How We Came to Know the Cosmos: Light & Matter

Discover How We Came to Know the Cosmos

Chapter 14. Nuclear Physics

14.1 History of early nuclear physics

14.1.1 Positively charged alpha rays

Ernest Rutherford and the 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.[1-3]

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

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 ions - 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.

14.1.2 Discovery of the neutron

Rutherford discovered the nucleus of the atom in 1911[9] (discussed in Chapter 9) and predicted the existence of the neutron in 1920.[10] The neutron was finally discovered by the British physicist James Chadwick in 1932.[11]

Earlier that year, the 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 the 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 predicted the existence of isotopes in 1913,[14] and it was now clear that heavier isotopes contain more neutrons.

14.1.3 Nuclear fission and nuclear 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, the Irish physicist Ernest Walton and the British physicist John Cockcroft. Walton and Cockcroft fired protons at lithium atoms to split them into two helium nuclei.[15]

The 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.

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

Figure 14.1
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The fission chain reaction of uranium, which splits into krypton, barium, and three separate neutrons.

A diagram showing how hydrogen fuses to make helium.

Figure 14.2
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The nuclear fusion chain reaction that occurs in stars, known as the proton-proton chain.

14.2 History of nuclear energy

14.2.1 Nuclear binding energy

The 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 (discussed in Chapter 23).

A plot of atomic weight against binding energy per neutron. Energy is released from fusion for atoms less massive than iron, and from fission in more massive atoms.

Figure 14.3
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The binding energy of different elements.

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 Albert Einstein’s theory of special relativity[20] (discussed in Book I). Special relativity states that the mass (m) of an object equals E/c2, 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 iron undergo fission 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.

14.2.2 Nuclear chain reactions

In 1934, the 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 even more neutrons.

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, the 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] The Austrian physicist Lise Meitner showed that this provided evidence of nuclear fission,[24] and the Austrian-British physicist Otto Robert Frisch confirmed this experimentally in January of 1939.[25] That same year, the German-American physicist Hans Bethe[26] and the Indian-American physicist Subrahmanyan Chandrasekhar[27] showed how stars are fuelled by nuclear fusion chain reactions.

In 1956, the 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]

14.2.3 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 the 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.

14.2.4 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.

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

Figure 14.4
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A diagram of a nuclear power plant.

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]

14.3 References

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