Abnormal behavior of the optical potential for the halo nuclear system 6 He+ 209 Bi

In a recent transfer reaction measurement of Pb-208(Li-7, He-6)Bi-209 at energies around and below the Coulomb barrier, the optical model potentials of the halo nuclear system He-6 + Bi-209 were extracted by fitting the experimental data with the theoretical frameworks of the distorted-wave Born approximation and coupled reaction channels, respectively. With the high-precision result, a complete picture of the behavior of the optical potential for this halo system is clearly derived for the first time. The real potential presents a bell-like shape around the barrier as a normal threshold anomaly. However, for the imaginary part, it first increases with the energy decreasing below the barrier and then falls quickly to 0, hence the threshold energy can be determined by fitting the variation trend. Moreover, the result also provides some evidence that the dispersion relation does not hold for this halo nuclear system, which calls for further investigation of the underlying physics.