The discoveries of the late XIX — early XX century (X-rays, radioactivity, electron, quanta) attracted the main attention of physicists to the study of the structure of atoms and the processes of the microcosm. E. Rutherford put forward a nuclear model of the atom, and N. Bohr formulated the theory of the structure of the atom, which organically combined quantum theory and Rutherford’s model of the atom.
Finally, the first artificial transformations of atoms of a non-radioactive element (nitrogen) under the influence of bombardment were performed alpha particles, which led to the formation of a new element – oxygen, and the discovery by Rutherford in 1919 of the second elementary particle — the proton.
Blackett, Skobeltsyn, and Wilson’s Camera
In 1925, the British physicist Patrick Blackett (1897-1974), having improved the Wilson camera, for the first time obtained photographs of the splitting of atomic nuclei of nitrogen by alpha particles in it and experimentally recorded traces of a proton. Important research in the field of atomic nucleus physics and cosmic ray physics belongs to D. V. Skobeltsyn, who for the first time in 1924 developed a new method for studying the atomic nucleus using the Wilson camera method placed in a magnetic field.
By 1929, D. V. Skobeltsyn conducted a series of studies on the interaction of gamma rays of radium with matter and for the first time experimentally confirmed the quantum theory of the Compton phenomenon. With the help of Wilson’s camera, he first discovered traces of the propagation of cosmic rays in space. At the same time, he discovered a phenomenon called a “shower of cosmic rays.”
These works by D. V. Skobeltsyn were, in fact, the first studies that laid the foundation for one of the most relevant areas of physics development — elementary particle physics, or high-energy physics. The initiated studies of cosmic rays were of great importance for nuclear physics since it was cosmic rays that contain high-energy particles that were used to bombard nuclei.
Secondly, it was in cosmic rays in the 30–the 50s that positrons, positive and negative nu-mesons were discovered, charged and neutral pi mesons, heavy ka mesons, lambda hyperons, etc. An important place in the study of processes in nuclear physics and cosmic ray physics also belongs to the method of thick-layer photographic plates, which was developed and improved in the 20s by Soviet physicists L. V. Mysovsky and A. P. Zhdanov.
Using this method, they obtained the first photographs of the complete splitting of atoms into elementary particles under the action of cosmic rays (the so-called “stars”). These experiments once again confirmed the fact of the complex structure of the atomic nucleus.
Discovery of the neutron
By the early 30s, the hypothesis prevailed in science that the nucleus consists of protons and electrons. The assumption of the existence of electrons in the nucleus encountered several insurmountable theoretical obstacles, and experimental data obtained at that time testified to the discrepancy between the properties of nuclei and the properties of their constituent components — proton and electron.
At the same time, it was shown that the nucleus cannot consist only of protons themselves, and therefore scientists began to look for new elementary particles that enter the nucleus.
The first step was the research of German physicists Walter Bothe and Georg Becker. In 1930, they experimentally discovered a new penetrating radiation, which they initially mistook for hard gamma rays.
Further work in this direction, carried out in early 1932 by Irene and Federico Joliot-Curie, showed that this radiation is capable of knocking protons out of hydrogen-containing substances, giving them greater speed. Hard gamma rays could not do this due to insufficient energy, and therefore the assumption that this radiation is hard gamma rays turned out to be incorrect.
Based on the analysis and his experiments, the English physicist Chadwick James Chadwick (1891-1974) concluded that the photonic interpretation of beryllium radiation is incompatible with the law of conservation of energy, and showed that all difficulties disappear if we assume that this radiation is particles with zero charge, whose mass is approximately equal to the mass of a proton. He called these particles neutrons.
D. Chadwick announced the discovery of a new elementary particle, the neutron, at the end of February 1932. The discovery of the neutron allowed the Soviet physicist D. D. Ivanenko to formulate a proton-neutron model of the nucleus. In June of the same year, independently of Ivanenko, V. Heisenberg also published an article on the proton-neutron model.
Most physicists reacted with distrust to the new model of the nucleus, because it seemed to contradict the phenomenon of beta decay: how can there be a flow of electrons from the nucleus if they are not there? In addition, the neutron was considered by many physicists, including Rutherford, as a complex formation of a proton and an electron.
But in the same 1932, as part of cosmic rays, the American physicist Carl David Anderson discovered the positron predicted by Dirac in 1928. It also followed Dirac’s theory that an electron and a positron, interacting with each other, should pass into a pair of photons. The reverse process is also possible, when a photon, interacting with the nucleus, turns into a pair of particles.
