A Fingerprint of α Clustering Configurations in Atomic nucleus

Cluster phenomenon widely exits in various fields of physics, from galaxy structure to Nano level, and nuclear theory predicts the existence of cluster or molecular made of nucleons at femtometer (1 fm =10^-15 m) scale in subatomic field. The newest theory predicts the cluster configurations appear not only in the excited states of light nuclei and nuclei far from β stability line, but also exist in the ground states of some light nuclei, such as the possible exotic triangle configuration with three α’s in ¹²C near ground state and tetrahedral configuration of ¹⁶O ground state made of  four α’s.  It is well known that α clustering plays a very important role in Bose-Einstein condensation in nucleus,  nuclear equation of state at low density and nucleosynthesis in nuclear astrophysics. However, it is very difficult now to confirm the cluster configuration in nuclei and study the underlying collective dynamics of cluster, due to lacking the effective observable from the experimental side. Therefore, it is high needed to put forward new sensitive probe to diagnose the cluster in nuclei.

 Recently, Yu-Gang Ma’s group proposed a new observable for alpha-cluster configuration of light nuclei which has just been published in Phys. Rev. Lett. 113, 032506 (2014). In this work, Mr. Wan-Bing He, a PhD student from nuclear physics division of Shanghai Institute of Applied Physics, CAS, under the guidance of Prof. Yu-Gang Ma and Dr. Xi-Guang Cao, who also collaborate with Dr. Xiangzhou Cai and Guoqiang Zhang put forward a new observable of cluster configuration – the γ spectrum emitted from giant dipole resonance (GDR) by studying the collective dynamics of cluster nuclei in the framework of an extended quantum molecular dynamics model.  

 The spectrum of high energetic photons is produced from bremsstrahlung in one of the nuclei oscillation modes called giant dipole resonance (GDR), where the collective motion of neutrons and protons against each other. The GDRs of ⁸Be, ¹²C and ¹⁶O with differentαconfigurations are studied. It is found that GDRs fragment into several apparent peaks due to the existence of α cluster. The different α cluster configurations have corresponding characteristic spectra of GDR. The number and centroid energies of peaks in the GDR spectra can be well explained by the geometrical and dynamical symmetries of α clustering configurations. The GDR peak located around 30 MeV is a feasible way to confirm the existence of α cluster degree of freedom. Analysis of other low- lying peaks can be used to diagnose the different configurations formed by α clusters. In addition, in the GDR spectra of chain ¹²C and kite ¹⁶O, there exist similar GDR spectra of ⁸Be and triangle ¹²C, respectively, which can also used to clearly distinguish the different configurations of ¹²C and ¹⁶O. Therefore, the GDR can be regarded as a very effective probe to diagnose the different α cluster configurations in light nuclei.

 Making use of the probe obtained in this work, the experiments aiming at extracting the α cluster configurations of ¹²C and ¹⁶O in ground and excited states, are feasible to be carried out on future Shanghai Laser Electron Gamma Source (SLEGS) terminal belong to Shanghai Synchrotron Radiation Facility (SSRF), also on the terminal of High Intensity Gamma-Ray Source (HIγs) located in the campus of Duke University and operated by Triangle Universities Nuclear Laboratory (TUNL).  

 This work is partly supported by the Ministry of Science and Technology of China (the Major State Basic Research Development Program) and National Natural Science Foundation of China.

Fig.1 The γ spectrum of giant dipole resonance from 8Be, 12C and 16O with different cluster configuration.