Flerov, Georgii Nikolaevich

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(b. Rostov-on-Don, Russia, 2 March 1913; d. Moscow, USSR, 19 November 1990),

nuclear physics, nuclear chemistry.

Flerov is known for his experimental investigations of the physics of the atomic nucleus, including the discovery of the spontaneous fission of the uranium nucleus. Together with Igor V. Kurchatov, he directly participated in the development of Soviet atomic weapons. In addition, Flerov helped develop the field of the physics of heavy ions. With his coworkers, he successfully conducted experiments on the synthesis of chemical elements with atomic numbers 102–107 and discovered new physical phenomena, such as the accelerated spontaneous fission of isomeric nuclei, the delayed fission of nuclei, decay of nuclei with the emission of delayed protons, and a new class of nuclear reactions—the reactions of deep inelastic transmission of nucleons.

Scientific Path . After finishing secondary school in 1929 in his hometown, Flerov worked at a multitude of different jobs—as an unskilled worker, as an engine-house greaser, and then as an electrician. In 1931 he left Rostov-on-Don and went to Leningrad, where he took a position as an electrician at the metal-working plant named “Krasnii Putilovets” (Red Putilov). In 1933 the factory sent the young worker to study at the M. I. Kalinin Leningrad Polytechnical Institute in the engineering-physics department. During his years of study Flerov specialized in the area of thermophysics and experimental nuclear physics. In the upper courses he also studied at the Leningrad Physico-Technical Institute (LFTI). His diploma thesis, supervised by the well-known physicist Kurchatov, was a study of the interaction of slow neutrons with nuclei. Upon graduation Flerov was selected to continue work in Kurchatov’s laboratory.

Spontaneous Fission of Nuclei . At the end of 1939, physicists throughout the world were surprised by news about the discovery of nuclear fission. This problem attracted Flerov’s attention and, together with his colleague Lev I. Rusinov, he conducted an experiment that attempted to obtain a chain reaction of fissioning uranium nuclei. Although this experiment was not successful, it provided important results about determining the key parameters of chain reactions—the number of secondary neutrons arising from the fission of a uranium nucleus. This experiment was the first step in Flerov’s major work on nuclear fission.

The next question to consider was with what probability would the naturally occurring isotopes of uranium (238, 235, 234) fission under the impact of neutrons of various energies. For these experiments Flerov and Konstantin A. Petrzhak developed a technique with extremely high sensitivity, and in 1940 they showed, for the first time, that uranium nuclei could split spontaneously. This fundamental discovery served as the basis for a whole new field of nuclear physics. In 1946 Flerov (together with Petrzhak) was awarded the State (Stalin) Prize, second class, for the discovery of the spontaneous fission of uranium.

Participation in the “Atomic Project.” . Soon after the German attack on the Soviet Union in the summer of 1941, Flerov volunteered to serve in the army at the front. He was sent for training at the Military-Aviation Academy, located near Leningrad, and later was evacuated to Ioshkar-Ola, where he studied the servicing of military aircraft. After completing his studies Flerov was commissioned as a lieutenant in the Ninetieth Reconnaissance Aviation Squadron of the Military-Aviation Academy of the Southwest Front. Even while on active duty in the army, Flerov took advantage of every spare moment and tried to follow the research being published on uranium. He soon noticed the disappearance of scientific articles about this topic, whereas before the war such articles appeared regularly in foreign publications. He came to the conclusion that all such work had become classified as a military project and, consequently, that foreign countries had become engaged in creating an atomic bomb. In 1941–1942, while still at the front, Flerov sent a series of letters about his suspicions to well-known scientists and government leaders, including Kurchatov, Sergei V. Kaftanov (the plenipotentiary for science at the State Defense Committee), and even Joseph Stalin himself. These letters, according to the recollections of Kaftanov, were one of the essential factors that prompted the Soviet government’s decision to initiate a program for the creation of atomic weapons at such a difficult period for the country.

