With the participation of the Nuclear Research Institute in Debrecen (Atomki), an international research group found a solution to the 40-year-old mystery of why fission products rotate after nuclear fission even when the nucleus has not rotated.
Although the study of nuclear fission has a long history, there are still unexplored, exciting phenomena on this topic – read the publication of the Institute for Nuclear Research.
As they write, one of these is the origin of the spinning motion of fissures. Both halves of a split core rotate even if the parent core has not rotated. This phenomenon has been known for more than 40 years, but so far it has not been understood. Different theoretical ideas compete with each other, between which experimental observation could not do the truth. Until now, there has been roughly a consensus among rival models in one respect: it was thought that the collective vibration of the parent nucleus before fission occurred was responsible for the resulting momentum.
An article published in the journal Nature in late February refutes the above theory. According to the authors, including an employee of Atomki, the vortex does not arise before the fission, but after it.
Nuclear fission was discovered in 1938 by Otto Hahn and Fritz Strassmann during the study of uranium nuclei, and a theoretical explanation was given by Lise Meitner. During fission, two (or more) smaller parts of the nucleus are released, other more or less neutrons are released, and the process is accompanied by gamma radiation. Energy is released during the fission of heavy nuclei.
Nuclear nuclei do not always split in the same way, fission products can be diverse. Before the moment of nuclear fission, the shape of the nucleus changes: from a slightly elongated shape to a strongly elongated one, it tapers in the middle, it can be said that its neck is formed, which becomes thinner and thinner. What fission products are formed depends on where the “neck” is randomly broken.
Nuclear fission can occur spontaneously, i.e., without external intervention or in an induced manner, when a particle (e.g., a neutron) collides with the nucleus and causes it to split. Shortly after the discovery of the phenomenon, the application was born: in 1942 the first nuclear reactor was completed, and in 1945 the first atomic bomb. Both utilize the energy released during fission, the difference being in the course of time.
In a nuclear reactor, fission takes place under controlled conditions, constantly controlling the number of fission nuclei involved in the chain reaction. Most nuclear reactors use uranium (235U isotope) as a fissile material. In contrast, in the case of a bomb, the chain reaction is not limited, so the huge energy released during a large number of fission in a short time causes an explosion. The best known fissile material in the atomic bomb is plutonium (239Pu).
In the present study, an international research team analyzed spontaneous fission californium (252Cf) and fast neutron-induced fission on thorium (232Th) and uranium (238U) isotopes in experiments at the IJC Laboratory in France. The analysis of the obtained large amount of data, their comparison with the results of the theoretical calculations and the simulations carried out eventually led the researchers to conclude that the rotation of the two fissures formed in nuclear fission is independent of each other after the nuclear fission. Thus, the rotational motion of the fissures is not due to the collective vibration that occurs before nuclear fission.
According to their explanation, before nuclear fission, the “neck” connecting the parts in the separator first elongates, then breaks, and finally the torn, deformed fissures acquire their spherical shape. Meanwhile, similar to a rubber band stretched to tear, the potential energy initially stored in the elongated neck is converted into kinetic / rotational energy.
According to the article, the rotation of fissures depends on how many nucleons are located in the neck threaded during the fission process and exactly where the rupture occurs.
The results of this paper are important for a better understanding and theoretical description of nuclear fission, for studying the structure of neutron-rich isotopes, for understanding the formation and stability of superheavy nuclei, and for the heating problem caused by fission gamma radiation in nuclear reactors.
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