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MOST IMPORTANT SCIENTIFIC DISCOVERIES
artificial radioactivity. History and essence of scientific discovery
Directory / The most important scientific discoveries Artificial radioactivity was discovered by the spouses Irene (1897–1956) and Frederic (1900–1958) Joliot-Curie. On January 15, 1934, their note was presented by J. Perrin at a meeting of the Paris Academy of Sciences. Irene and Frederick were able to establish that after being bombarded with alpha particles, some light elements - magnesium, boron, aluminum - emit positrons. Further, they tried to establish the mechanism of this emission, which differed in character from all cases of nuclear transformations known at that time. The scientists placed a source of alpha particles (polonium preparation) at a distance of one millimeter from aluminum foil. They then exposed her to radiation for about ten minutes. The Geiger-Muller counter showed that the foil emits radiation whose intensity falls off exponentially with time with a half-life of 3 minutes 15 seconds. In experiments with boron and magnesium, the half-lives were 14 and 2,5 minutes, respectively. But in experiments with hydrogen, lithium, carbon, beryllium, nitrogen, oxygen, fluorine, sodium, calcium, nickel and silver, no such phenomena were found. Nevertheless, the Joliot-Curies concluded that the radiation caused by the bombardment of aluminum, magnesium and boron atoms could not be explained by the presence of any impurity in the polonium preparation. “An analysis of the radiation of boron and aluminum in a cloud chamber showed,” K. Manolov and V. Tyutyunnik write in their book “Biography of the Atom,” that it is a stream of positrons. It became clear that scientists were dealing with a new phenomenon that was significantly different from of all known cases of nuclear transformations.The nuclear reactions known up to that time were of an explosive nature, while the emission of positive electrons by some light elements subjected to irradiation with polonium alpha rays continues for some more or less long time after the removal of the source of alpha rays. boron, for example, this time reaches half an hour. The Joliot-Curies came to the conclusion that here we are talking about real radioactivity, manifested in the emission of a positron. New evidence was needed, and, above all, it was required to isolate the corresponding radioactive isotope. Building on Research Rutherford and Cockcroft, Irene and Frédéric Joliot-Curie were able to establish what happens to aluminum atoms when they are bombarded with polonium alpha particles. First, alpha particles are captured by the nucleus of an aluminum atom, the positive charge of which increases by two units, as a result of which it turns into the nucleus of a radioactive phosphorus atom, called radiophosphorus by scientists. This process is accompanied by the emission of one neutron, which is why the mass of the resulting isotope increases not by four, but by three units and becomes equal to 30. The stable isotope of phosphorus has a mass of 31. "Radiophosphorus" with a charge of 15 and a mass of 30 decays with a half-life of 3 minutes 15 seconds , emitting one positron and becoming a stable isotope of silicon. The only and indisputable evidence that aluminum turns into phosphorus and then into silicon with a charge of 14 and a mass of 30 could only be the isolation of these elements and their identification using their characteristic qualitative chemical reactions. For any chemist working with stable compounds, this was a simple task, but for Irene and Frederick, the situation was completely different: the phosphorus atoms they obtained lasted a little more than three minutes. Chemists have many methods for detecting this element, but they all require lengthy determinations. Therefore, the opinion of chemists was unanimous: it is impossible to identify phosphorus in such a short time. However, the Joliot-Curies did not recognize the word "impossible". And although this "unsolvable" task required overwork, tension, virtuoso dexterity and endless patience, it was solved. Despite the extremely low yield of products of nuclear transformations and the absolutely negligible mass of the substance that underwent the transformation - only a few million atoms, it was possible to establish the chemical properties of the resulting radioactive phosphorus. The discovery of artificial radioactivity was immediately regarded as one of the greatest discoveries of the century. Prior to this, the radioactivity that was inherent in some elements could not be caused, destroyed, or somehow changed by man. The Joliot-Curies were the first to artificially cause radioactivity by obtaining new radioactive isotopes. Scientists foresaw the great theoretical significance of this discovery and the possibility of its practical applications in the field of biology and medicine. The very next year, the discoverers of artificial radioactivity, Irene and Frederic Joliot-Curie, were awarded the Nobel Prize in Chemistry. Continuing these studies, the Italian scientist Fermi showed that neutron bombardment induces artificial radioactivity in heavy metals. Enrico Fermi (1901–1954) was born in Rome. Even as a child, Enrico showed great aptitude for mathematics and physics. His outstanding knowledge in these sciences, acquired mainly as a result of self-education, allowed him to receive a scholarship in 1918 and enter the Higher Normal School at the University of Pisa. Then Enrico received a temporary position as a teacher of mathematics for chemists at the University of Rome. In 1923, he went on a business trip to Germany, to Göttingen, to Max Born. Upon returning to Italy, Fermi worked at the University of Florence from January 1925 until the autumn of 1926. Here he receives his first degree of "free associate professor" and, most importantly, creates his famous work on quantum statistics. In December 1926 he took up the post of professor in the newly established chair of theoretical physics at the University of Rome. Here he organized a team of young physicists: Rasetti, Amaldi, Segre, Pontecorvo and others, who made up the Italian school of modern physics. When the first chair of theoretical physics was established at the University of Rome in 1927, Fermi, who managed to gain international prestige, was elected its head. Here in the capital of Italy, Fermi rallied around him several prominent scientists and founded the country's first school of modern physics. In international scientific circles, it began to be called the Fermi group. Two years later, Fermi was appointed by Benito Mussolini to the honorary position of a member of the newly created