In December 1941 Flerov presented a talk to specialists from the USSR Academy of Sciences, which at that time had been evacuated to the city of Kazan, east of Moscow. In this presentation he emphasized the need to study nuclear chain reactions induced by fast neutrons. During the spring of 1942 Flerov sent a draft of an article to Kurchatov in which he repeated the main ideas of the talk he had given in Kazan. This article, “The Question of Using Inter-atomic Energy,” proposed a scheme for the realization of atomic explosions by very rapidly joining together two hemispheres of uranium-235 in order to achieve critical mass and included a calculation of the necessary velocity for joining the two halves together. (The article remained unpublished.) Kurchatov considered this work to be spectacular. Flerov was detached from the army in June 1942 and sent to work at the Physical-Technical Institute in Kazan, where he remained during 1942–1943.

On 28 September 1942 Stalin approved the resolution of the State Defense Committee, “About the Organization of Work on Uranium,” which gave the USSR Academy of Sciences the task of resuming the work on studies to realize the uses of atomic energy by the path of chain reactions of uranium nuclei that had been interrupted by the war. In February 1943 “Laboratory No. 2” was established under the direction of Kurchatov, who supervised the atomic project. The main group of the “Kurchatov team” was composed of Leningrad physicists who had graduated from the Leningrad Physico-Technical Institute—AnatoliÎ P. Alexandrov, Abram I. Alikhanov, Lev A. Artsimovich, IuliÎ B. Khariton, Yakov B. Zeldovich, and others, including Flerov. From 1943 to 1945 Flerov was a senior scientist at Laboratory No. 2 of the USSR Academy of Sciences and from 1945 to 1949 was a section chief there.

Already in 1943 two main strategies had been outlined for the realization of the atomic project: (1) a gun variant of an atomic bomb containing uranium-235 obtained from one of the methods of isotope separation; and (2) a nuclear bomb of the implosion type using plutonium-239, an isotope that had scarcely been investigated at that time, but which held the promise of providing the material for a self-sustaining chain reaction upon of slow neutrons irradiation of uranium-238 using graphite as moderator. These two paths were proposed based on the assumption of obtaining a sufficient amount of fissionable material for an atomic bomb. Flerov’s group studied the characteristic interactions of neutrons with the nuclei of heavy elements and the splitting of nuclei. The plutonium strategy won out. For a series of reasons, the gun variant (RDS-2) was inferior to a nuclear bomb of the implosion type (RDS-1), and work on the former type of bomb was ended in mid-1948. An experimental prototype RDS-1 was exploded by the United States in July 1945. The creation of the RDS-1, as is known, was based on information about the physical scheme of the American plutonium bomb.

The construction of the first atomic bombs and the necessary research for them was carried out in a special branch of Laboratory No. 2 (KB-11) in the small town of Sarov. The scientific research work of laboratory KB-11 began in the spring of 1947. In January 1948, Laboratory No. 9 was organized to measure critical masses and was headed by Flerov. This laboratory studied the transmission and reflection of fast neutrons by layers of various materials. It was important to select the optimal material for reflecting neutrons in order to minimize the critical mass of metallic plutonium-239 that was needed. At the same time, the laboratory constructed a special apparatus that provided an experimental estimation of similar critical measurements. These quite dangerous measurements of the critical mass of plutonium were carried out by a group of researchers under the direction of Flerov in June 1949 at a metallurgical plant (Cheliabinsk-40) in the Urals that prepared the metallic plutonium. On the basis of these experiments, the group of theoreticians led by Zeldovich was able to determine the necessary mass of the plutonium sphere that would satisfy the basic parameters for a nuclear explosion. The successful realization of these experiments was an important step on the path toward the testing of the first Soviet atomic bomb on 29 August 1949.

The test of the first Soviet thermonuclear bomb was conducted on 12 August 1953. Flerov’s research team actively participated in conducting nuclear-physical experiments connected with developing the thermonu-clear system in the years up to the test. This team studied nuclear constants that could be used for calculating and utilizing the modeling of widely varied systems.

