The fusion of three alpha particles to form the excited Hoyle state in 12C, and subsequent electromagnetic decay to the ground state, is the only known pathway to synthesis of stable carbon in the Universe. This process takes place in stars in which the helium density, as well as temperature is sufficiently high for the triple alpha process to occur, i.e. in helium burning red giants. The Hoyle state is a resonance for the 3α process, and greatly enhances the production of carbon. However, the state is located energetically above the α decay threshold, and will therefore disintegrate 99.92% of the time by the emission of an α particle to 8Be, which will further decay into two α particles. There is also a very small probability (<0.043%) that the Hoyle state will disintegrate directly into three α particles. The probability that the Hoyle state decays to a stable configuration of 12C is very small, about 0.04%, and after more than 60 years of research this branching ratio is only known with about 10% accuracy.
In recent years, numerous experiments have been performed to identify and characterise structural properties of the Hoyle state, such as rotational excitations built on top of it, and the roles of different alpha decay channels. These studies motivated and challenged theoretical models, particularly on the description of the spatial configuration of the Hoyle state. The discussions also sparked a new interest to remeasure and improve the knowledge on the triple alpha production rate, which mainly relies on measurements from the 1960-70’s. The 3α rate depends directly on the tiny radiative decay branch of the Hoyle state. My thesis focuses on a series of pair conversion measurements carried out with the Super-e spectrometer at ANU, using the 14UD tandem accelerator. Our results reduced the uncertainty of the E0 pair branching ratio of the Hoyle state by almost a factor of two, which is the component of the radiative decay branch introducing most of the uncertainty. Also, our results were combined with data from a recent proton-gamma-gamma measurement, carried out in collaboration with the Nuclear Physics group in Oslo, Norway. Two new values for the radiative width were deduced; one is a conservative estimate by the average of all measurements, and the other was deduced from the new measurements only. The latter is likely to have significant impact on the predictions of various astrophysical models.
In addition, I will report on the high precision spectroscopy of excited 0+ states and E0 transitions in the Z=26, N=28 nucleus, 54Fe, an isotone of the doubly magic 56Ni. Our results allow for studies of shape co-existence and the evolution of collectivity in the vicinity of the Z=N=28 closed shells. These phenomena are continuously challenging our basic understanding of the nuclear structure, and high precision experimental data are essential.