Investigation of the Hoyle state in 12C and the related triple alpha reaction rate

Abstract

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 red giant stars, in which the helium density and temperature are su ciently high for three α particles to fuse. The Hoyle state is located energetically above the ⁸Be + α and 3α energies, which makes it a resonance for the triple-α process, and thus greatly enhances the production of carbon. This also means that the Hoyle state is unstable to α breakup, and consequently, the probability that the Hoyle state decays electromagnetically to a stable con guration of ¹²C is very small, only about 0.04%. After over 60 years of research, the electromagnetic branching ratio is only known with 10% accuracy, and the adopted value mainly relies on measurements from the 1960-70s. The rate of the triple-α process depends directly on the tiny radiative decay branch of the Hoyle state, and it is imperative for astrophysical modeling to reduce its uncertainty. The present work focuses on a series of pair conversion measurements of transitions from the two first excited states in ¹²C, populated by the (p; p') reaction at 10.5 MeV beam energy. The measurements were carried out with the Super-e spectrometer at the Australian National University, where the beam was delivered by the 14UD tandem accelerator. The experiments were conducted with aim to deduce an accurate value on the radiative width of the Hoyle state, by a novel method. Another goal was to deduce a new, improved value on the partial E0 decay branching ratio. Two new values on the radiative width, based on new and averaged measurements, are discussed, and a new value on the E0 branching ratio is recommended. The values are Γrad = 2:29(24) meV, Γrad = 3:27(57) meV, and Γ(E0)/Γ = 7:19(37)*10⁶, respectively. In order to have confidence in the measurements, a great deal of work was put into characterization of the spectrometer transmission and detection efficiency. A part of this characterization involved the analysis of conversion electron and internal pair spectra of transitions in ⁵⁴Fe, which has a clean energy spectrum that includes a strong E0 transition. Besides being a test case, the experiment is also part of a campaign to obtain high precision spectroscopy data on excited 0+ states and E0 transitions in the N ≈ Z ≈ 28 region of the nuclear chart. Shape co-existence and collective vibrations in the vicinity of the Z = N = 28 closed shells are continuously challenging our basic understanding of the nuclear structure, and experimental data are essential. As physics results emerged from the data, a more comprehensive analysis became a part of this thesis. Two deformed band structures, built on the 0+2 and 2+2 levels, were identfi ed, and properties of the 0+3 state were deduced.