RESUMO
The 12C(α,γ)^16O reaction plays a fundamental role in astrophysics and needs to be known with accuracy better than 10%. Cascade γ transitions through the excited states of 16 O are contributing to the uncertainty. We constrained the contribution of the 0+ (6.05 MeV) and 3- (6.13 MeV) cascade transitions by measuring the asymptotic normalization coefficients for these states using the α-transfer reaction 6 Li(12C,d)^16O at sub-Coulomb energy. The contribution of the 0+ and 3- cascade transitions at 300 keV is found to be 1.96 ± 0.3 and 0.12 ± 0.04 keV b for destructive interference of the direct and resonance capture and 4.36 ± 0.45 and 1.44 ± 0.12 keV b for constructive interference, respectively. The combined contribution of the 0+ and 3- cascade transitions to the 12C(α,γ)16O reaction cross section at 300 keV does not exceed 4%. Significant uncertainties have been dramatically reduced.
RESUMO
In this review, we discuss the present status of three indirect techniques that are used to determine reaction rates for stellar burning processes, asymptotic normalization coefficients, the Trojan Horse method and Coulomb dissociation. A comprehensive review of the theory behind each of these techniques is presented. This is followed by an overview of the experiments that have been carried out using these indirect approaches.
Assuntos
Astronomia/métodos , Evolução Química , Modelos Químicos , Física Nuclear/métodos , Radiometria/métodos , Astros Celestes , Termodinâmica , Simulação por Computador , Doses de RadiaçãoRESUMO
The (13)C(α,n)(16)O reaction is the neutron source for the main component of the s-process, responsible for the production of most nuclei in the mass range 90~A~204. It is active inside the helium-burning shell in asymptotic giant branch stars, at temperatures ~10(8) K, corresponding to an energy interval where the (13)C(α,n)(16)O is effective from 140 to 230 keV. In this region, the astrophysical S(E)-factor is dominated by the -3 keV subthreshold resonance due to the 6.356 MeV level in (17)O, giving rise to a steep increase of the S(E)-factor. Notwithstanding that it plays a crucial role in astrophysics, no direct measurements exist inside the s-process energy window. The magnitude of its contribution is still controversial as extrapolations, e.g., through the R matrix and indirect techniques, such as the asymptotic normalization coefficient (ANC), yield inconsistent results. The discrepancy amounts to a factor of 3 or more right at astrophysical energies. Therefore, we have applied the Trojan horse method to the (13)C((6)Li,n(16)O)d quasifree reaction to achieve an experimental estimate of such contribution. For the first time, the ANC for the 6.356 MeV level has been deduced through the Trojan horse method as well as the n-partial width, allowing to attain an unprecedented accuracy in the (13)C(α,n)(16)O study. Though a larger ANC for the 6.356 MeV level is measured, our experimental S(E)-factor agrees with the most recent extrapolation in the literature in the 140-230 keV energy interval, the accuracy being greatly enhanced thanks to this innovative approach.
RESUMO
We formulate scattering theory in the framework of a surface-integral approach utilizing analytically known asymptotic forms of the three-body wave functions. This formulation is valid for both short-range and Coulombic potentials. The post and prior forms of the breakup amplitude are derived without any reference to renormalization procedures.
RESUMO
The 18O(p,alpha)15N reaction is of primary importance in several astrophysical scenarios, including fluorine nucleosynthesis inside asymptotic giant branch stars as well as oxygen and nitrogen isotopic ratios in meteorite grains. Thus the indirect measurement of the low energy region of the 18O(p,alpha)15N reaction has been performed to reduce the nuclear uncertainty on theoretical predictions. In particular the strength of the 20 and 90 keV resonances has been deduced and the change in the reaction rate evaluated.
RESUMO
Off-energy-shell effects in p - p scattering have been investigated at p - p relative energies from 600 down to 80 keV applying the Trojan horse method (THM) to the p + d --> p + p + n reaction at 5 MeV. In contrast with the on-energy-shell case, no Coulomb-nuclear interference minimum has been found in the extracted THM p - p cross section, due to the suppression of the Coulomb amplitude as predicted by the half-off-energy shell calculations. This hypothesis is strengthened by the agreement between THM p - p data and calculated on-energy-shell n + n, n + p and nuclear p + p cross sections.
RESUMO
The reaction 13C(alpha,n) is considered to be the main source of neutrons for the s process in asymptotic giant branch stars. At low energies, the cross section is dominated by the 1/2+ 6.356 MeV subthreshold resonance in (17)O whose contribution at stellar temperatures is uncertain by a factor of 10. In this work, we performed the most precise determination of the low-energy astrophysical S factor using the indirect asymptotic normalization (ANC) technique. The alpha-particle ANC for the subthreshold state has been measured using the sub-Coulomb alpha-transfer reaction ((6)Li,d). Using the determined ANC, we calculated S(0), which turns out to be an order of magnitude smaller than in the nuclear astrophysics compilation of reaction rates.
RESUMO
We show how the charge symmetry of strong interactions can be used to relate the proton and neutron asymptotic normalization coefficients (ANCs) of the one-nucleon overlap integrals for light mirror nuclei. This relation extends to the case of real proton decay where the mirror analog is a virtual neutron decay of a loosely bound state. In this case, a link is obtained between the proton width and the squared ANC of the mirror neutron state. The relation between mirror overlaps can be used to study astrophysically relevant proton capture reactions based on information obtained from transfer reactions with stable beams.
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We present results of the first calculation of the double differential cross section for the 208Pb(8B,(7)Bep)208Pb Coulomb breakup reaction which treats the postdecay acceleration of the ejectiles within a genuine three-body approach. From this we conclude that, in order to minimize postdecay Coulomb acceleration effects, experiments should be performed at as small as possible scattering angles, not too low 7Be-p relative energies, and high incident energy.
RESUMO
It is shown that existing problems with the formal theory of ionization can be effectively resolved. An integral representation for the ionization amplitude free of ambiguity and divergence problems is given. Moreover, the ionization amplitude in the new formulation is shown directly to have an ideal form for practical calculations.