For his work on the atomic project G. N. Flerov was awarded the State (Stalin) Prize, first class (1949) and received the title of Hero of Socialist Labor with the Order of Lenin (1949). He was also awarded the Order of the Red Banner of Labor in 1948 and 1953.

Flerov also rose in his scientific career. In 1943 he was awarded the degree of candidate of physical-mathematical sciences without the defense of a dissertation, and in 1949 he received the degree of doctor of physical-mathematical sciences. In 1953 he was elected a corresponding member of the USSR Academy of Sciences.

The Synthesis of New Elements . In the early 1950s Flerov (who from 1949 to 1957 was the head of Section No. 7 at the Laboratory of Instrumental Measurement of the Institute of Atomic Energy, USSR Academy of Sciences) decided to return to the investigation of spontaneous nuclear fission. Whereas in 1940 only uranium was known to undergo spontaneous fission, by the early 1950s a series of artificially created elements had been produced that exhibited spontaneous fission. In order for Flerov to study the properties of spontaneous fission in these new heavy nuclei, it was necessary to synthesize them. Analyzing the known methods of synthesis of transuranium elements, Flerov concluded that the best way would be to use accelerated heavy ions. His conclusion, essentially, was to irradiate the target nuclei by accelerated heavy ions and increase the atomic number of the nucleus in that way by several units in a single step. As a consequence, the question arose about how to obtain accelerated heavy ions with the needed energy and intensity.

At that time the available sources could produce ions with only a single or double charge, which would not achieve the needed energies with classical methods. Therefore Flerov decided to switch to a different method of accelerating ions—the method of peeling the electron shells by small charged ions in molecules of a residual gas by acceleration in a cyclotron. Experiments with the acceleration of heavy ions in the institute’s 150-centimeter cyclotron, using short frequencies with supplementary “peeling,” showed, on the one hand, the utility of using heavy ions for the synthesis of transuranium elements, and, on the other hand, the need for more intense beams of accelerated particles. This need could be satisfied only with the help of intense sources of multiply charged ions. By 1955 the first such source was created and was employed in the 150 centimeter cyclotron at the institute. Research was conducted there on the physics of heavy ions, including the synthesis and investigation of the properties of transuranium elements, such as the element with the atomic number 102.

In 1957 this work was transferred to the Ob- edinennyÎ Institut Iadernykh IssledovaniÎ (OIIaI -Joint Institute for Nuclear Research [JINR]) in Dubna, 120 kilometers from Moscow. This institute was created on the basis of an agreement signed 26 March 1956 in Moscow by representatives of the governments of eleven founding nations, with the goal of unifying their scientific and material potential for the study of the fundamental properties of matter. The scientific council of the institute included well-known physicists from Germany, Italy, the United States, and France. In 1957 Flerov proposed that a laboratory for the study of nuclear reactions be established at JINR under his supervision.

From the first days of work in his laboratory, Flerov paid special attention to modernizing its accelerators, which could produce powerful sources of multiply charged ions. The results of this work included the creation in the laboratory of a new generation of heavy ion accelerators with record sizes: a 300-centimeter cyclotron U-300, the isochronous cyclotron U-200, then the tandem cyclotrons U-300 and U-200; in the late 1970s the U-400, one of the largest isochronous cyclotrons in the world at that time, came online. These very powerful accelerators allowed the researchers to work on cutting-edge problems in the physics of heavy ions.

The research conducted in the laboratory of Flerov and his students over the course of three decades led to the synthesis of new chemical elements with the atomic numbers 102–109, the production of a large number of new nuclei at the border of nuclear stability, the discovery of new types of radioactivity, and new mechanisms of nuclear interactions.

The basis for the success of Flerov and his coworkers in their work on the synthesis of new elements was the careful employment of nuclear-physical methods for the identification of isotopes. In order to identify the newly created elements, the group developed an original method of rapid gas chromatography for separating transactinium elements (in particular, 104 and 105) from a mixture of lighter products of the reactions. One of the most important results of this work was the discovery of the high stability of extremely heavy nuclei with atomic numbers greater than 104 in regard to spontaneous fission, which opened new methods for the further development of research in this area. At the same time, workers in Flerov’s laboratory decided to conduct experiments on the synthesis of hypothetical transuranium elements with very large atomic numbers and to realize the attempts to search for these elements in nature (for example, the element number 114).

