Threshold Anomaly in the ⁶He+²⁰⁹Bi System

Full text: A main task in the study of nuclear reactions is to understand the nucleus-nucleus interaction. A phenomenological interaction potential, i.e. the optical potential, is universally adopted to describe the interaction of nuclear collisions. For tightly-bound nuclear systems, optical potentials show a threshold anomaly behavior at energies around the Coulomb barrier, which can be explained well by the dispersion relation. For weakly-bound nuclear systems, however, optical potentials show a different behavior, the so-called breakup threshold anomaly, which is not yet understood both experimentally and theoretically. To this end, angular distributions of elastic scattering and one-proton stripping reaction for the ⁷Li+²⁰⁸Pb systems have been measure at energies around the Coulomb barrier as well as well-below the barrier by using the Q3D magnet spectrograph and silicon-strip detector array, respectively. Optical potentials for the ⁷Li+²⁰⁸Pb system were extracted by fitting angular distributions of the incoming channel. After that, Optical potentials for the outgoing channel of ⁶He+²⁰⁹Bi were extracted with good precision by fitting angular distributions of ²⁰⁸Pb(⁷Li, ⁶He) reaction. The imaginary potentials increase when energies decrease below the barrier, and quickly drop down to zero at extremely low energy. The threshold energy was determined to about 13.7 MeV, which is corresponding to about 0.73 VB. This is the first time to determine the threshold energy for the weakly-bound system. Moreover, the traditional dispersion relation cannot describe the behavior of imaginary and real potentials. Some possible underlying physical reasons are pointed out. A new type of dispersion relation is strongly called for. Details of the experiment and analysis as well as discussions will be presented in the conference. (author)