Direct measurement of B 11 ( p , γ ) C 12 astrophysical S factors at low energies

We directly measure the absolute cross section of $^{11}\mathrm{B}(p,\ensuremath{\gamma})^{12}\mathrm{C}$ in the energy region of ${E}_{\mathrm{c}.\mathrm{m}.}=130$--257 keV by using a thin target for the first time. This work is performed on a 320-kV platform at the Institute of Modern Physics in Lanzhou. The astrophysical $S$ factors of this reaction are obtained for capture to the ground and first excited states of $^{12}\mathrm{C}$. The properties of the known resonance at $\ensuremath{\sim}150$ keV are derived and agree with the previous results. However, in the energy region of 170--240 keV, our $S$ factors are about 15%--50% larger than the adopted values in NACRE II and are also larger than the upper limits of NACRE II by up to $\ensuremath{\sim}20%$. This indicates that our new reaction rate is enhanced by about 15%--50% compared to the NACRE II adopted rate in the temperature region 0.32--0.62 GK.