Slope of the astrophysical S factor for the Li-7(p, gamma)Be-8 reaction - art. no. 015802

Polarized proton beams of 80 and 60 keV, and a biased target have been used to determine the slope of the astrophysical S factor for the Li-7(p, gamma )Be-8 reaction from direct measurements at proton energies from 40 to 100 k eV for capture to both the ground and the first excited states of Be-8. The results indicate a negative slope for both cases. Vector analyzing power m easurements at 900 were also obtained for proton energies from 40 to 80 keV . It is shown that the negative slope can be accounted for by considering t he effect of the subthreshold resonance state at 16.6 MeV if the capture in to this weakly bound, subthreshold state occurs at an effective radius of a bout 46 fm. The effect of this state increases the extrapolated value of th e S factor at E-p = 0 from the previous value of 0.3 keV b to the present v alue of 0.50 keV b.