The second 2+ state of the C12 nucleus is of great importance to nuclear astrophysics reaction rate calculations and also to nuclear cluster structure studies. The triple-alpha process, which is responsible for C12 production, primarily proceeds through a resonance in the C12 nucleus, famously known as the Hoyle state. The cluster nature of the Hoyle state allows the formation of a rotational band built upon it. The first member of the band is thought to be in the 9 -11 MeV excitation energy region, with 2+ spin-parity. Further knowledge of this state would help not only to understand the debated structure of the C12 nucleus in the Hoyle state, but also to better constrain the high-temperature (>1 GK) reaction rate of the triple-alpha process. The precise evaluation of the triple-alpha reaction rate is required to be able to understand the final stages of stellar nucleosynthesis and the elemental abundances in the universe.
In the past decade, several experiments have been performed in search for the second 2+ state of the C12 nucleus. But the results from different measurements have been so different from each other that it has not been possible to reach a consensus on the resonance parameters. Due to the significance of the resonance, a reconciliation of the data from different available probes is highly desirable.
In this work, the first clear observation of the 2+ rotational excitation of the Hoyle state via a N12 beta-decay experiment has been presented. The experiment, N12(beta-decay)C12(alpha1)Be8(alpha2)alpha3, was performed at the IGISOL facility at JYFL, Jyväskylä, Finland. The identification of the 2+ strength relative to that of the 0+ in the 9 - 12 MeV energy region has been done through the novel technique of coincident detection of beta-3alpha particles from the cascade, followed by the beta-alpha1 angular correlation studies.
