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Nuclear processes not only generate the energy and drive the evolution of stars, but also are responsible for the synthesis of the elements in the Universe. Helium ( 4 He, or α) is the second most abundant element after hydrogen, thus α -particle induced reactions such as (α , γ) , (α , n) and (α , p) play a crucial role in nuclear astrophysics, especially for understanding stellar helium burning. Direct measurement of the cross sections for these α -capture reactions at stellar energies is greatly hindered because of the strong Coulomb repulsion. Alpha-cluster transfer reaction is a powerful tool for investigation of astrophysical (α , γ) , (α , n) and (α , p) reactions since it can preferentially populate the natural-parity states with an α -cluster structure which dominantly contribute to these astrophysical α -capture reactions during stellar helium burning. In this review we summarize the theoretical scheme, the experimental technique, astrophysical applications and the future perspectives of such approach based on α -cluster transfer reactions. [ABSTRACT FROM AUTHOR]

The 12C(α,γ)16O reaction is one of the most crucial reactions in nuclear astrophysics. The E2 external capture to the 16O ground state (GS) has not been emphasized in previous analyses but may make a significant contribution to the 12C(α,γ)16O cross section depending on the value of the GS asymptotic normalization coefficient (ANC). In the present work, we determine this ANC to be 337±45 fm-1/2 through the 12C(11B,7Li)16O reaction using a high-precision magnetic spectrograph. This sheds light on the existing large discrepancy of more than 2 orders of magnitude between the previously reported ANC values. Based on the new ANC, we experimentally constrain the GS external capture and show that through interference with the high energy tail of the 2+ subthreshold state, a substantial enhancement in the GS SE2(300) factor can be obtained (70±7 keV b) compared to that of a recent review (45 keV b), resulting in an increase of the total S factor from 140 to 162 keV b, which is now in good agreement with the value obtained by reproducing supernova nucleosynthesis calculations with the solar-system abundances. This work emphasizes that the external capture contribution for the ground state transition cannot be neglected in future analyses of the 12C(α,γ)16O reaction. [ABSTRACT FROM AUTHOR]

We present the first experimental determination on the spectroscopic amplitudes (SAs) for the α -cluster in the B 11 ground state via the Li 7 (Li 6 , d) 11 B reaction using a high-precision magnetic spectrograph. This is believed to have a strong effect on the studies of α -induced reactions which are crucial in nuclear astrophysics. It is found that the previous SAs of B 11 from shell model calculations cause overestimations of up to 23% for the C 12 (α , γ) 16 O S E 2 (300) factor and up to 34% for the C 13 (α , n) 16 O S (190) factor with respect to the results from the present experimentally determined SAs. Due to the importance of these two reactions, for example the uncertainty in the C 12 (α , γ) 16 O cross section at E c.m. = 300 keV is required to be better than 10% by stellar modeling, the new S -factors could result in a potential influence on the astrophysical network calculations. [ABSTRACT FROM AUTHOR]

The 12C(α,γ)16O reaction plays a key role in the evolution of stars with masses of M>0.55M⊙. The cross-section of the 12C(α,γ)16O reaction within the Gamow window (Ec.m.=300keV, T9=0.2) is extremely small (about 10-17 barn), which makes the direct measurement in a ground-based laboratory with existing techniques unfeasible. Up until now, the cross-sections at lower energies can only be extrapolated from the data at higher energies. However, two subthreshold resonances, located at Ex=7.117MeV and Ex=6.917MeV, make this extrapolation more complicated. 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. The experiment was performed using the Q3D magnetic spectrograph at the HI-13 tandem accelerator. We measured the angular distribution of the 12C(11B,7Li)16O transfer reaction leading to the 6.917 MeV state. Based on the finite-range distorted wave Born approximation (FRDWBA) and coupled-reaction-channel (CRC) analysis, we derived the asymptotic normalization coefficient (ANC) of the 6.917 MeV level in 16O to be (1.10±0.29) ×1010fm-1, with which the reduced α width was computed to be 18.0±4.7keV at the channel radius of 6.5 fm. Finally, we calculated the astrophysical SE2(300) factor of the ground-state transitions to be 46.2 ± 7.7 keV b. The result for the astrophysical SE2(300) factor confirms the values obtained in various direct and indirect measurements and presents an independent examination of the most important data in nuclear astrophysics. [ABSTRACT FROM AUTHOR]

JUNA measured the SP 19 sp F(p, ) SP 16 sp O channel down to the lowest energy of E SB c.m. sb 72 keV, and the JUNA rate deviates significantly from the previous expectations by a factor of 0.2-1.3 [[21]]. Thanks to the advantages provided by CJPL, the Jinping Underground Nuclear Astrophysics (JUNA) experiment collaboration has proposed to exploit the extremely low background and try to reach the Gamow region in CJPL-II A1 Hall (see Figure 1) [[9]]. Previously, this reaction was thought to be weak compared to the SP 19 sp F(p, ) SP 16 sp O reaction, so most of the SP 19 sp F produced by the CNO cycle would be recycled back into SP 16 sp O, with no substantial chemical abundance changes. The background mainly consists of the neutron-induced background from SP 13 sp C( ,n) SP 16 sp O reaction and the gamma background from other ( , ) reactions, making the analysis complicated. [Extracted from the article]

Abstract: Angular distributions of 2H(6He,7Li)n and 12C(7Li,6He)13N reactions were measured at the HI-13 tandem accelerator, Beijing. Asymptotic normalization coefficient (ANC) of 13N →12C + p was derived to be through distorted wave Born approximation (DWBA) analysis, which was then used to deduce the astrophysical S(E) factors for direct capture in 12C(p,γ)13N at energies of astrophysical relevance. [Copyright &y& Elsevier]

Abstract: Systematic studies of nuclear astrophysical reactions induced by lithium isotopes using the unstable ion beam facility GIRAFFE in CIAE were performed. We have measured the angular distributions of some single nucleon transfer reactions, such as 8Li(d,p)9Li, 8Li(d,n)9Be and 8Li(p,d)7Li in inverse kinematics, and derived the astrophysical S-factors or reaction rates for 8Li(n,γ)9Li and 8Li(p,γ)9Be by using asymptotic normalization coefficient (ANC) or spectroscopic factor. [Copyright &y& Elsevier]