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 ${}^7\mathrm{Li}(p,\gamma {)}^8\mathrm{Be}$ reaction from direct measurements at proton energies from 40 to 100 keV for capture to both the ground and the first excited states of ${}^8\mathrm{Be}.$ The results indicate a negativeslope for both cases. Vector analyzing power measurements at $90°$ were also obtained for proton energies from 40 to 80 keV. It is shown that the negative slope can be accounted for by considering the effect of the subthreshold resonance state at 16.6 MeV if the capture into this weakly bound, subthreshold state occurs at an effective radius of about 46 fm. The effect of this state increases the extrapolated value of the S factor at $E_p=0\mathrm{}$ from the previous value of 0.3 keV b to the present value of 0.50 keV b.

• Received 25 January 1999

DOI:https://doi.org/10.1103/PhysRevC.61.015802