- Introduction - British physicist James Chadwick won the 1935 Nobel Prize in Physics for the discovery of neutrons. Why were Chadwick and not Joliot Curie the first to discover neutrons? The answer is simple: because Chadwick was a rutherford student, he had long known that there could be a particle in nature with strong interaction properties similar to that of protons, called neutrons. This is an excellent example of working at the master's side and making it easier to become a master.
Written by | Xing Zhizhong (Researcher, Institute of High Energy Physics, Chinese Academy of Sciences)
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On October 20, 1891, 130 years ago, the British physicist James Chadwick was born in an ordinary home in the small town of Borrington in northwest England. His childhood was spent mostly with his grandparents, somewhat similar to that of is the childhood of the giant of science, Isaac Newton. Around the age of 11, Chadwick came to Manchester to reunite with his parents and began his secondary education. In 1907, Chadwick, a secondary school graduate, was awarded a scholarship to the University of Manchester and successfully entered the university. In May of that year, Ernest Rutherford, a 36-year-old British physicist of New Zealand descent, joined the University of Manchester, bringing the gospel to Chadwick.
In fact, Chadwick originally wanted to study mathematics in college rather than physics. By mistake, he attended an interview conducted by a physics faculty member in the fall of 1908. The shy Chadwick became an undergraduate in the physics department. He took Rutherford's electromagnetics course in the second academic year, and was immediately impressed by the charm of the master of science, and then decided to follow Rutherford to do a specific scientific research project, that is, to study the radioactivity of radium. In the summer of 1911, after completing his undergraduate studies, he became a graduate student at Rutherford. In 1912, Chadwick published his first academic paper in collaboration with his supervisor.
Rutherford's outstanding scientific talent and influence have made the University of Manchester a research centre for nuclear physics, attracting young scholars from all over the world to worship the "Manchester School". In March 1912, niels Bohr, a 27-year-old Danish physicist, came to the University of Manchester for postdoctoral research, and he and Chadwick soon became good friends. A year later, in July 1913, Bohr published a major paper in the prestigious British Journal of Philosophy and Science, first proposing a model of the quantized hydrogen atom. This work became one of the milestones in the history of quantum theory, and Bohr himself won the 1922 Nobel Prize in Physics.
In such an academic climate at the University of Manchester, it was difficult for a young Chadwick to succeed.
In the summer of 1912, Chadwick received his master's degree with an excellent scientific record. Although Rutherford wanted Chadwick to stay with him and do research, for other reasons, Chadwick came to Berlin, Germany, in the fall of 1913 to join the laboratory of Hans Geiger, the inventor of the Geiger counter.
Geiger also worked in Manchester and was one of Rutherford's key collaborators, so he loved House and U, and took good care of Chadwick. Berlin was one of the world's research centers for nuclear physics and radiochemistry, and big scientists like Otto Hahn and Lise Meitner, who later became famous for discovering nuclear fission, worked there, prompting Chadwick to choose beta decay of atomic nuclei as his new research topic.
Academics have long thought that beta decay of an atomic nucleus is a two-body process: the parent nucleus splits into a subnucleus and emits an electron, so the latter has a definite energy, that is, its energy spectrum should present a unipolar discrete spectrum. But by 1913, the initial observations made by the Manchester School and Hahn's laboratory contradicted this expectation. Using a Geiger counter that was more advanced than previous photosensitive film detection techniques, Chadwick remeasured the electron energies of beta decay and found that it presented a continuously changing spectral pattern. He published the measurements in 1914 as a single author, which was immediately endorsed by Rutherford and Hahn, among others, but questioned by Meitner. In 1927, Charles Ellis and William Wooster of manchester laboratories completed more reliable measurements of beta decay spectra, confirming that the energy spectra of electrons were continuous. Their results were subsequently confirmed by Meitner's group. Thus the question of whether energy is strictly conserved during beta decay, the so-called "energy crisis," became a dark cloud floating in the skies of nuclear physics in the 1920s and 1930s.
To explain the continuous energy spectrum problem of beta decay, Bohr proposed that conservation of energy in the microscopic world may only be a statistically average law, that is, there may be a situation where energy is not strictly conserved for a single microscopic reaction process. This view undoubtedly contradicts the results of the 1923 experiment on the scattering of photons and electrons published by the American physicist Arthur Compton, which clearly shows that microscopic scattering processes such as these strictly adhere to the laws of conservation of energy and momentum. In fact, to explain the results of the beta decay experiment of that year, theorists also face another challenge: how to ensure the conservation of the total angular momentum of the primary and final states of particles?
The person most qualified to speak at this time was the Austrian physicist Wolfgang Pauli, who proposed the "exclusion principle" in January 1925, because he was too sensitive to the spin angle momentum of atomic nuclei and elementary particles. In December 1930, in an open letter to his colleagues studying nuclear radioactivity, Pauli proposed his solution to the "energy crisis" of beta decay. He hypothesized that during beta decay of the nucleus, in addition to producing subnuclearies and electrons, a small, electrically neutral new particle with a spin quantum number equal to 1/2 would be released. Pauli called this invisible, untouchable imaginary particle "neutron," apparently unaware that the concept of "neutron" had been invented and occupied by Rutherford as early as 1920—to describe another imaginary particle of electrical neutrality, mass comparable to protons, and that could act as a fundamental component of the nucleus. Later, the Italian physicist Enrico Fermi renamed the "neutron" conceived by Pauli as "neutrino", which means a tiny "neutron".
