1935, 'Zur theorie der kernmassen' ('The theory of nuclear masses'), Zeitschrift für Physik A Hadrons and Nuclei, 96, pp.431-458.

  19. Bethe, H. A. and Bacher, R. F., 1936, 'Nuclear Physics A. Stationary States of Nuclei', Reviews of Modern Physics, 8, pp.82.

  20. Einstein, A., 1905, 'On the electrodynamics of moving bodies', Annalen der Physik, 17, pp.891-921, reprinted in in 'The Principle of Relativity: A Collection of Original Memoirs on the Special and General Theory of Relativity', 1920, Courier Corporation.

  21. Szilard, L. and Chalmers, T. A., 1934, 'Detection of Neutrons Liberated from Beryllium by Gamma Rays: a New Technique for Inducing Radioactivity', Nature, 134, pp.494–495.

  22. Fermi, E., 1934, 'Possible production of elements of atomic number higher than 92', Nature, 133, pp.898-899.

  23. Hahn, O. and Strassmann, F., 1939, 'Über den Nachweis und das Verhalten der bei der Bestrahlung des Urans mittels Neutronen entstehenden Erdalkalimetalle' ('On the detection and characteristics of the alkaline earth metals formed by irradiation of uranium with neutrons'), Naturwissenschaften, 27, pp.11-15.

  24. Meitner, L. and Frisch, O. R., 1939, 'Products of the fission of the uranium nucleus', Nature, 143, pp.1939.

  25. Frisch, O. R., 1939, 'Physical evidence for the division of heavy nuclei under neutron bombardment', Nature, 143, pp.276-276.

  26. Bethe, H. A., 1939, 'Energy production in stars', Physical Review, 55, pp.434-456.

  27. Chandrasekhar, S., 1939, 'The Dynamics of Stellar Systems. I-VIII', The Astrophysical Journal, 90, pp.1-50.

  28. Kuroda, P. K., 1956, 'On the nuclear physical stability of the uranium minerals', The Journal of Chemical Physics, 25, pp.781-782.

  29. Neuilly, M., et al, 1972, 'Existence of a chain fission nuclear reaction in the distant past in the Oklo (Galbon) uranium deposit', Compt. Rend., Ser. D, 275, pp.1847-1849.

  30. Anderson, H. L., Fermi, E. and Szilard, L., 1939, 'Neutron production and absorption in uranium', Physical Review, 56, pp.284.

  31. Einstein, A., 1939, 'Einstein's Letter to President Roosevelt'.

  32. ESA, 'Helium-3 mining on the lunar surface', last accessed 15-02-16.

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