130 years ago1October 20th, British physicist james chadwick was born in an ordinary family in Burlington, a town in the northwest of England. His childhood was mainly spent with his grandparents, which was somewhat similar to that of the great scientist isaac newton. 1 1 years old, chadwick came to Manchester to reunite with his parents and began to receive secondary education. From 65438 to 0907, chadwick, who graduated from middle school, won a scholarship from Manchester University and successfully entered the university. In May this year, ernest rutherford, a 36-year-old New Zealand-born British physicist, joined Manchester University, bringing good news to chadwick.
In fact, chadwick University originally wanted to study mathematics, not physics. I missed an interview hosted by a physics teacher in the autumn of 1908. Shy chadwick became an undergraduate in physics. He took Rutherford's electromagnetism course in the second academic year and was immediately moved by the charm of the Master of Science. Then he decided to follow Rutherford to do a specific scientific research project, which is to study the radioactivity of radium. 19 1 1 In the summer of, he completed his undergraduate studies and became a graduate student of Rutherford. 19 12, chadwick published his first academic paper in cooperation with his tutor.
Rutherford's outstanding scientific talent and influence made Manchester University a research center of nuclear physics, which attracted young scholars from all over the world to worship Manchester School. 1912 In March, 27-year-old Danish physicist niels bohr came to Manchester University for postdoctoral research, and he and chadwick soon became good friends. A year later, in July of 19 13, Bohr published an important paper in the famous British journal of philosophy and the journal of science, and put forward a quantized hydrogen atom model for the first time. This work became one of the milestones in the development history of quantum theory, and Bohr himself won the Nobel Prize in Physics from 65438 to 0922.
Being in such an excellent academic atmosphere of Manchester University, it is difficult for young chadwick to succeed.
19 12 In the summer, chadwick obtained a master's degree with excellent scientific research results. Although Rutherford hoped that chadwick would stay with him to do research, for other reasons, chadwick came to Berlin in the autumn of 19 13 and joined the laboratory of Hans Geiger, the inventor of Geiger counter.
Geiger also works in Manchester, and is one of Rutherford's important collaborators, so he loves me and dogs very much, and takes good care of chadwick. At that time, Berlin was one of the research centers of nuclear physics and radiochemistry in the world. Later, great scientists such as otto hahn and lise meitner, who are famous for discovering nuclear fission, all worked there, which prompted chadwick to choose nuclear beta decay as his new research topic.
For a long time, the academic circles think that the beta decay of nuclear is a two-body process: the mother nucleus splits into daughter nuclei and releases an electron, so the latter has certain energy, that is, its energy spectrum should present a single-energy discrete spectrum. But by 19 13, the preliminary observation results given by Manchester School and Hahn Laboratory were contrary to this expectation. Using Geiger counter, which is more advanced than the previous photographic film detection technology, chadwick re-measured the electron energy of β decay and found that it showed a continuously changing spectrum. As a single author, he published this measurement result in 19 14, which was immediately recognized by Rutherford and Hahn, but was questioned by Maitenaz. 1927, Charles Ellis and William Worcester of Manchester Laboratory completed a more reliable measurement of β decay energy spectrum, which confirmed that the energy spectrum of electrons is continuous. Their experimental results were subsequently confirmed by Maitenaz's research team. Therefore, the question of whether energy is strictly conserved in the process of β decay, the so-called "energy crisis", became a dark cloud floating in the sky of nuclear physics in the 1920s and 1930s.
In order to explain the continuous energy spectrum of β decay, Bohr put forward the view that the conservation of energy in the microscopic world may only be a statistical average law, that is, there may be a situation in which energy is not strictly conserved in a single microscopic reaction. This view is undoubtedly in contradiction with the experimental results of photon and electron scattering published by American physicist arthur compton in 1923, which clearly shows that such microscopic scattering process strictly abides by the law of conservation of energy and momentum. In fact, in order to explain the experimental results of β decay, theorists are faced with another challenge: how to ensure the conservation of the total angular momentum of the initial and final particles?
At this time, the most qualified speaker was Austrian physicist Wolfgang Pauli, who put forward the "incompatibility principle" in June 1925, because he was too sensitive to the spin angular momentum of the nucleus and elementary particles. 1930 In February, Pauli put forward his plan to solve the "energy crisis" of beta decay in an open letter to colleagues who studied nuclear radiation. He hypothesized that in the process of β decay of the nucleus, in addition to producing daughter nuclei and electrons, a new particle with extremely small mass and electric neutrality will be released, and its spin quantum number is equal to 1/2. Pauli called this invisible imaginary particle "neutron". Obviously, he didn't know that the concept of "neutron" was invented and occupied by Rutherford as early as 1920-used to describe another hypothetical particle that is electrically neutral and has the same mass as proton and can be used as the basic component of the nucleus. Later, Italian physicist Enrico Fermi changed Pauli's concept of neutron to neutrino, which means tiny neutron.