In 1933, at the Leningrad Conference on the Atomic Nucleus, F. Joliot demonstrated a photograph taken in Wilson’s camera, on which the birth was registered electron-positron pairs. At the same conference, D. D. Ivanenko made a report on the neutron-proton model of the nucleus. He firmly formulated the basic position: the nucleus consists only of heavy particles — protons and neutrons. Both of these particles can pass into one another, releasing an electron or positron. Since then, the proton-neutron model of the nucleus has become generally accepted.
Particle accelerators and artificial radioactivity
In 1932, another important discovery was made in nuclear physics — the splitting of the lithium nucleus by artificially accelerated protons.
Helium nuclei are scattered with an energy of about 8.5 MeV. It was the first nuclear reaction obtained at an accelerator, and its authors, the English physicist George Cockcroft and the Irish physicist Ernest Walton were awarded the Nobel Prize in 1951. In October 1932, K. D. Sinelnikov, A. I. Leipunsky, A. K. Walter and G. D. Latyshev
They carried out the first in the USSR and the second in the world artificial nuclear reaction at the Ukrainian Institute of Physics and Technology, bombarding the nucleus of natural lithium consisting of a mixture of two isotopes with accelerated protons. 1934 brought discoveries to nuclear physics.
In January, at a meeting of the Paris Academy of Sciences, Frederic Joliot and Irene Curie reported on their discovery of artificial radioactivity in several light elements (boron, beryllium, aluminum) when bombarded with alpha particles.
As a result of these experiments, the first artificial radioactive isotopes were obtained. In 1935, the Swedish Academy of Sciences awarded them the Nobel Prize for this discovery. After the discovery of artificial radioactivity, work on its research and the production of new isotopes unfolded on a broad front.
The most fruitful in this direction were the experiments of the great Italian physicist Enrico Fermi. The turbulent theoretical work of the scientist until 1933 was focused on three main areas. Firstly, having mastered quantum mechanics, he effectively developed it and explained and disseminated it in scientific circles. Secondly, Fermi was engaged in the construction of statistical mechanics. Thirdly, with his theoretical works, this physicist made a significant contribution to the doctrine of the structure of atoms and molecules.
He taught his students and students not only physics but also a passionate love for it, and an understanding of the spirit and ethics of this science. He always emphasized the high moral responsibility of a scientist for published works.
After 1933, Fermi completely immersed himself in nuclear physics. He developed the theory of beta decay, which consisted of the fact that during beta decay, in addition to an electron, a neutrino is also released – a “small neutron”, as V. Pauli foresaw.
This Fermi model formed the basis of the modern theory of elementary particles. In 1934, the scientist performed the first large-scale experimental work in the field of nuclear physics related to the bombardment of elements by neutron flux.
Fermi concluded that neutrons should be the most effective means to produce radioactive elements. With his usual enthusiasm, he began to irradiate almost all elements with neutrons. As a result of these experiments, more than 60 new radioactive isotopes were created and neutron deceleration was detected.
The work of the Fermi team was highly appreciated by the scientific community. The detection of neutron deceleration has opened a new branch of nuclear physics, as well as a new branch of technology — atomic engineering.
For experiments on the creation of radioactive elements by irradiation with neutrons and for the detection of nuclear reactions under the influence of slow neutrons, Fermi was awarded
The Nobel Prize.
Launch of the first nuclear reactors
In 1938, the works of Irene and Frederic Joliot-Curie, Pavle Savich, Otto Hahn, Friedrich Strassmann, Lisa Meitner, and Otto Frisch established that the uranium nucleus is divided by the action of a neutron into two “fragments”, which acquire energy equal to the mass defect as a result of electrostatic repulsion.
When A. Frisch reported this conclusion to Niels Bohr, the great physicist hit himself on the forehead, indignant and wondering why it had not been noticed earlier. In 1939, N. Bohr, speaking at a conference in Washington, announced from a high rostrum the discovery of uranium fission.
In the spring of 1941, E. Fermi completed the development of the theory of a chain reaction in a uranium-graphite system, after which a series of experiments began, which culminated in the construction of the first reactor in Chicago. In the Soviet Union, the work in the field of nuclear physics was led by academician I.V. Kurchatov (1903-1960).
Together with K. D. Sinelnikov, A. K. Walter, and A. I. Leipunsky, Kurchatov performed important experiments in Kharkiv on the study of the atomic nucleus, thorough studies of the processes of neutron deceleration and absorption.
On December 25, 1946, I. V. Kurchatov and his collaborators I. S. Panasyuk, B. G. Dubovsky, E. M. Babulevich, and A. K. Kondratiev launched the first Soviet experimental uranium-graphite reactor. In 1948, the first industrial reactor was launched. Thus, thanks to the research of outstanding scientists around the world, the foundations for the industrial use of nuclear energy were laid.
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