Speaker
Description
The Hoyle state, the triple-$\alpha$ resonance in $^{12}$C, plays a central role in stellar helium burning and is responsible for the production of nearly all carbon. The formation of $^{12}$C requires the Hoyle state to decay to the ground state through radiative channels, including a cascade of two gamma rays (both E2 transitions) via the first excited 2$^+$ state and internal pair emission (known as the E0 branch). The probability of this relaxation to the ground state through gamma emission is known as the branching ratio. Two recent direct measurements of the E2 radiative branching ratio performed before this work differ by approximately a factor of 1.5. This introduces an uncertainty of as much as 25% in stellar nucleosynthesis calculations that determine the relative production of $^{12}$C and $^{16}$O and influencing subsequent r-, p-, and i-process reaction networks. Resolving this discrepancy is therefore important for improving astrophysical reaction rate calculations.
The coincident detection of scattered charged particles with high-resolution $\gamma$ spectroscopy enables clean identification of reactions populating the Hoyle state and allows the direct observation of the two $\gamma$ rays cascade populating the ground state. GODDESS combines the ORRUBA silicon detector array for quasi-$4\pi$ charged-particle detection with the GRETINA gamma-ray tracking array, providing high energy resolution and approximately 33% of $4\pi$ solid-angle coverage. This capability provides a powerful approach for determining the E2 radiative branching ratio while suppressing competing backgrounds.
We report on a new measurement of the Hoyle-state E2 radiative branching ratio using GODDESS. The experiment was performed at the ATLAS facility at Argonne National Laboratory using a 10.5 MeV proton beam impinged on a 99% enriched $^{12}$C target, delivered under multiple beam current conditions over 120 hours of beam time. The Hoyle state was populated via 12C(p,p’), and the ratio of the observed number of two gamma-cascade to the number of particles detected gives us our branching ratio.
Experimental results and analysis of the E2 radiative decay branch of the Hoyle state will be presented.
Work supported by US Department of Energy and National Science Foundation. Some of this work was also supported through the US Department of Energy Office of Science Graduate Student Research (SCGSR) Program.
| Contribution category | Experiment |
|---|---|
| Presenter status | Postdoc |