With the existence of neutrinos, the conservation of energy, momentum and angular momentum of β decay reaction is no longer a problem; The energy spectrum of electrons is continuous because electrons share the reaction energy corresponding to the mass difference between the parent nucleus and the daughter nucleus with neutrinos. In this three-body decay process, neutrinos escape with some energy and momentum. But the experimental technology of that year could not confirm Pauli's hypothesis at all. It was not until 1956 that neutrinos were first confirmed as hypothetical particles in reactor experiments.
Back in August 19 14, chadwick's scientific research work was interrupted by the outbreak of the First World War. Despite the protection of his German colleagues, chadwick, a citizen of a hostile country to the war, was arrested by the authorities in June of that year and put into a concentration camp in the west of Berlin. However, he was not alone in prison, and even had the opportunity to teach electromagnetism and radioactivity to his cellmates regularly. Coincidentally, another Rutherford student, Ellis, was also imprisoned in this concentration camp, so he became a good friend of chadwick. Due to the food shortage caused by the war, chadwick suffered from digestive tract diseases due to severe malnutrition in prison. 1918165438+October, the war finally ended. Chadwick and Ellis returned to their native England, and they later became colleagues at Cambridge University.
1930, Cambridge University Press published the book Radiation of Radioactive Substances, which was co-authored by Rutherford, chadwick and Ellis, systematically summarized the experimental results of the scattering of helium nuclei (α particles) with helium nuclei, protons and heavy nuclei, and laid a preliminary experimental foundation for the establishment of the strong interaction theory. 1935, Japanese physicist Hideki Yukawa put forward a theoretical picture of the interaction between nuclei of exchange light mesons. This work is his first scientific research work, and it became an instant hit, so it won the 1949 Nobel Prize in Physics.
In 1930, German scientists Walter Bothe and Herbert Becker observed a strong penetrating ray in the scattering experiment of helium and beryllium nuclei, and they naturally interpreted it as gamma ray. Two years later, 1932, Madame Curie's eldest daughters Irene Joliot-Curie and Frederic Joliot-Curie repeated the experiment. They found that when a substance containing hydrogen atoms is bombarded by rays observed by Bert and Baker, it will produce 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 experimental results of Mr. and Mrs. Joliot-Curie could be interpreted as Compton scattering of protons and photons. Chadwick immediately set out to design an experiment and got his own measurement results within three weeks. He found that this new type of ray is not a gamma ray, but a ray composed of new particles, which are electrically neutral and have the same mass as protons. 1932 On February 27th, the British magazine Nature published the experimental results of chadwick. His paper entitled "The Possible Existence of a Neutron" is less than one page and contains only about 700 words, without any formulas or charts. At the end of the paper, chadwick clearly pointed out that "up to now, all the evidence is in favor of neutrons, and the quantum hypothesis (that is, the gamma-ray hypothesis) is untenable unless the conservation of energy and momentum is abandoned to some extent". So neutrons were discovered as another basic component of the nucleus! 1935, 44-year-old chadwick won the Nobel Prize in physics for discovering neutrons.
Why did chadwick, not Aurio Curie, discover neutrons first? The answer is simple: because chadwick was a student of Rutherford, he knew for a long time that there might be a particle with strong interaction properties similar to protons in nature, and its name was neutron. This is a good example that it is easier to become a master by working with him. In contrast, Mr. and Mrs. Iorio Curies have to admit that although they are also in the research environment of masters (Curies, etc.). ), they knew nothing about the concept of neutrons, so they failed to correctly explain their experimental results in the first time, thus missing the opportunity to discover neutrons.
However, it is gratifying that two years later, on February 1934 10, Nature published a paper entitled "Artificial production of a new radioactive element", which was jointly completed by Aurio and Curie. This paper is less than one page, only contains about 620 words, 1 chemical reaction equation, but it is the first work of artificial radioactivity. With this discovery, Joliot-Curie and his wife won the 1935 Nobel Prize in chemistry at an extraordinary speed! People can't help asking an interesting question: If Mr. and Mrs. Aurio Curie correctly understood their experimental results in 1932 and announced the discovery of neutrons, would it be possible for them to win the Nobel Prize in Physics and Chemistry at one fell swoop?
1In the autumn of 935, before winning the Nobel Prize, chadwick was hired as a professor at the University of Liverpool. He promoted the construction of cyclotron there, making Liverpool one of the European nuclear physics research centers. Chadwick is also a key figure in the Manhattan Project of Anglo-American cooperation, because the discovery of neutrons is one of the important prerequisites for building an atomic bomb. From 65438 to 0948, chadwick returned to Cambridge University and became the dean of Kewell and Case College. /kloc-retired at the end of 0/958 and moved to North Wales with his wife. Ten years later, they moved back to Cambridge and lived not far from their daughter.
Main references:
Brown, Neutrons and Atomic Bombs: Biography of Sir james chadwick, Oxford University Press, new york, 1997.
2) G. Eck, james chadwick: a leader of his time, arXiv:2007.06926.
3) J. chadwick, the possible existence of a neutron, property129 (1932) 312.
4) F. Iorio and I. Curie, artificially made a new radioactive element, Nature133 (1934) 201.