Is the dispersion relation applicable for exotic nuclear systems? Scientists from the Department of Nuclear Physics of CIAE have recently found the answer to this important question. The scientists are from the department’s research team on nuclear reaction, which is headed by researcher Lin Chengjian. Their research results were published in Physical Review Letters on 26th July, 2017, the paper titled “Is the Dispersion Relation Applicable for Exotic Nuclear Systems? The Abnormal Threshold Anomaly in the 6He+209Bi System” 

With the rise of radioactive nuclear beam, unstable nuclear-induced nuclear reaction has become a hot-spot issue in nuclear physics. Unstable nuclei are usually weakly bound, with exotic structures such as halo (skin) or cluster. Nuclear collisions of unstable nuclei will lead to new reaction mechanisms such as rupture and strong-coupling of continuous states. The phenomenological expression of the nuclear-nuclear interaction potential is the optical potential. As a basic physical quantity, it is the starting point of experimental research. The optical potential of the weakly bound nuclei system is very different from that of the traditional tightly bound system, especially the fact that significant imaginary potential exists in the below-barrier region of the former system, which has aroused great attention of scientists. However, serious uncertainties are seen in optical potential if it is extracted from elastic scattering in the below-barrier region. And the quality of extracted optical potential is further worsened by radioactive nuclear beam featured with low current intensity and poor quality. These problems seriously hinder scientists to conduct in-depth study of the optical potential of these exotic nuclear systems. 

Lin and his research team proposed a new method to measure optical model potential (OMP) of halo nuclear systems via transfer reactions of stable nuclei beams. They established wide-scope detector matrix in the tunnel end R60 of BRIF through which they measured angular distribution of transfer reactions 208Pb(7Li,6He)209Bi in the incoming and outgoing channels at energies ranging from above-barrier region to deep below-barrier region, and precisely extracted OPMs of halo nuclear system 6He+209Bi. The researchers for the first time observed decrease of imaginary potential in the extremely low energies, and found its threshold of about 0.68 times the Coulomb barrier. Comparing the imaginary potential from the experiment with real potential calculated according to the dispersion relation, it was found that there appeared significant deviation between the two values, which indicated that the dispersion relation is inapplicable to the halo nuclear system. This might be a universal phenomenon for exotic nuclear systems.   

Dispersion relation (known as the Kramers-Kronig relation in the general case) is a natural consequence of the causality principle in stable systems, which describes dispersion effect when a wave travels through a medium. The research provided an evidence of inapplicability of the dispersion relation to exotic nuclear systems, and explanations to possible reasons for such anomaly. The results may bring extensive impact on a number of areas including quantum mechanics, optics, particle physics and electronic engineering.