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MOST IMPORTANT SCIENTIFIC DISCOVERIES
X-ray radiation. History and essence of scientific discovery
Directory / The most important scientific discoveries In January 1896, a typhoon of newspaper reports swept over Europe and America about the sensational discovery of Wilhelm Conrad Roentgen, professor at the University of Würzburg. It seemed that there was no newspaper that would not have printed a picture of the hand, which, as it turned out later, belonged to Bertha Roentgen, the professor's wife. And Professor Roentgen, having locked himself in his laboratory, continued to intensively study the properties of the rays he had discovered. The discovery of X-rays gave impetus to new research. Their study led to new discoveries, one of which was the discovery of radioactivity. German physicist Wilhelm Konrad X-ray (1845-1923) was born in Lennep, a small town near Remscheid in Prussia, and was the only child of a successful textile merchant, Friedrich Conrad Roentgen, and Charlotte Constance (nee Frowijn) Roentgen. In 1862, Wilhelm entered the Utrecht technical school. In 1865, Roentgen was enrolled as a student at the Federal Institute of Technology in Zurich, as he intended to become a mechanical engineer. Three years later, Wilhelm received a diploma, and a year later he defended his doctoral dissertation at the University of Zurich. After that, Roentgen was appointed by Kundt as the first assistant in the laboratory. Having received the chair of physics at the University of Würzburg (Bavaria), Kundt took his assistant with him. The move to Würzburg was the beginning of an "intellectual odyssey" for Roentgen. In 1872, together with Kundt, he moved to the University of Strasbourg and in 1874 began his teaching career there as a lecturer in physics. In 1875, Roentgen became a full (real) professor of physics at the Agricultural Academy in Hohenheim (Germany), and in 1876 he returned to Strasbourg to begin reading a course in theoretical physics there. The experimental research carried out by Roentgen in Strasbourg touched various branches of physics and, in the words of his biographer Otto Glaser, earned Roentgen a reputation as a "subtle classical experimental physicist". In 1879, Roentgen was appointed professor of physics at the University of Hesse, where he remained until 1888, refusing offers to take the chair of physics successively at the universities of Jena and Utrecht. In 1888 he returned to the University of Würzburg as professor of physics and director of the Physics Institute. In 1894, when Roentgen was elected rector of the university, he began experimental research on electric discharge in glass vacuum tubes. On the evening of November 8, 1895, Roentgen was working as usual in his laboratory, studying cathode rays. Around midnight, feeling tired, he was about to leave. Having looked around the laboratory, he turned off the light and was about to close the door, when he suddenly noticed some kind of luminous spot in the darkness. It turns out that a screen made of barium synergistic was glowing. Why is he glowing? The sun had long since set, the electric light could not cause a glow, the cathode tube was turned off, and in addition it was covered with a black cardboard cover. Roentgen looked again at the cathode tube and reproached himself: it turns out he forgot to turn it off. Feeling for the switch, the scientist turned off the receiver. Disappeared and the glow of the screen; turned on the receiver again - and the glow appeared again. This means that the glow is caused by the cathode tube! But how? After all, cathode rays are delayed by a cover, and the air gap between the tube and the screen for them is armor. Thus began the birth of the discovery. Recovering from his momentary amazement, Roentgen began to study the discovered phenomenon and the new rays, which he called x-rays. Leaving the case on the tube so that the cathode rays were covered, he began to move around the laboratory with a screen in his hands. It turns out that one and a half to two meters is not an obstacle for these unknown rays. They easily penetrate a book, glass, frame... And when the scientist's hand was in the path of unknown rays, he saw on the screen the silhouette of her bones! Fantastic and creepy! But this is only a minute, because Roentgen's next step was a step to the cabinet where the photographic plates lay: it is necessary to fix what he saw on the picture. Thus began a new night experiment. The scientist discovers that the rays illuminate the plate, that they do not diverge spherically around the tube, but have a certain direction ... In the morning, exhausted, Roentgen went home to rest a little, and then start working with unknown rays again. Most scientists would immediately publish such a discovery. Roentgen, on the other hand, believed that the message would be more impressive if it were possible to give some data on the nature of the rays discovered by him, by measuring their properties. So he worked hard for fifty days, testing every assumption that came to his mind. X-rays proved that the rays came from the tube and not from any other piece of apparatus. Just before the New Year, on December 28, 1895, Roentgen decided to acquaint his colleagues with the work done. On thirty pages, he described the experiments performed, printed the article in the form of a separate brochure and sent it along with photographs to the leading physicists of Europe. “Fluorescence is visible,” wrote Roentgen in his first communication, “with sufficient darkening and does not depend