The 7 Li+ 28 Si elastic scattering was studied at near-barrier energies, namely, 8, 8.5, 9, 10, 11, 13, 15, and 16 MeV, with the aim to map the real and imaginary part of the optical potential and therefore probe the threshold anomaly. Angular distributions were measured over a wide angular range of ( lab = 25°to 150°) for the lower energies and of ( lab = 10°to 100°) for the higher energies. The present data, together with previous ones on heavier targets ( 138 Ba and 208 Pb) at near barrier energies, were analyzed by using optical potentials obtained in a double-folding framework. The results were compared with previous measurements of 6 Li on the same targets. It was found that a striking difference occurs between the imaginary potentials of 6 Li and 7 Li, which, respectively, present an increasing and decreasing behavior approaching the barrier from higher to lower energies. On the other hand, this energy variation is not fully reflected to the real part of the potential, as it is described by dispersion relations. The strength of the real potential remains almost constant with a weak declining and uprising trend for the 6 Li and 7 Li, respectively. For a better understanding of our results, continuum-discretized-coupled-channel calculations were also performed and are discussed.
The production of alpha particles in the 6Li+28Si reaction was studied at near-barrier energies. Angular distributions were performed at four bombarding energies, namely, 7.5, 9, 11, and 13 MeV. The distributions were characterized by a Gaussian shape, which was integrated in order to obtain alpha-particle cross sections. Our results were compared with previous data of 6Li scattering on various heavier targets and found to exhibit a universal behavior. Present continuum-discretized-coupled-channel calculations support the obtained data. The consequences of the systematic behavior of the alpha-particle production on the unusual behavior of the imaginary potential observed previously in elastic scattering of weakly bound systems is discussed.
The cross section of the l~B(n, a)8Li reaction has been measured at E,=7.6 to 12.6 MeV. The neutron beam was produced via the D(d, n)3He reaction and a BF3 counter (with natural B isotopic composition) served both as target for the I~B nuclides and as detector for the observation of the delayed a-activity of SLi. The data match well with previous results obtained at E,= 12.5 to 20.0 MeV. Using the principle of detailed balance the data were converted to the case of the SLi(a, no) 11B reaction. The associated astrophysical S(E) factor is dominated by a resonance at ER=0.58MeV of width FR =200 keV, with S(ER)=8400 MeVb. This S(ER) value for the no channel alone is already three times higher than the constant S(E) factor assumed previously and, thus, strengthens the significance of inhomogeneous Big Bang nucleosynthesis.
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