The ${}^{11}\mathrm{B}(\overrightarrow{p},\gamma {)}^{12}\mathrm{C}$ reaction was studied by measuring the $\gamma$ rays that were produced when 80–100-keV polarized protons were stopped in a thick ${}^{11}\mathrm{B}$ target. Cross sections and vector analyzing powers at $90°$ were determined as a function of energy for capture to the ground and first excited states of ${}^{12}\mathrm{C}.$ These analyzing powers are particularly sensitive to the interference between s- and p-wave contributions, and to the relative phase between direct and resonance amplitudes. The results were used to produce a reliable extrapolation of the astrophysical S factor at 0 keV by means of a direct-capture-plus-resonances model calculation. The value of $S\left(0\right)\mathrm{}$ that was obtained for ${}^{11}\mathrm{B}(p,{\gamma }_0),$ $1.8\pm 0.4\hspace{0.5em}\mathrm{keV}\mathrm{}\mathrm{b},$ is in agreement with previously determined values, but for ${}^{11}\mathrm{B}(p,{\gamma }_1)$ the value of $S\left(0\right)\mathrm{}$ is $3.5\pm 0.6\hspace{0.5em}\mathrm{keV}\mathrm{}\mathrm{b}$ and is more than twice as large as previously determined values.

• Received 5 April 2000

DOI:https://doi.org/10.1103/PhysRevC.62.025803