Nuclear Structure and Dynamics

Role of the continuum in Beryllium-11

Role of the continuum in Beryllium-11: The exotic nucleus beryllium-11 (11Be) is a one-neutron halo nucleus, one in which the last proton or neutron appears to be in a large, extended orbit around an otherwise normal core. The extent to which 11Be truly behaves as a lone neutron in a large orbit around a 10Be core has been the subject of two important TIGRESS experiments, both of which employed scattering of 11Be off of heavy metal thin foil targets. The first, also using TRIUMF's Bambino, as measured the ratio of elastic scattering to Coulomb excitation, the process in which energy is transferred to a nucleus from the electric field of another, and (in most cases) re-emitted as a gamma ray. As reported in Physical Review Letters (2017), this high-precision measurement revealed the influence of coupling to the continuum, the role of virtual states corresponding to unbound, free neutrons interacting with 10Be. A second experiment with TIGRESS, published in Physics Letters B (2014), also identified breakup reactions, those in which enough energy is transferred to break 11Be into 10Be and a free neutron, a “continuum” state.  The first measurement was accurate enough to validate a first-principles calculation of the gamma-decay rate, which revealed the influence of the coupling of virtual, continuum states to the overall structure of the bound states of 11Be.  The second showed that the overall reaction dynamics depended not just on the continuum but also on highly excited states in the 10Be core itself.