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MOST IMPORTANT SCIENTIFIC DISCOVERIES
Radioactivity. History and essence of scientific discovery
Directory / The most important scientific discoveries Discovery of Roentgen remarkable not only for the opportunity to understand the structure of matter and numerous practical applications. This discovery excited the thought of scientists who had already decided that the edifice of physics had been built and that in nature there was nothing more known to man. A member of the French Academy Becquerel was also excited about the discovery of x-rays. Henri Becquerel (1852-1908) first worked as a road engineer, but soon became interested, like his father and grandfather, in scientific research. At 35, Henri Becquerel defended his doctoral dissertation, at 40 he became a professor. He studies the phenomenon of fluorescence. He really wants to unravel the nature of the mysterious glow of certain substances under the influence of solar radiation. Becquerel collects a huge collection of luminous chemicals and natural minerals. In his report at the congress, Becquerel pointed out that it seemed to him very unlikely that X-rays could exist in nature only under those difficult conditions in which they are obtained in experiments. X-ray. Becquerel, who was closely acquainted with his father's research on luminescence, drew attention to the fact that the cathode rays in Roentgen's experiments produced, on impact, both glass luminescence and invisible X-rays. This led him to the idea that all luminescence is accompanied simultaneously by the emission of x-rays. This idea was first expressed by A. Poincaré. In his doctoral dissertation M. Curie-Sklodowska writes on this occasion "The first X-ray tubes did not have a metal anticathode: the source of X-rays was a glass wall exposed to cathode rays; at the same time, it strongly fluoresced. One could wonder whether the emission of X-rays is an indispensable companion of fluorescence, regardless of the cause of the latter" . For several days, Becquerel ponders the experiment he planned, then selects from his collection the double sulfate salt of uranium and potassium, pressed into a small cake, puts the salt on a photographic plate hidden from the light in black paper, and exposes the plate with salt to the sun. Under the influence of sunlight, the double salt began to glow brightly, but this glow could not fall on the protected photographic plate. Becquerel barely waited until the photographic plate could be removed from the developer. The image of a salt cake was clearly visible on the plate. Is everything correct, and salt, in response to irradiation with sunlight, emits not only light, but also X-rays? Becquerel checks himself again and again. On February 26, 1896, cloudy days came, and Becquerel regretfully hides the photographic plate prepared for the experiment with salt in the table. Between the cake of salt and the photographic plate this time he placed a small copper cross to see if the x-rays would pass through it. Probably few discoveries in science owe their origin to bad weather. If the end of February 1896 had been sunny in Paris, one of the most important scientific phenomena, the solution of which led to a revolution in modern physics, would not have been discovered. On March 1, 1896, Becquerel, without waiting for the sun to appear in the sky, took out the same photographic plate from the box, on which the cross and salt had lain for several days, and just in case developed it. What was his surprise when he saw on the developed photographic plate a clear image of both a cross and cakes with salt! So the sun and fluorescence have nothing to do with it? As a first-class researcher, Becquerel did not hesitate to put his theory to a serious test and began to investigate the effect of uranium salts on a plate in the dark. So it was discovered, and this Becquerel proved by successive experiments, that uranium and its compound continuously emit, without attenuation, rays that act on a photographic plate and, as Becquerel showed, are also capable of discharging an electroscope, i.e., creating ionization. This discovery caused a sensation. Particularly striking was the ability of uranium to radiate spontaneously, without any external influence. Ramsay says that when in the fall of 1896 he, together with Lord Kelvin (W. Thomson) and D. Stokes, visited Becquerel's laboratory, "these famous physicists wondered where the inexhaustible supply of energy in uranium salts could come from. Lord Kelvin was inclined to the assumption that that uranium serves as a kind of trap that catches the otherwise undetectable radiant energy that reaches us through space, and converts it into a form in which it is made capable of producing chemical effects. The world's first report on the existence of radioactivity was made by Henri Becquerel at a meeting of the Paris Academy of Sciences on February 24, 1896. The discovery of the phenomenon of radioactivity by Becquerel can be attributed to the most outstanding discoveries of modern science. It was thanks to him that man was able to significantly deepen his knowledge in the field of the structure and properties of matter, understand the patterns of many processes in the Universe, and solve the problem of mastering nuclear energy. The doctrine of radioactivity had a tremendous impact on the development of science, and in a relatively short period of time. Studying the properties of new rays, Becquerel tried to explain their nature. However, he could not come to clear conclusions and for a long time held the erroneous view that radioactivity might be a form of long-term phosphorescence. Soon, other scientists joined the study of the new phenomenon, and, above all, the spouses Pierre and Marie Curie. The young Polish researcher Maria Sklodowska (1867-1934), having shown outstanding abilities and great diligence, in 1894 received two licentiate diplomas - in physics and mathematics - at the famous Sorbonne, the University of Paris. At first, she takes a research topic from Professor G. Lippmann, and begins to study the magnetic properties of hardened steel. The development of the topic leads her to the Paris School of Industrial Physics and Chemistry. There she met Pierre Curie (1859–1906) and continued her experiments in his laboratory. In July 1895, Pierre and Maria became spouses. After