Soft E1 excitation of neutron-halo nuclei

Soft electric dipole (E1) response of 11Li has been investigated by a kinematical complete measurement of the Coulomb breakup at about 70 MeV/nucleon. A two-wall neutron detector setup allowed us to employ a kinematical condition to exclude fully the cross talk events even for the low relative energies (Erel) down to 0 MeV, thereby enhancing the experimental sensitivities in this energy region. In the Erel spectrum, a strong peak corresponding to the soft E1 excitation has been observed at about Ex=0.6 MeV with B(E1)=1.42(18) e2 fm2 (4.5(6) Weisskopf units) for 0⩽Erel⩽3 MeV. The spectral shape as well as its amplitude is in excellent agreement with a three-body model with strong neutron-neutron correlation in the halo. Such correlation is further supported by the E1 non-energy weighted cluster sum rule.