Experiments to Further the Understanding of the Triple-Alpha Process in Hot Astrophysical Scenarios

Abstract

In astrophysics, the first excited 0+ state of 12C at 7.654 MeV (Hoyle state) is the most important in the triple‐α process for carbon nucleosynthesis. In explosive scenarios like supernovae, where temperatures of several 109 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.