Abstract
The ^{12}C+^{12}C fusion reaction plays a critical role in the evolution of massive stars and also strongly impacts various explosive astrophysical scenarios. The presence of resonances in this reaction at energies around and below the Coulomb barrier makes it impossible to carry out a simple extrapolation down to the Gamow window-the energy regime relevant to carbon burning in massive stars. The ^{12}C+^{12}C system forms a unique laboratory for challenging the contemporary picture of deep sub-barrier fusion (possible sub-barrier hindrance) and its interplay with nuclear structure (sub-barrier resonances). Here, we show that direct measurements of the ^{12}C+^{12}C fusion cross section may be made into the Gamow window using an advanced particle-gamma coincidence technique. The sensitivity of this technique effectively removes ambiguities in existing measurements made with gamma ray or charged-particle detection alone. The present cross-section data span over 8 orders of magnitude and support the fusion-hindrance model at deep sub-barrier energies.
Highlights
The 12C þ 12C fusion reaction plays a critical role in the evolution of massive stars and strongly impacts various explosive astrophysical scenarios
The presence of resonances in this reaction at energies around and below the Coulomb barrier makes it impossible to carry out a simple extrapolation down to the Gamow window—the energy regime relevant to carbon burning in massive stars
We show that direct measurements of the 12C þ 12C fusion cross section may be made into the Gamow window using an advanced particle-gamma coincidence technique
Summary
An approach, which provides a unique signature and which can circumvent these experimental limitations, is to detect evaporated charged particles and gamma rays in coincidence This effectively removes ambiguities such as those associated with protons created in reactions on target contaminants. Experiment.—Here, we report on measurements of 12C fusion well into the Gamow window relevant to the most massive stars (M⊙ ≈ 25) in the energy regime Ebeam 1⁄4 2.2 to 5.4 MeV (in the center-of-mass system) with the STELLA apparatus [24] for coincident gamma-particle detection. To calculate the effective beam energy Eeff, the varying fusion cross section within the target was interpolated with an exponential response function:
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