on whether the paper is brought up with the side coated or not coated with platinum-cyanogen barium. Fluorescence is noticeable even at a distance of two meters from the tube.” "It is easy to verify that the causes of fluorescence come from the discharge tube, and not from any place in the conductor." Roentgen suggested that the fluorescence was caused by some kind of rays (he called them X-rays) passing through the black cardboard of the tube cover, which was impenetrable to ordinary visible and invisible light rays. Therefore, he, first of all, investigated the absorptive capacity of various substances in relation to X-rays. He found that all bodies are permeable to this agent, but to varying degrees. The beams passed through a bound book of 1000 pages, through a double deck of playing cards. Spruce boards from 2 to 3 centimeters thick absorbed the rays very little. An aluminum plate about 15 millimeters thick, although it greatly weakened the rays, did not completely destroy them. "If you hold your hand between the discharge tube and the screen, you can see the dark shadows of the bones in the faint outlines of the shadow of the hand itself." The rays act on a photographic plate, and "you can take pictures in a lighted room, using a plate enclosed in a cassette or in a paper shell." Roentgen, however, could not detect either reflection or refraction of X-rays. However, he established that, if correct reflection "does not take place, yet various substances behave in relation to X-rays in the same way as turbid media in relation to light." Thus, Roentgen established the important fact of X-ray scattering by matter. However, all his attempts to detect the interference of x-rays gave a negative result. Negative results were also given by attempts to deflect rays by a magnetic field. From this Roentgen concluded that X-rays are not identical with cathode rays, but are excited by them in the glass walls of the discharge tube. In conclusion of his report, Roentgen discusses the question of the possible nature of the rays he discovered: “If we ask what X-rays actually are (they cannot be cathode rays), then, judging by their intense chemical action and fluorescence, we can attribute them to ultraviolet light. But in this case we immediately face serious obstacles. Indeed, if X-rays are ultraviolet light, then this light should have the properties: a) when passing from air to water, carbon disulfide, aluminum, rock salt, glass, zinc, etc., not experience any noticeable refraction; b) not experience any noticeable correct reflection from these bodies; c) not to be polarized by all common means; d) its absorption does not depend on any properties of the body, except for density. Hence, it would be necessary to accept that these ultraviolet rays behave quite differently from the hitherto known infrared, visible and ultraviolet rays. I could not decide on this and began to look for another explanation. Some relationship between new rays and light rays seems to exist. This is indicated by shadow images, fluorescence, and the chemical effects produced by both types of rays. It has long been known that, in addition to transverse light vibrations, longitudinal vibrations are also possible in the ether. Some physicists believe that they must exist. Their existence, of course, has not yet been clearly proven, and therefore their properties have not yet been experimentally studied. Shouldn't the new rays be attributed to longitudinal vibrations in the ether? I must confess that I am more and more inclined to this opinion, and I take the liberty of expressing this assumption here, although I know, of course, that it needs further substantiation. In March 1896 Roentgen made a second communication. In this communication he describes experiments on the ionizing action of rays and on the study of the excitation of X-rays by various bodies. As a result of these studies, he stated that "there was not a single solid body that, under the action of cathode rays, would not excite X-rays." This led Roentgen to redesign the tube to produce intense X-rays. "I have been successfully using the discharge tube of the following device for several weeks. Its cathode is a concave aluminum mirror, in the center of the curvature of which, at an angle of 45 degrees to the axis of the mirror, a platinum plate is placed, which serves as an anode." "In this tube, the X-rays exit from the anode. Based on experiments with tubes of various designs, I came to the conclusion that it does not matter for the intensity of the X-rays whether the place of excitation of the rays is the anode or not." In this way, Roentgen established the basic design features of X-ray tubes with an aluminum cathode and a platinum anticathode. The discovery of Roentgen caused a huge resonance not only in the scientific world, but throughout society. Despite the modest title given to his article by Roentgen: "On a new kind of rays. Preliminary communication", it aroused great interest in different countries. The Viennese professor Eksper reported the discovery of new invisible rays to the New Free Press newspaper. In St. Petersburg, already on January 22, 1896, Roentgen's experiments were repeated during a lecture in the physics auditorium of the university. Roentgen rays quickly found practical applications in medicine and technology, but the problem of their nature remained one of the most important in physics. X-rays rekindled the controversy between the corpuscular and wave nature of light, and many experiments were carried out to solve the problem. In 1905, Charles Barkla, 1917 Nobel laureate