Polarized proton beams with energies from 80 to $130\mathrm{keV}$ have been used to determine the slope of the astrophysical $S$ factor for the ${}^6\mathrm{Li}\left(p,{\gamma }_0\right){}^7\mathrm{Be}$ and ${}^6\mathrm{Li}\left(p,{\gamma }_1\right){}^7\mathrm{Be}$ reactions. The slope was determined from the relative yields at five incident proton energies. The slope of the $S$ factor was found to be negative. The analyzing power measurements indicate that the reaction proceeds predominately by $s$-wave capture. The negative slope does not appear to be due to low-binding-energy effects, the effects of nearby resonances, or electron screening of the nuclei. A resonating group model calculation demonstrates a new mechanism for producing a negative slope at astrophysically relevant energies in radiative capture reactions.

• Received 13 August 2004

DOI:https://doi.org/10.1103/PhysRevC.70.055801