R-matrix analysis of elastic scattering, phase shifts, and radiative capture reaction cross sections in the ? plus ? system

The unstable nucleus 8Be, with its two alpha-cluster configuration, is the doorway to the formation of heavier alpha-cluster nuclei. Most importantly, its the precursor of the production of 12C through the Hoyle state, a resonance state of three alpha clusters, in the helium burning phase of a massive star. The nucleus exhibits a ground state band of rotational states established through alpha-alpha scattering experiments. A subsequent precision particle-gamma coincidence measurement of the electromagnetic transition between the 4+ -> 2+ excited states also corroborated the evidence for a highly deformed dumb-bell shaped structure of 8Be. A simultaneous phenomenological R-matrix analysis of the measured capture reaction cross sections along with the elastic excitation function and phase shift data has been performed. The resulting reduced transition strength of 21.96 +/- 3.86 e2fm4 compares well with the estimated experimental value of 21.0 +/- 2.3 e2 fm4. The R-matrix yield of the B(E2) value is closer to the prediction of cluster model but about 19% less than the ab initio result.