for the study of X-rays (1877-1944), measured these scattered rays by taking advantage of the ability of X-rays to discharge electrified bodies. The intensity of the rays could be determined by measuring the speed with which, under their action, an electroscope was discharged, say, with golden leaves. Barkla in a brilliant experiment investigated the properties of scattered radiation, causing its secondary scattering. He found that radiation scattered at 90 degrees could not be scattered again at 90 degrees. This was convincing evidence that the X-rays were transverse waves. Proponents of the corpuscular point of view also did not stay idle. William Henry Bragg (1862–1942) considered his data to be proof that Roentgen rays were particles. He repeated Roentgen's observations and became convinced of the ability of X-rays to discharge charged bodies. It was found that this effect is due to the formation of ions in the air. Bragg found that too much energy is transferred to individual gas molecules for it to be transferred only by a small part of a continuous wave front. This period of apparent contradictions - for the results of Barkle and Bragg could not be reconciled with each other - was suddenly brought to an end in 1912 by a single experiment. This experiment was carried out by a happy combination of ideas and people and can be considered one of the greatest achievements in physics. The first step was taken when graduate student Ewald turned to the theoretical physicist Max Laue (1879–1960). Ewald's idea, which interested Laue, was as follows. To check whether X-rays are waves, a diffraction experiment must be carried out. However, any artificial diffracting system is obviously too crude. But the crystal is a natural diffraction grating, much finer than any artificially made. Could X-rays be diffracted by crystals? Laue was not an experimenter and needed help. He turned to Sommerfeld (1868–1951) for advice, but he did not support him, saying that thermal motion must greatly disturb the correct structure of the crystal. Sommerfeld refused to allow one of his assistants, Friedrich, to waste time on such meaningless experiments. Fortunately, Friedrich had a different view and, with the help of his friend Knipping (1883–1935), carried out this experiment in secret. They chose a copper sulphate crystal—these crystals were available in most laboratories—and assembled the setup. The first exposure did not give any result; the plate was placed between the tube—the source of the x-rays—and the crystal, since it was believed that the crystal should act as a reflective diffraction grating. In the second experiment, Knipping insisted on placing photographic plates on all sides around the crystal: after all, every possibility had to be taken into account. On one of the plates, located behind the crystal in the path of the X-ray beam, the effect they were looking for was found. This is how X-ray diffraction was discovered. In 1914, Laue was awarded the Nobel Prize for this discovery. In 1913, G. V. Wulff in Russia, father and son Bragg in England, repeated the experiments of Laue and his friends with one significant change: they directed X-rays at crystals at different angles to their surface. Comparison of x-ray images obtained in this case on photographic plates allowed researchers to accurately determine the distances between atoms in crystals. The Braggs were awarded the Nobel Prize in 1915. So two fundamental scientific facts came to physics: x-rays have the same wave properties as light rays; With the help of X-rays, you can explore not only the internal structure of the human body, but also look into the depths of the crystals. Using x-rays, scientists could now easily distinguish crystals from amorphous bodies, detect shifts in atomic chains in the depths of metals and semiconductors that are opaque to light, determine what changes in the structure of crystals occur during strong heating and deep cooling, during compression and tension. Roentgen did not take a patent, giving his discovery to all mankind. This made it possible for designers from around the world to invent a variety of X-ray machines. Doctors wanted to learn as much as possible about the ailments of their patients with the help of X-rays. Soon they were able to judge not only about bone fractures, but also about the structural features of the stomach, about the location of ulcers and tumors. Usually the stomach is transparent to x-rays, and the German scientist Rieder suggested feeding the sick before photographing ... barium sulphate porridge. Barium sulphate is harmless to the body and much less transparent to x-rays than muscles or internal tissues. The pictures showed any narrowing or expansion of the human digestive organs. In more recent X-ray tubes, a hot tungsten spiral radiates a stream of electrons, against which an anti-cathode of thin plates of iron or tungsten is located. Electrons knock out a strong flow of X-rays from the anticathode. Powerful sources of X-rays have been found outside the Earth. In the depths of new and supernovae there are processes during which high-intensity X-rays are produced. By measuring the X-ray fluxes coming to the Earth, astronomers can judge the phenomena occurring many billions of kilometers from our planet. A new field of science has arisen - X-ray astronomy ... The technology of the XNUMXth century could not, without X-ray analysis, have at its disposal that magnificent constellation of various materials that it has at its disposal today. Author: Samin D.K.
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