Carbon, the fourth most abundant element in our universe, is produced in the triple-alpha process in helium-burning red-giant stars. In 1953 Fred Hoyle realized that the fact that there is carbon in the universe requires a resonant state in 12C very near 7.7 MeV energy. The subsequent observation of the state in 1953 is often cited as the beginning of experimental nuclear astrophysics. In the so-called triple-α process, the unstable 8Be nucleus, which decays back to two a particles with a half-life of T1/2=6.7 x 10-17 s, is combined with a third α-particle to form the 7.654 MeV resonant state in the stable nucleus 12C. However 99.96% of time the resonant state decays back to 8Be by alpha emission, producing no stable carbon nuclei. A small fraction of the time, the resonant state decays to lower states in the carbon nucleus, which remain stable against α-emission. These decay paths, the only source of carbon in the universe, proceed by the emission of a 3.215 MeV electric quadrupole (E2) and a 7.654 MeV electric monopole (E0) transition.
The sum of absolute E2 and E0 decay rates is known to only 12% accuracy, which has been identified as a major obstacle to improve current stellar models. Previous measurements are based on rather indirect observations. We are planning to measure the relative E0 and E2 decay rates by observing the electron-positron pairs emitted from the Hoyle state. While the E0 decay was observed in the initial ANU experiments, the E2 transition is below our detection limit.
The core part of the project will involve to understand the instrumental background through Monte-Carlo simulations and to carry out experiments to observe the E2 and E0 transitions in the lalboratory.