Royal Academy of Italy. In 1938, Fermi was awarded the Nobel Prize in Physics. The decision of the Nobel Committee stated that the prize was awarded to Fermi "for evidence of the existence of new radioactive elements obtained by irradiation with neutrons, and the discovery of nuclear reactions caused by slow neutrons." Enrico Fermi learned about artificial radioactivity immediately, in the spring of 1934, as soon as the Joliot-Curies published their results. Fermi decided to repeat the Joliot-Curie experiments, but went in a completely different way, using neutrons as bombarding particles. Later, Fermi explained the reasons for the distrust of neutrons by other physicists and his own lucky guess: "The use of neutrons as bombarding particles suffers from a disadvantage: the number of neutrons that can be practically disposed of is immeasurably less than the number of alpha particles obtained from radioactive sources, or the number of protons and deuterons accelerated in high-voltage devices. But this disadvantage is partially offset by the greater efficiency of neutrons in conducting "artificial nuclear transformations" Neutrons also have another advantage. They are capable of causing nuclear transformations to a large extent. The number of elements that can be activated by neutrons far exceeds the number of elements that can be activated by other types of particles." In the spring of 1934, Fermi began to irradiate elements with neutrons. Fermi's "neutron guns" were small tubes a few centimeters long. They were filled with a "mixture" of finely dispersed beryllium powder and radium emanation. Here is how Fermi described one of these neutron sources: "It was a glass tube only 1,5 cm in size ... in which there were beryllium grains; before soldering the tube, it was necessary to introduce a certain amount of radium emanation into it. Alpha particles emitted by radon collide in large numbers with beryllium atoms and give neutrons... The experiment is carried out as follows. In the immediate vicinity of the neutron source, a plate of aluminum or iron, or in general of the element that is desired to be studied, is placed and left for several minutes, hours or days (depending on the specific case). Neutrons emitted from the source collide with the nuclei of matter. In this case, many nuclear reactions of various types take place ... " How did all this look in practice? The sample under study was subjected to intense exposure to neutron irradiation for a specified time, then one of Fermi's employees literally ran the sample to a Geiger-Muller counter located in another laboratory and recorded the counter pulses. After all, many new artificial radioisotopes were short-lived. In the first communication, dated March 25, 1934, Fermi reported that by bombarding aluminum and fluorine, he obtained sodium and nitrogen isotopes that emit electrons (and not positrons, as in Joliot-Curie). The method of neutron bombardment proved to be very effective, and Fermi wrote that this high fission efficiency "completely compensates for the weakness of existing neutron sources in comparison with sources of alpha particles and protons." In fact, much was known. The neutrons hit the nucleus of the shelled atom, turning it into an unstable isotope, which spontaneously decayed and radiated. The unknown was hidden in this radiation: some of the artificially obtained isotopes emitted beta rays, others - gamma rays, and still others - alpha particles. Every day the number of artificially produced radioactive isotopes increased. Each new nuclear reaction had to be comprehended in order to understand the complex transformations of atoms. For each reaction, it was necessary to establish the nature of the radiation, because only knowing it, one could imagine the scheme of radioactive decay and predict the element that would be the final result. Then came the turn of the chemists. They had to identify the resulting atoms. This also took time. With his "neutron gun" Fermi bombarded fluorine, aluminium, silicon, phosphorus, chlorine, iron, cobalt, silver and iodine. All of these elements were activated, and in many cases Fermi could indicate the chemical nature of the resulting radioactive element. He succeeded in activating 47 of the 68 elements studied by this method. Encouraged by his success, he, in collaboration with F. Rasetti and O. D'Agostino, undertook the neutron bombardment of heavy elements: thorium and uranium. "Experiments have shown that both elements, previously purified from the usual active impurities, can be strongly activated when bombarded with neutrons." On October 22, 1934, Fermi made a fundamental discovery. By placing a paraffin wedge between the neutron source and the activated silver cylinder, Fermi noticed that the wedge did not decrease the neutron activity, but slightly increased it. Fermi concluded that this effect was apparently due to the presence of hydrogen in the paraffin, and decided to test how a large number of hydrogen-containing elements would affect the splitting activity. Having carried out the experiment first with paraffin, then with water, Fermi stated an increase in activity hundreds of times. Fermi's experiments revealed the enormous efficiency of slow neutrons. But, in addition to remarkable experimental results, in the same year Fermi achieved remarkable theoretical achievements. Already in the December issue of 1933, his preliminary thoughts on beta decay were published in an Italian scientific journal. Early in 1934, his classic paper "On the Theory of Beta Rays" was published. The author's summary of the article reads: "A quantitative theory of beta decay based on the existence of neutrinos is proposed: in this case, the emission of electrons and neutrinos is considered by analogy with the emission of a light quantum by an excited atom in radiation theory. Formulas are derived from the lifetime of the nucleus and for the form of the continuous spectrum of beta- rays; the obtained formulas are compared with the experiment". Fermi in this theory gave life to the neutrino hypothesis and the proton-neutron model of the nucleus, also accepting the isotonic spin hypothesis proposed by Heisenberg for this model. Based on the ideas expressed by Fermi, Hideki Yukawa predicted in 1935 the existence of a new elementary particle, now known as the pi-meson, or pion. Commenting on Fermi's theory, F Razetti wrote: "The theory he built on this basis turned out to be able to withstand almost unchanged two and a half decades of the revolutionary development of nuclear physics. One might notice that a physical theory is rarely born in such a final form." Author: Samin D.K.
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