Improved information on electron screening in Li-7(p, alpha)alpha using the Trojan-horse method

The available astrophysical S(E) factor data for the reaction Li-7(p, alpha )a at 10 < E < 1000keV exhibit an exponential increase at low energies due to the effects of electron screening. A parametrisation of the data using a non-resonant, direct process and two subthreshold resonances reproduces t he data at energies E greater than or equal to 100keV, while at lower energ ies this calculated S-b(E) factor curve for bare nuclides drops below the d ata, which in turn represent the case of electron-shielded nuclides, i.e. t he electron-shielded S-s(E) factor. The comparison between S-b(E) and S-s(E ) leads to an electron-screening potential energy U-e = 350 eV, which is mu ch higher than the adiabatic limit of 175 eV and not understood at present. The deduced value of S-h(0) is considerably smaller than the previously ad opted value of 59 keV b, significantly increasing thf calculated abundance of Li-7 in big-bang nucleosynthesis. The Trojan-horse method was applied to the reaction Li-7(p,alpha)alpha to determine the energy dependence of the Sb(E) factor for 10 < E < 370 keV,. free from the effects of the Coulomb ba rrier and electron screening. Thr THM results are close to the calculated S -b(E) curve and suggest that the THM may become a powerful way; to obtain i mproved information on low-energy cross-sections and associated electron-sc reening effects in a model-independent way.