Abstract
The 12 C( α , γ ) 16 O reaction plays a key role in the evolution of stars with masses of M > 0.55 M ⊙ . At the Gamow peak ( E c.m. = 300 ke V, T 9 = 0.2), the cross section of the 12 C( α , γ ) 16 O reaction is so small (about 10 −17 barn) that the direct measurement in ground laboratory is not feasible with the existing technology. Up to now, the cross sections at lower energies can only be extrapolated from the data at higher energies. However, two subthreshold resonances, locating at E x = 7.117 MeV and E x = 6.917 MeV, make this extrapolation more complicated. In this work the 6.917 MeV subthreshold resonance in the 12 C( α , γ ) 16 O reaction was investigated via the 12 C( 11 B, 7 Li) 16 O reaction. The experiment was performed using the Q3D magnetic spectrograph at HI-13 tandem accelerator. We measured the angular distribution of the 12 C( 11 B, 7 Li) 16 O transfer reaction leading to the 6.917 MeV state. Based on DWBA analysis, we derived the square of ANC of the 6.917 MeV level in 16 O to be (2.45± 0.28) ×10 10 fm −1 , with which the reduced- α width can be computed. Finally, we calculated the astrophysical S E 2 factor of the 6.917 MeV resonance to be 67.6 ± 7.7 ke V b.
Highlights
The 12C(α, γ)16O reaction is believed to be one of the most crucial reactions in nuclear astrophysics [1, 2]
In this work the 6.917 MeV subthreshold resonance in the 12C(α, γ)16O reaction was investigated via the 12C(11B, 7Li)16O reaction
We measured the angular distribution of the 12C(11B, 7Li)16O transfer reaction leading to the 6.917 MeV state
Summary
The 12C(α, γ)16O reaction is believed to be one of the most crucial reactions in nuclear astrophysics [1, 2]. At energies corresponding to the Gamow peak, the 12C(α, γ)16O cross sections are too low (at the order of 10−17 barn) to be measured directly in the ground laboratory. The other is the E2 capture which includes the contributions from the direct capture and the subthreshold 2+ resonance at Ex = 6.917 MeV. To date considerable experimental methods have been utilized to study these two subthreshold resonances, such as the α + 12C elastic scattering [8], the β-delayed α decay of 16N [9], radiative capture cross sections [10], transfer reactions [11] and Coulomb dissociation [12]. In this work measurement of the 12C(11B, 7Li)16O reaction is performed to derive the reduced α-width of the 6.917 MeV 2+ subthreshold resonance. The astrophysical S E2 factor at the Gamow peak of the 12C(α, γ)16O reaction is studied
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