New measurement of the ${}^8\mathrm{Li}(\alpha ,n){}^{11}\mathrm{B}$ reaction in a lower-energy region below the Coulomb barrier

The ${}^8\mathrm{Li}(\alpha ,n){}^{11}\mathrm{B}$ reaction is regarded as the key reaction in the inhomogeneous big bang and in type-II supernova nucleosynthesis. Recently, the importance of this reaction to solving the ${}^7\mathrm{Li}$ problem, i.e., the inconsistency between the predicted and the observed primordial ${}^7\mathrm{Li}$ abundances, has also been noted. The most recent cross-section data published by our collaboration group in 2006 [H. Ishiyama et al., Phys. Lett. B 640, 82 (2006)] cover the 0.7- to 2.6-MeV energy region in the center-of-mass system. Here, we present additional data spanning the 0.45- to 1.8-MeV energy region. Thus, the predominant energy region for the big bang nucleosynthesis, corresponding to $T_9=1$ (where $T_9$ is a temperature unit equivalent to ${10}^9$ K), is almost completely spanned by the previous [H. Ishiyama et al., Phys. Lett. B 640, 82 (2006)] and present results together.

Figure 1

Cross sections of the ${}^8\mathrm{Li}(\alpha ,n){}^{11}\mathrm{B}$ reaction. The present and previous experimental results are plotted using the indicated markers. Gamow energies corresponding to $T_9=1$, 2, and 3 are drawn for reference.

Figure 2

Level scheme of the ${}^8\mathrm{Li}+\alpha \to {}^{11}\mathrm{B}+n$ reaction. $n_i$ indicates a neutron going to each state of ${}^{11}\mathrm{B}$, assuming that a reaction occurs at the center-of-mass energy, $E_{\mathrm{c}.\mathrm{m}.}=1.05$ MeV.

Figure 3

Neutron energy as a function of laboratory angle. Assuming that reaction occurs at $E_{\mathrm{c}.\mathrm{m}.}=1.05$ MeV and that ${}^{11}\mathrm{B}$ is excited in each state.

Figure 4

Solid-angle ratio of the center-of-mass system $\left({\mathrm{\Omega }}_{\mathrm{c}.\mathrm{m}.}\right)$ with respect to the laboratory system $\left({\mathrm{\Omega }}_{\mathrm{lab}.}\right)$ as a function of laboratory angle. Assuming that the reaction occurs at $E_{\mathrm{c}.\mathrm{m}.}=1.05$ MeV and that ${}^{11}\mathrm{B}$ is excited in each state.