In 1967 Flerov was awarded the Lenin Prize for the synthesis and investigation of transuranium elements, and in 1968 he was elected a full member of the USSR Academy of Sciences. In 1975 he was awarded the USSR State Prize for his work on nuclei close to the nucleon stability boundary.

Two other scientific groups also participated in the synthesis and investigation of elements 102–109—the Lawrence Berkeley National Laboratory (United States) and the Gesellschaft für Schwerionenforschung mbH (Association for Heavy Ion Research) in Darmstadt (Germany). Upon Flerov’s initiative, an independent international commission of experts was created that recognized JINR’s priority for the discovery or codiscovery of five of the eight elements from 102 through 109. The element with number 105 received the name “Dubnium.”

Flerov achieved considerable success in these years with his work on the physics of nuclear fission. In 1961 Flerov’s group discovered a new kind of nuclear isomerism: spontaneously fissile isomers. In these isomers, the nucleus spontaneously fissions with a probability about 1025 times greater than for the ground state. The new phenomenon served as the basis for the prediction of the possibility of an island of stability of super-heavy nuclei with greater than 184 neutrons and an atomic number of 110. In 1962–1963 Flerov and his collaborators discovered the phenomenon of delayed proton emission. Experiments over the course of many years confirmed that the phenomenon was a widespread property of supersaturated by protons isotopes. It has been successfully used to obtain valuable information about the structure of the nucleus.

In 1966–1967 the team discovered yet another phenomenon: the fission of nuclei from high-lying excited states formed after beta-decay (electron capture). The first observation of this phenomenon was in isotopes of americium. It has been established that such phenomena play an important role in nucleosynthesis. In 1969–1970 a new class of nuclear reaction was discovered in the Flerov laboratory—deep inelastic reactons. There has been a veritable boom in research on these new types of reactions since 1973. Researchers at Dubna, Orsay (France), and Berkeley (United States) have obtained results demonstrating that for massive ions, such as ions of argon and krypton, these nuclear reactions are the dominant nuclear processes in the collision of two nuclei. The study of the nuclei of reactions of deep inelastic scattering of nucleons is one of the main directions of nuclear physics research with heavy ions.

Practical Applications of Nuclear Physics . Flerov actively participated in the development of nuclear methods for prospecting for oil and for the most efficient use of the oil fields in the USSR. For several years after 1951 he directed a group of scientists at the Moscow Petroleum Institute (later the State Academy of Petroleum and Gas) in research that eventually developed an improved apparatus for neutron and gamma-logging of oil reservoirs. This group also developed an original impulse method of neutron-logging.

In his Laboratory of Nuclear Reactions (Laboratoriia Iadernykh Reaktsiî, LIaR), Flerov organized a section for applied nuclear physics, devoted to studies on polynuclear filters and on radiation aspects of materials sciences. These scientists have developed methods using heavy ion beams for analyzing the durability and other properties of construction materials for thermonuclear plants. This section also has conducted work on obtaining the shortest-lived radionuclides for biological studies and medical diagnostics. After the Chernobyl nuclear disaster of 26 April 1986, 187 workers from JINR participated in the efforts to deal with the consequences of this catastrophe. The institute not only offered its specialists but also knowledge of radiation containment and various types of apparatus and protective gear, such as gas masks and respirators with Flerov filters (nuclear membranes), which were successfully used in the zone of radiation contamination.

The scientific achievements of G. N. Flerov have been highly praised not only in his native country but also abroad. He was elected a foreign member of the Royal Danish Academy of Sciences, an honorary member of the German Academy of Sciences Leopoldina, and was awarded honorary doctorates from a variety of foreign universities. Flerov also was given awards from the governments of various countries, including Mongolia and Bulgaria.



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Elena Zaitseva

Nathan Brooks