Line-shape analysis of Doppler-broadened γ lines following the β decay of 11 Li

The β decay of 11 Li is studied at the TRIUMF Isotope Separation and Acceleration (ISAC) facility using the 8n y-ray spectrometer coupled with an inner array of 20 plastic scintillators for β detection. Doppler-broadened line shapes resulting from the decay of the excited states in 10 Be populated by β-delayed one-neutron emission are analyzed using Monte Carlo simulations. New β-delayed neutron decay branches are shown to contribute to the complex decay of 11 Li. The half-lives of all but one bound excited state in 10 Be are also deduced from this work. Among them, the half-life of the 2- state in 10 be is shown to be much shorter than previously thought, yielding a new experimental B(E1) now well within the range of theoretical predictions and providing further evidence that the 2- state in 10 Be is an excited halo state. The nature of the 8.82-MeV state in 11 Be and its decay paths to excited states in 10 Be are found to be consistent with the β decay of the core proceeding through this particular state.