The11B(p→,γ)12Creaction below 100 keV

The ${}^{11}\mathrm{B}(\stackrel{\ensuremath{\rightarrow}}{p},\ensuremath{\gamma}{)}^{12}\mathrm{C}$ reaction was studied by measuring the $\ensuremath{\gamma}$ 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 $90\ifmmode^\circ\else\textdegree\fi{}$ 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 $S(0)$ that was obtained for ${}^{11}\mathrm{B}(p,{\ensuremath{\gamma}}_{0}),$ $1.8\ifmmode\pm\else\textpm\fi{}0.4 \mathrm{keV}\mathrm{}\mathrm{b},$ is in agreement with previously determined values, but for ${}^{11}\mathrm{B}(p,{\ensuremath{\gamma}}_{1})$ the value of $S(0)$ is $3.5\ifmmode\pm\else\textpm\fi{}0.6 \mathrm{keV}\mathrm{}\mathrm{b}$ and is more than twice as large as previously determined values.