4. 2  Investigation of the Clustering Structure of Neutron-rich Nuclei through the Study of Fragmentation
 


Fig. 4-2 Change of the density distributions of B isotopes with increasing neutron number calculated using antisymmetrized molecular dynamics (AMD). Within the AMD framework spatial distribution functions of single-particle states are expressed by Gaussians. The positions of the centers of such Gaussians, which are indicated by red and blue points, are determined to minimize the energy of the system.


Fig. 4-3 The calculated dependence of coincidence cross sections between He and Li fragments on incident energy in 13B + 14N (blue line) and 19B + 14N (red line) reactions, which are simulated using antisymmetrized molecular dynamics.

A nucleus is a many-body system consisting of protons and neutrons. Many nuclear properties are describable in terms of the idea that the nucleons in a nucleus can be considered as independent particles moving in some average potential created by all the other nucleons. The occurrence of the so-called magic numbers 2, 8, 20, 28, 50, 82, and 126 has, from the experimental point of view, been one of the strongest motivations for the development of the nuclear shell model. Recently, numerous experiments using unstable beams have succeeded in extending the observed neutron drip line, and various features not expected in nuclei near the stability line have been revealed. Not only is the halo structure of weakly bound neutrons anomalous, but the change of the shell structure is as well. Shell-structure anomalies are closely related to the change of structure with increase of neutron number. Figure 4-2 shows the theoretical prediction of the change of structure for the isotopes of boron. The neutron closed-shell nucleus 13B has a spherical structure, the deformation of which becomes progressively greater with increase of neutron number. The neutron drip-line nucleus 19B has a well-developed He-Li cluster structure. However, the occurrence of such a clustering structure as a result of neutron-rich composition has not been confirmed experimentally up to now.
We have proposed an experimental method, using fragmentation reactions, to confirm the clustering structure of 19B. The difference between the 13B and 19B structures, as shown in Fig. 4-2, will be reflected in fragments produced in heavy-ion reactions. It is expected that He and Li isotopes produced from 19B fragmentation are abundant compared with those from 13B, because 19B has a He-Li clustering structure, while 13B contains no seeds for He or Li clusters. We have checked this expectation using antisymmetrized molecular dynamics. Figure 4-3 shows the dependence of the coincidence cross section between He and Li fragments on incident energy in 13B + 14N and 19B + 14N reactions. As is clearly seen, the coincidence cross section for 19B + 14N is larger than that for 13B + 14N, reflecting the cluster structure of 19B. Accordingly, we suggest that a coincidence experiment between He and Li isotopes produced in heavy-ion reactions may be used to confirm the existence of a cluster structure for the isotope 19B.


Reference
H. Takemoto* et al., Incident-Energy Dependence of the Fragmentation Mechanism Reflecting the Cluster Structure of the 19B Nucleus, Phys. Rev. C63, 034615 (2001). *post doctoral fellow.

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Persistent Quest - Research Activities 2001
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