the birth of her daughter in September 1897, Marie Skłodowska-Curie decides to start working on her doctoral dissertation. It was important to clearly formulate the task of the study. At this time, she learns about the discovery of Becquerel. Marie Curie began her research by patiently examining a large number of chemical elements: are some of them, like uranium, sources of "Becquerel rays"? The study of the radioactivity of uranium compounds led her to the conclusion that radioactivity is a property belonging to the atoms of uranium, regardless of whether they are included in the chemical compound or not. At the same time, she "measured the intensity of uranium rays, using their property to impart electrical conductivity to the air." By this ionization method, she became convinced of the atomic nature of the phenomenon. “Then I began to investigate whether there were other elements with the same property, and for this purpose I studied all the elements known at that time, both in pure form and in compounds. I found that among these rays only thorium compounds emit rays like those of uranium." The experiments of Maria Sklodowska-Curie on the study of ores showed that some uranium and thorium ores have "anomalous" radioactivity: their radioactivity turned out to be much stronger than what could be expected from uranium and thorium. “Then I put forward a hypothesis,” wrote Maria Sklodowska-Curie, “that minerals with uranium and thorium contain a small amount of a substance much more radioactive than uranium and thorium; this substance could not belong to the known elements, because all of them have already been investigated; it had to be a new chemical element." Realizing the importance of testing this hypothesis, Pierre Curie left his research on crystals and joined the work conceived by Marie. For their experiments, they chose uranium pitch, mined in the city of St. Joachimsthal in Bohemia. Despite the difficulties, research progressed successfully. Although Pierre Curie's salary was barely enough to cover various expenses, they nevertheless decided to take on an assistant to conduct chemical research. They became the young Jacques Bemon. The main efforts of scientists were directed to the isolation of radium from the waste of uranium pitch, since it was shown that it is easier to separate. Four years were spent on this difficult work, which was carried out in adverse conditions and required a lot of labor and strength. As a result, Maria and Pierre managed to obtain the world's first decigram of radium from 8 tons of Joachimsthal uranium tar waste, then estimated at 75 gold francs ($800). Hard work brought generous results. On July 18, 1898, Pierre and Marie Curie at a meeting of the Paris Academy of Sciences made a report "On a new radioactive substance contained in resin blende." The scientists said: "The substance that we extracted from the resin blende contains a metal that has not yet been described and is a neighbor of bismuth in its analytical properties. If the existence of a new metal is confirmed, we propose to call it polonium, after the name of the homeland of one of us." In this work, for the first time, the phenomenon being studied is called radioactivity, and the rays are called radioactive. The activity of the new element - polonium - turned out to be 400 times higher than the activity of uranium. As a result of chemical analysis, it was also possible to isolate the element barium from uranium pitch, which had a relatively strong radioactivity. When barium chloride was isolated from an aqueous solution in crystalline form, radioactivity passed from the mother liquor into crystals. Spectral analysis of these crystals showed the presence of a new line, "which, apparently, does not belong to any of the known elements." On December 26, 1898, the following article by the Curie and J. Bemont spouses appeared - "On a new, highly radioactive substance contained in tar ore" The authors reported that they had succeeded in isolating a substance from uranium waste containing some new element, giving it the property of radioactivity and very close in its chemical properties to barium. They proposed to call the new element radium. The activity of the isolated radium chloride was 900 times higher than the activity of uranium. The discovery of polonium and radium begins a new stage in the history of radioactivity. At the end of January 1899, Sklodowska-Curie suggested the essence of radioactive radiation, its material nature. She believed that radioactivity might turn out to be a property inherent only in heavy elements. In the same year, A. Debjorn, testing Marie Curie's hypothesis about the presence of other radioactive elements in uranium pitch, except for radium and polonium, made another discovery: a highly radioactive substance can be isolated from pitch, which is separated during fractionation with rare earth elements and titanium. The chemical properties of the new substance differed from those of radium and polonium, and its activity was 100 times higher than that of uranium. In 000, A. Debjorn announced the isolation of this new radioactive element, called actinium. Thus, by the beginning of the 1900th century, five radioactive substances were known: uranium, thorium, polonium, radium, actinium. Marie and Pierre Curie were not the only scientists to study the phenomenon of radioactivity. Henri Becquerel continued research on uranium in Paris. G. Schmidt in Germany simultaneously with the Curie discovered the radioactivity of thorium. In 1899, German scientists S. Meyer, E. Schweidler and, independently of them, F. Gisel demonstrated the deflection of "Becquerel rays" in a magnetic field. In Germany, J. Elster and G. Geitel in 1899 reported the first observed case of the chemical inseparability of radioelements and confirmed the atomic nature of radioactivity. In England, a new phenomenon became the focus of attention in the laboratories of W. Crookes and W. Ramsay. Radioactivity was also studied in other scientific centers in Europe. In 1906, Pierre Curie died in an accident. Marie Curie, recovering from this shock, continued to work on the study of the phenomenon of radioactivity, which soon became one of the most important areas of modern science and attracted the attention of many talented researchers. Author: Samin D.K.
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