Li7andBe7production in theα+αreaction

Abstract

Differential cross sections for the $^{4}\mathrm{He}(\ensuremath{\alpha},p)$ 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 $^{4}\mathrm{He}(\ensuremath{\alpha},p)$. Comparison of the ground state cross sections with $^{7}\mathrm{Li}(p,\ensuremath{\alpha})$ data using the principle of detailed balance indicates good agreement with the most recent $^{7}\mathrm{Li}(p,\ensuremath{\alpha})$ measurements but not with some older results. Cross sections for $^{4}\mathrm{He}(\ensuremath{\alpha},d)^{6}\mathrm{Li}$ (ground state), extracted at three energies, are consistent with the most recent $^{6}\mathrm{Li}(d,\ensuremath{\alpha})$ measurements. Cross sections for $^{7}\mathrm{Be}$ production via $^{4}\mathrm{He}(\ensuremath{\alpha},n)^{7}\mathrm{Be}$ were determined at nine energies between 39.4 and 47.4 MeV by direct collection of the $^{7}\mathrm{Be}$ and detection of the $\ensuremath{\gamma}$ 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 $^{4}\mathrm{He}(\ensuremath{\alpha},p)$ and $^{4}\mathrm{He}(\ensuremath{\alpha},n)$ 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 $^{4}\mathrm{He}(\ensuremath{\alpha},p)$ and $^{4}\mathrm{He}(\ensuremath{\alpha},n)$ excitation functions are discussed in terms of resonances in $^{8}\mathrm{Be}$. Parameters determined by Kumar and Barker in a fit to $^{7}\mathrm{Li}(p,\ensuremath{\alpha})$ data are inadequate to explain our $^{4}\mathrm{He}(\ensuremath{\alpha},p)^{7}\mathrm{Li}$ (478 keV) cross sections.NUCLEAR REACTIONS $^{4}\mathrm{He}(\ensuremath{\alpha},p)$, $E=39.0\ensuremath{-}49.5$ MeV; measured $\ensuremath{\sigma}(E;\ensuremath{\theta},{E}_{p})$, $\ensuremath{\sigma}(E)$. $^{4}\mathrm{He}(\ensuremath{\alpha},n)$, $E=39.4\ensuremath{-}47.4$ MeV; measured $\ensuremath{\sigma}(E)$. $^{4}\mathrm{He}(\ensuremath{\alpha},d)$, $E=46.7\ensuremath{-}49.5$ MeV; measured $\ensuremath{\sigma}(E;\ensuremath{\theta})$, $\ensuremath{\sigma}(E)$. Astrophysical implications for $^{7}\mathrm{Li}$ production considered. Discussion of $^{8}\mathrm{Be}$ levels in $R$-matrix formalism.