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MOST IMPORTANT SCIENTIFIC DISCOVERIES
fission reaction. History and essence of scientific discovery
Directory / The most important scientific discoveries In 1938, I. Joliot-Curie and P. Savich noticed that in uranium activated by the method Fermi, there is an element similar to lanthanum. These experiments were repeated in the same year by O. Hahn and F. Strassmann, who confirmed the results of their French colleagues and established that the new element they noticed was precisely lanthanum. Together with Hahn and Strassmann, Lisa Meitner, a graduate of the University of Vienna, a talented theorist and specialist in the field of atomic physics, worked at the Kaiser Wilhelm Institute in Berlin. But, being a Jewess of German origin, she was forced to flee to Denmark in Copenhagen to Niels Bohr and Otto Frisch, another German physicist. And then the events are described in detail in the book "The World of the Atom": "In the calm creative atmosphere of the Institute of Theoretical Physics, she quickly forgot the anxieties and fears of the past days. Now the problem of the atomic nucleus again became the main thing for her. Two days before her departure, Lise Meitner received a letter from Otto Hahn, in which he wrote about research into radioactive barium. After reading the letter, she instinctively clenched her fists. She wanted to crush it and throw it away. Inside, everything boiled: "Nonsense! What nonsense!" When the first excitement passed, she thought: "If Hahn claims that uranium turns into barium, maybe it really is so. He cannot be mistaken. Perhaps Irene Curie was right ..." Meitner could doubt the work of others, but results Ghana - no. This means that neutrons cause some new kind of transformation of the uranium nucleus. She took a pencil and began to write quickly. The mathematical symbols with which she filled out the sheet would have looked incomprehensible to an ordinary person. The nucleus of the uranium atom broke up into about two parts. In the letter, Gan used the word "split". Now it is not so important, the fact itself is important. Is it possible to understand the possibility of such splitting on the basis of the known laws of physics? The very first calculations that she made gave a positive answer. Meitner felt unsure - what if she's wrong?" Lisa asks to check Otto Frisch's calculations. He skimmed through the crumpled sheets, then took out a pencil, squatted down and began to quickly make calculations. - But it's wonderful and incredible. You are really right! Frisch slipped the sheet into his pocket. - We're coming back. We need to check everything immediately. So their holidays ended before they even started. The festivities promised to be extremely merry, but now they were not interested. They locked themselves in a room where one of the most remarkable theoretical studies began. Huge hardships awaited them. Endless calculations, complex and time-consuming conclusions, verification of the results obtained, comparison with the derived formulas and patterns ... They did not notice how seven days had passed and how the year 1939 had come. The new year brought a new theory. Meitner and Frisch were the first to give a theoretical explanation of the results obtained by Hahn and Strassmann. If their conclusions are confirmed and everything turns out to be correct, humanity will follow a new path, will have a new source of energy. They were fully aware that they had made an epoch-making discovery, so they hurried to prepare articles. An article by Lise Meitner and Otto Frisch entitled "Uranium Fission by Neutrons: A New Type of Nuclear Reaction" was sent to press on January 16, 1939, and appeared in Priroda a month later. Here, another article of theirs was soon published - "Products of the Fission of the Uranium Nucleus" and then Frisch's work on the results of experiments carried out in Denmark. In fact, this phenomenon was explained almost simultaneously in late 1938 - early 1939 by several physicists. In less than a month in four laboratories around the world - in Copenhagen, New York, Washington and Paris. Hahn and Strassmann, Meitner and Frisch have already been mentioned. In the dungeon of Columbia University, John Dunning and two assistants also carry out the fission of the uranium nucleus. Besides them, in the laboratory of the Collège de France in Paris, the couple Irene and Frédéric Joliot-Curie, together with their collaborators Pavle Savich, Hans Halban, and Lev Kovarsky, came to the same discovery. According to this explanation, a uranium atom bombarded by neutrons experiences a new type of fission, with the atom hit by the neutron splitting into two more or less equal parts. This phenomenon was soon given the name fission. Joliot-Curie immediately realized the extreme importance of this new type of atomic decay. In the nuclei of light elements, the number of protons and neutrons is approximately the same, and with an increase in the atomic number, the relative number of neutrons increases. If in the nucleus of uranium the ratio of the number of neutrons to the number of protons is 1,59, then for the elements of the middle of the periodic system it fluctuates between 1,2 and 1,4. This means that if a uranium atom decays into two parts, then the total number of neutrons in the fission fragments must, in order to achieve the stability of the fission