A research group led by Graduate Student Maito Okada, Associate Professor Wataru Horiuchi, and Professor Naoyuki Itagaki of the Graduate School of Science at Osaka Metropolitan University, has revealed that in titanium 48, the nucleus of titanium changes from a shell structure to an alpha-cluster structure depending on the distance from the center of the nucleus. This result is expected to overturn conventional understanding of the nuclear structure and provide clues into the α-decay mechanism that occurs in heavy nuclei, which has not been solved for about 100 years. The result was published in the online bulletin of Physical Review C.
Helium nucleus (α-particles) is a system of two strongly bound protons and two strongly bound neutrons (nucleons) that form a clump, called an α-cluster. This α-cluster structure is considered to be particularly important in lighter nuclei and is closely related to the origin of elements. Furthermore, as a nucleus becomes heavier, it is known that its shell structure, the standard view of nuclei, becomes better established. However, in the shell structure, each nucleon moves independently, which cannot explain the α-decay observed in many heavy nuclei.
In recent years, there has been a debate over whether nuclei in the medium-mass region of nuclear systems have a shell or an α-cluster structure. For example, assuming that the titanium 48 nucleus has an α-cluster structure made up of an α-particle and of calcium 44, one can explain the results of the experiment (α-knockout reaction) wherein α-particles were knocked out from the nucleus, but one cannot reproduce other fundamental properties associated with the nucleus such as its radius. However, if it is assumed to have a shell structure, then, it can explain the basic properties but cannot explain the experimental results of the α-knockout reaction.
To determine which structure the titanium 48 nucleus actually possesses, the research group constructed a theoretical model that can represent the shell and cluster structures in the same framework. Using the general property of nuclear reactions, namely that proton bombardment on a nucleus reveals the structure near the surface of the target nucleus and α-particle bombardment on a nucleus provides the structure of the outer region, they calculated the process of the bombardment of titanium 48 with highly accelerated protons and α-particles. By comparing the results of the theoretical calculations of titanium 48, namely by assuming that it has a shell structure and a cluster structure, with experimental data, it was determined that titanium 48 has an α-cluster component outside the surface, although the shell structure is dominant, which is the standard view.
The results of this study are the first to present the possibility that the nuclear structure in the medium-heavy region can shift from a shell structure to an α-cluster structure depending on the distance from the center of the nucleus, thereby overturning the conventional understanding of a nuclear structure. The phenomenon of the α-decay of heavy nuclei has long been known, but its fundamental understanding (whether α particles exist on the surface of nuclei) remained unknown since the Gamow's theory about 100 years ago. The present study indicates that in nuclei in the medium-heavy region and beyond, α particles may be deposited on the surface, which is beyond the understanding of the usual shell model. It provides a solution to the major question of why heavy nuclei undergo α decay and opens up new horizons in the study of the nuclear structure.
Horiuchi said, "The analytical method we developed in this study has demonstrated that medium-heavy mass systems such as titanium 48 have a shell structure at the center and an α-cluster structure at their very surface. This is the first time such a structural transition has been demonstrated, and this result offers a new way of looking at nuclei. In the future, we would like to extend the results obtained in this study and challenge the unsolved problems of heavier nuclei."
Journal Information
Publication: Physical Review C
Title: Shell-cluster transition in 48Ti
DOI: 10.1103/PhysRevC.109.054324
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