An international research team with the Nuclear Research Institute has found a solution to a 40-year-old mystery: why fission products rotate after nuclear fission even when the nucleus has not rotated. A recent article in the journal Nature proves that the rotation of the two fissures created during the process is independent of each other, they spin up after the fission. This was achieved by the researchers after analyzing the large amount of data, comparing them with the results of theoretical calculations and running Monte Carlo simulations.

There are attractive and repulsive interactions between protons and neutrons (collectively: nucleons) that make up nuclei. up. Protons are positively charged, they repel each other electrically. Neutrons are neutral, so there is no electrostatic (Coulomb) interaction between them. Yet most of the nuclei that occur in nature are stable, and the protons in them do not run apart despite repulsion. This is because there is another very short-range but extremely strong attractive interaction between the nucleons that make up the nucleus, called the nuclear force. The stability of nuclei is thus shaped by the interplay of repulsive electric and attractive nuclear force. Very heavy nuclei do not exist because they are cleaved by Coulomb repulsion.

Nuclear fission was discovered in 1938 by Otto Hahn and Fritz Strassmann during the study of uranium nuclei, and was theoretically explained by Lise Meitner. During fission, the nucleus breaks into two (or more) smaller parts, while 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. The type of fission products produced depends on the location of the neck at random

Nuclear fission can occur spontaneously, ie without external intervention or induced when a particle (such as 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 restricted, 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).

Although the study of nuclear fission has a long history, there are some unexplored, exciting phenomena in this subject.

One such the origin of the spinning motion of fissures. Both halves of a split core rotate even if the parent core had no momentum, i.e., did not rotate. 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 has so far failed to do truth. Until now, there has been a broad consensus among rival models in one respect: the collective vibration of the parent nucleus prior to fission has been thought to be responsible for the resulting momentum.

A recent article in the journal Nature refutes the above theory. According to the authors, including a staff member of Atomki, the momentum (impulse moment) does not occur before but after fission

An international research team studied spontaneous fission californium (252Cf) in experiments at the IJC Laboratory in France. , and induced fission induced by fast neutrons on thorium (232Th) and uranium (238U) isotopes. The rotation of the resulting fissures ceases very quickly, in a few nanoseconds (10-9 s), because the fissure tries to get rid of its excess energy by emitting gamma radiation. This gamma radiation was detected during the measurements. The analysis of the large amount of data, their comparison with the results of the theoretical calculations and the Monte Carlo simulations, eventually led the researchers to conclude that the rotation of the two fissures in nuclear fission is independent of each other after the nuclear fission. Thus, the rotational motion of the fissures is not responsible for the collective vibration that occurs before nuclear fission.

The explanation is instead as follows. Prior to nuclear fission, the neck connecting the parts in the separator first elongates, then ruptures, and finally the ruptured, deformed fissures acquire their spherical shape. Meanwhile, similarly to a rubber band stretched to tear, the potential energy initially stored in the elongated neck is converted into kinetic / rotational energy.

According to this article, the rotation of fissures depends on how many nucleons receive in the neck during the fission process. location and exactly where the rupture occurs. In the classic case, the neck would rupture in the weakest place, in the middle, but in the world of atoms, rupture, if not with equal probability, could occur anywhere. When the rupture occurs, the nucleons that make up the neck are logically distant from the center of mass of the fissure that has just formed, but they try to get as close to it as possible, thus taking up your rounded nuclei. The approach is not exactly in the direction of the center of gravity, but in one way or another slightly different from it due to the random nature of the rupture. Because of this, the fissure begins to rotate. (Like when a football player kicks the ball slightly sideways and spins it.) In the resulting two fissures, different numbers of cervical nucleons, due to their random location, create spins of different magnitudes and orientations, without, of course, violating the law of momentum retention. This model explains the researchers’ current observations.

The result of this article is important for a better understanding and theoretical description of nuclear fission, study of the structure of neutron-rich isotopes, understanding the formation and stability of superheavy nuclei, and fission in nuclear reactors.

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