fragments themselves, become less than the number of neutrons contained in the original nucleus. The fission of a uranium atom releases neutrons, which can in turn cause the fission of other atoms. Thus, there is a possibility of a chain reaction similar to chemical chain reactions in an explosion. F. Perrin in the same 1939 made and published the first calculation of the "critical mass" needed to start a chain reaction. True, that was only a preliminary assessment. Today it is known that no amount of ordinary uranium can start a chain reaction. The neutrons produced by the fission of uranium-235 atoms are absorbed by the so-called "resonance capture" by uranium-238 atoms to form uranium-239. The latter, as a result of two successive decays, passes into neptunium and plutonium. Only for such fissile substances as uranium-235 and plutonium, there is a critical mass. The calculation of the mass loss during the fission of the uranium atom made it possible, moreover, to foresee that the fission process must be accompanied by the release of enormous energy of 165 MeV. Joliot-Curie's ideas were soon confirmed experimentally. It has been proven that the nucleus of uranium captures slow neutrons and then fissions. Niels Bohr after theoretical consideration, he came to the conclusion that it is not ordinary uranium with a mass of 238 that undergoes fission, but its isotope with a mass of 235. In 1940, A.O. Nier confirmed Bohr's prediction experimentally, also finding that another easily fissile atom was the atom of plutonium. The idea of using atomic energy for military purposes was put forward by a group of foreign scientists who fled fascism to the United States, of which L. Szilard, E. Wigner, E. Teller, W. R. Weisskopf, E. Fermi are named in the report. This group managed to interest President Roosevelt of the United States. These scientists used Einsteinwho wrote a letter to the president. As a result, Roosevelt decided to provide state support for these studies, and they were immediately classified. “Efforts to produce atomic energy in large quantities had two different goals: the controlled slow release of energy for industrial needs and the creation of a super-high explosive,” writes Gliozzi. “The second goal was completely urgent in that tragic period of world history. However, very soon scientists realized that the fastest way to achieve the second goal is to achieve the first.As we have already said, atoms of plutonium and uranium-235, which are only 0,7 percent in natural uranium, are subject to fission.The atomic bomb required huge amounts of uranium-235, which is very difficult to separate. slow energy generation requires no prior separation, only large quantities of uranium are needed, and plutonium is produced as a by-product. Hence the idea of the "atomic pile", so named, perhaps because of the simplicity of its design. This name is now only of historical interest, because it has been superseded by the more appropriate name "nuclear tor". The original purpose of the atomic pile was not to obtain energy, but to produce plutonium in the quantities needed to create an atomic bomb. An important problem was to reduce the number of neutrons captured by uranium-238 due to resonance; they fall out of the chain reaction, although they are useful as enrichers, that is, in the production of uranium-239, which then turns into neptunium and plutonium. Therefore, it was necessary to remove fast neutrons from the mass of uranium as soon as possible, take away their kinetic energy and again direct them into uranium in the form of thermal neutrons in order to cause the fission of uranium-235. This function of moderators could be performed by atoms of those light elements in collision with which neutrons lose a significant part of their energy, without at the same time causing changes in these atoms. So far, only two substances suitable for these purposes have been found: heavy hydrogen (in the form of heavy water) and carbon. Heavy water is very expensive, so we settled on carbon in the form of graphite. The first atomic boiler, or nuclear reactor, of alternating layers of uranium and graphite, designed and constructed by Fermi in collaboration with Anderson, Zinn, L. Woods and G. Weil, began operation on December 2, 1942, on the tennis court of the University of Chicago. Its power was 0,5 watts. Ten days later it was brought up to 200 watts. It was the first installation of nuclear power, which has now become one of the most developed branches of modern industry." There is a memorial plaque on the outer wall of the tennis court at the University of Chicago. The inscription on the board reads: "Here on December 2, 1942, a man for the first time carried out a chain reaction and this marked the beginning of mastering the released nuclear energy." The first pilot plant made it possible to conduct an accurate experimental study of the plutonium production process. It led to the conclusion that this method gives a real possibility of manufacturing plutonium in quantities sufficient to make an atomic bomb. At the end of 1943, the atomic bomb project entered the implementation stage. The first experimental explosion was successfully carried out at 17:30 pm on July 16, 1945, at Alamogordo Air Force Base, about 200 kilometers from Albuquerque, in the New Mexico wilderness. Author: Samin D.K.
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