Differential cross sections for the ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,p)$ reaction leading to the ground and 478-keV states of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ have been measured at 11 energies between 39.0 and 49.5 MeV and used to determine integral cross sections for ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ production in ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,p)$. Comparison of the ground state cross sections with ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(p,\alpha )$ data using the principle of detailed balance indicates good agreement with the most recent ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(p,\alpha )$ measurements but not with some older results. Cross sections for ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,d){}_{}{}^6{}_{}{}^{}\mathrm{Li}$ (ground state), extracted at three energies, are consistent with the most recent ${}_{}{}^6{}_{}{}^{}\mathrm{Li}(d,\alpha )$ measurements. Cross sections for ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$ production via ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,n){}_{}{}^7{}_{}{}^{}\mathrm{Be}$ were determined at nine energies between 39.4 and 47.4 MeV by direct collection of the ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$ and detection of the $\gamma$ rays following its decay. These cross sections are essentially identical to the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ production cross sections above 43 MeV but are smaller below this energy because of threshold effects. The ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,p)$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,n)$ reactions are involved in several mechanisms which have been proposed to explain the universal abundance of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, and calculations of these processes have assumed cross sections larger than our data. We therefore confirm earlier conclusions that ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ is unlikely to be produced by galactic cosmic-ray spallation alone, but the status of the other mechanisms remains unclear. Peaks in the ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,p)$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,n)$ excitation functions are discussed in terms of resonances in ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$. Parameters determined by Kumar and Barker in a fit to ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(p,\alpha )$ data are inadequate to explain our ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,p){}_{}{}^7{}_{}{}^{}\mathrm{Li}$ (478 keV) cross sections.

NUCLEAR REACTIONS ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,p)$, $E=39.0\mathrm{}-49.5$ MeV; measured $\sigma (E;\theta ,E_p)$, $\sigma \mathrm{}\left(E\right)\mathrm{}$. ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,n)$, $E=39.4\mathrm{}-47.4$ MeV; measured $\sigma \mathrm{}\left(E\right)\mathrm{}$. ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\alpha ,d)$, $E=46.7\mathrm{}-49.5$ MeV; measured $\sigma (E;\theta )$, $\sigma \mathrm{}\left(E\right)\mathrm{}$. Astrophysical implications for ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ production considered. Discussion of ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$ levels in $R$-matrix formalism.

• Received 20 June 1977

DOI:https://doi.org/10.1103/PhysRevC.16.1712