Three-body structure of low-lying 12Be states

We investigate to what extent a description of 12Be as a three-body system made of an inert 10Be core and two neutrons is able to reproduce the experimental 12Be data. Three-body wave functions are obtained with the hyperspherical adiabatic expansion method. We study the discrete spectrum of 12Be, the structure of the different states, the predominant transition strengths, and the continuum energy spectrum after high-energy fragmentation on a light target. Two 0+, one 2+, one 1-, and one 0- bound states are found; the first four are known experimentally, whereas the 0- is predicted as an isomeric state. An effective neutron charge, reproducing the measured B(E1) transition and the charge rms radius in 11Be, leads to a computed B(E1) transition strength for 12Be in agreement with the experimental value. For the E0 and E2 transitions, the contributions from core excitations could be more significant. The experimental 10Be-neutron continuum energy spectrum is also well reproduced, except in the energy region corresponding to the 3/2- resonance in 11Be where core excitations contribute