Experiments to further the understanding of the triple-alpha process in hot astrophysical scenarios
In astrophysics, the first excited 0 + state of 12C at 7.654 MeV (Hoyle state) is the most important in the triple-a process for carbon nucleosynthesis. In explosive scenarios like supernovae, where temperatures of several 10 9 K are achieved, the interference of the Hoyle state with the second 0 + state located at 10.3 MeV in 12C becomes significant. The recent NACRE compilation of astrophysical reaction rates assumes a 2 + resonance at 9.1 MeV for which no experimental evidence exists. Thus, it is critical to explore in more detail the 7-10 MeV excitation energy region, especially the minimum between the two 0 + resonances for carbon nucleosynthesis. The states in 12C were populated through the β-decay of 12B and 12N produced at the ATLAS (Argonne Tandem Linac Accelerator System) in-flight facility. The decay of 12C into three alphas is detected in a Frisch grid twin ionization chamber, acting as a low-threshold calorimeter. This minimizes the effects of β-summing and allowed us to investigate the minimum above the Hoyle state with much higher accuracy than previously possible. A detailed data analysis will include an R-matrix fit to determine an upper limit on the 2 + resonance width. © 2009 American Institute of Physics.