Three-body structure of low-lying 12Be states

We investigate to what extent a description of Be-12 as a three-body system made of an inert Be-10 core and two neutrons is able to reproduce the experimental Be-12 data. Three-body wave functions are obtained with the hyperspherical adiabatic expansion method. We study the discrete spectrum of Be-12, 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 Be-11, leads to a computed B(E1) transition strength for Be-12 in agreement with the experimental value. For the E0 and E2 transitions, the contributions from core excitations could be more significant. The experimental Be-10-neutron continuum energy spectrum is also well reproduced, except in the energy region corresponding to the 3/2(-) resonance in Be-11 where core excitations contribute.