New binding energy for the two-neutron halo of 11Li.

The extended radius of a halo nuclide is very sensitive to the minute binding energy of its valence nucleons. The binding energy of 11Li has been measured with high precision by using the radio-frequency spectrometer MISTRAL at CERN's ISOLDE facility. The new two-neutron separation energy of 378+/-5 keV is 25% higher than the previously accepted value with an uncertainty 5 times smaller.