With the presence of neutrinos, the conservation of energy, momentum, and angular momentum of the beta decay reaction are no longer a problem; and the energy spectrum of the electron is presented as a continuous spectrum because the electron has to share with the neutrino the reaction energy corresponding to the mass difference between the parent nucleus and the nucleus. During such a three-body decay, neutrinos carry a portion of their energy and momentum to escape. But the experimental techniques of that year could not confirm Pauli's hypothesis at all. It wasn't until 1956 that neutrinos as hypothetical particles were first identified in reactor experiments.
Back in August 1914, Chadwick's scientific work was interrupted by the outbreak of World War I. Despite the protection of his German colleagues, Chadwick, a citizen of a war-rival country, was arrested by the authorities in November of that year and imprisoned in a concentration camp west of Berlin. However, he did not live alone in prison, and even had the opportunity to regularly teach his fellow inmates about electromagnetism and radioactivity. Coincidentally, another of Rutherford's students, Ellis, was also imprisoned in the camp, and he became a good friend of Chadwick. Due to food shortages caused by the war, Chadwick suffered from severe malnutrition in prison and suffered from digestive tract diseases. In November 1918, the war was finally over. Chadwick and Ellis returned to their native England, where they later became colleagues at Cambridge University.
In 1930, the Cambridge University Press published the book "Radiation of Radioactive Materials" co-authored by Rutherford, Chadwick and Ellis, which systematically summarized the scattered experimental results of helium nuclei (that is, alpha particles) and helium nuclei, protons and heavy atomic nuclei, laying a preliminary experimental foundation for the establishment of the strong interaction theory. In 1935, the Japanese physicist Hideki Yukawa proposed theoretical images of interactions between atomic nuclei by exchanging light mesons, a work he made his scientific debut, and he became a hit, for which he won the Nobel Prize in Physics in 1949.
As recently as 1930, German scientists Walter Bothe and Herbert Becker observed a highly penetrating ray that would not deflect in the electric field in a scattering experiment between helium and beryllium nuclei, which they rightly interpreted as gamma rays. Two years later, in 1932, Marie Curie's eldest daughter, Irene Joliot-Curie, and her husband, Frederic Joliot-Curie, repeated the experiment. They found that bombarding substances containing hydrogen atoms with radiation observed by Bot and Baker produces high-energy protons. So, is this new type of ray a gamma ray?
Of course not! Neither Chadwick nor his mentor Rutherford believed that the results of the Jolio-Curie experiment could be explained by Compton scattering of protons and photons. Chadwick immediately set about devising an experiment and got his own measurements within three weeks. He discovered that the new type of ray was not gamma rays, but a beam of new particles of electrical neutrality and mass comparable to protons. On February 27, 1932, the British journal Nature published the results of Chadwick's experiment. His paper, titled "Possible existence of a neutron," is less than a page long, contains no formulas and charts, and contains only about 700 words. Chadwick makes it clear at the end of the paper that "to date, all the evidence has skewed toward neutrons, while the quantum hypothesis (i.e., the gamma-ray hypothesis) does not hold unless conservation of energy and momentum is abandoned to some extent". So neutrons were discovered as another fundamental component of the nucleus! In 1935, at the age of 44, Chadwick was awarded the Nobel Prize in Physics for the discovery of neutrons.
Why were Chadwick and not Joliot Curie the first to discover neutrons? The answer is simple: because Chadwick was a rutherford student, he had long known that there could be a particle in nature with strong interaction properties similar to that of protons, called neutrons. This is a great example of how it is easier to become a master by working next to a master. In contrast, Joliot Curie had to admit that although the two of them were also in a scientific research environment where masters (Curie and so on) gathered, they knew nothing about the concept of neutrons, so they failed to make a correct interpretation of their experimental results in the first place, thus missing the opportunity to discover neutrons.
Thankfully, two years later, on February 10, 1934, the journal Nature published a paper titled "Artificial production of a new kind of radio-element" by Joliot Curie. The paper is also less than a page long, containing only about 620 words and a chemical reaction equation, but it is the culmination of artificial radioactivity. With this discovery, the Jolio-Curies won the 1935 Nobel Prize in Chemistry at an extraordinary rate! One can't help but ask an interesting question: If joliot Curie had correctly understood the results of their experiments in 1932 and announced the discovery of neutrons, would it have been possible for them to win the 1935 Nobel Prize in Physics and Chemistry?
In the autumn of 1935, before winning the Nobel Prize, Chadwick was hired as a professor at the University of Liverpool. There he pushed for the construction of a cyclotron, making Liverpool one of the european research centres for nuclear physics. Chadwick was also a key figure in the cooperation between Britain and the United States in the Manhattan Project, because the discovery of neutrons was one of the important prerequisites for the construction of the atomic bomb. In 1948, Chadwick returned to Cambridge University and became Dean of Cowell and Caius College. He retired at the end of 1958 and moved with his wife to North Wales; a decade later they moved back to Cambridge, not far from their daughters.
On July 24, 1974, Chadwick, a generation of physics masters, died in his sleep at the age of 83.
Key References:
1) A. Brown, The neutron and the bomb: a biography of Sir James Chadwick, Oxford University Press, New York, 1997.
2) G. Ecker, James Chadwick: a head of his time, arXiv:2007.06926, 2020.
3) J. Chadwick, Possible existence of a neutron, Nature 129 (1932) 312.
4) F. Joliot and I. Curie, Artificial production of a new kind of radio-element, Nature 133 (1934) 201.